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Behavioral and Brain Sciences (under review)
Murder by Design:
The Evolution of Homicide
David M. Buss and Joshua D. Duntley
David M. Buss, Department of Psychology, University of Texas, Austin, Texas 78712. Email:
[email protected]. URL: www.davidbuss.com
Joshua D. Duntley, Department of Psychology, University of Texas, Austin, Texas 78712. Email:
[email protected].
Short Abstract
We propose Homicide Design Theory as a new theory to explain why people kill. Multiple homicide
mechanisms have evolved as effective context-sensitive solutions to distinct adaptive problems.
Killing historically conferred large fitness benefits: preventing premature death, removing rivals,
gaining resources, depriving rivals of mates, aborting rival’s prenatal offspring, eliminating
stepchildren, and winnowing future competitors of one’s children. Homicidal premeditations are part
of evolved killer design, functioning to mobilize attention, rehearse scenarios, calculate
consequences, and motivate behavior. Because getting killed inflicts temporally cascading costs on
victims, selection has forged anti-homicide defenses, producing coevolutionary arms races between
homicidal strategies and anti-homicide defenses.
Long Abstract
Why do humans kill other humans? Extant theories of homicide–those invoking social learning,
media influence, cultures of honor, brain pathology, genetic defects, and evolutionary
epiphenomena–fail to explain the varying motives for murder and the dramatically different
circumstances in which murders occur. We propose a new theory of killing--Homicide Design
Theory. Multiple homicide mechanisms have evolved as context-sensitive solutions to an array of
diverse adaptive problems. Murder in circumscribed contexts conferred large fitness benefits. These
include protecting self and kin from injury or death, depriving rivals of access to valuable mates,
gaining access to contested resources, terminating prenatal offspring of rivals, eliminating
genetically unrelated stepchildren, removing key antagonists, and winnowing future competitors of
one’s children. Homicidal premeditations are hypothesized to be central components of evolved
design for murder. Homicidal premeditations function to mobilize attention, construct and rehearse
scenarios, calculate consequences, evaluate costs and benefits, and motivate decisions about whether
or not to commit homicide. Getting killed, however, inflicts temporally cascading costs on victims.
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The fitness costs of premature death imposed selection pressure for the evolution of anti-homicide
defense mechanisms that function to avoid death and deter killers. This led to a coevolutionary arms
race between homicidal strategies and anti-homicide defense mechanisms, analogous to arms races
between predators and prey. Homicide Design Theory generates several dozen novel testable
predictions about the psychology of murder that are supported by empirical studies. The theory
parsimoniously accounts for paleontological, archeological, ethnographic, psychological, and
behavioral evidence that remains inexplicable on alternative theories.
Key words: homicide, murder, evolution, killing, victims, defenses, evolutionary psychology
"Now kill every male dependent, and kill every woman who has had intercourse with a man, but
spare for yourselves every women among them who has not had intercourse." (Numbers 31)–
Instructions by Moses after the conquest of the Midianites.
"To deprive others of their life is one of the most effective means of increasing one’s fitness" —
Joseph Lopreato (1984)
1. Introduction
No offense against another human being inflicts greater costs than murder. The dead cease to
contribute to their own affairs, and cannot actively influence the affairs of their families, friends, or
enemies. Wherever written laws exist, killing is always singled out as a crime. No other infraction
comes attached with greater punishment. Where written laws are absent, killing typically constitutes
a major cause of death, sometimes accounting for the mortality of a third of all males (Keeley, 1996).
Although cultures with written laws, hired police forces, and the prospect of imprisonment appear to
have produced substantially lower homicide rates than cultures lacking these modern penalties, even
in modern societies the odds of dying by the intentional hand of another run as high as one in twentysix for certain sub-groups (Ghiglieri, 1999).
Homicide may be defined as the killing of one human being by another (Wilbanks, 1982, p. 153). In
1998, 18,936 murders occurred in the United States alone--a fairly typical year for killings (Federal
Bureau of Investigation, 1999). These figures include only "successful" murders, and ignore the
large, but currently unquantifiable, numbers of attempted murders that fail for one reason or another.
These figures also don’t include homicides committed in self-defense. Nor do they include many of
the assaults that do not lead to immediate death, but rather to the victim’s death days or weeks after a
police report is filed. The official murder rates also fail to include the potentially large number of
infanticides officially classified as Sudden Infant Death Syndrome (SIDS), a portion of which are
turning out to be intentional homicides (Becroft, Lockett, Etzel, Montaña, Dearborn, Smith, Infeld,
Dahms, & Carroll-Pankhurst, 1998). For all these reasons, current estimates of homicide rates are
likely to substantially underestimate the numbers of deaths produced at the hands of other humans.
Even given these underestimates, if we assume that the average American lives to be 72, the odds of
being killed by the hands of another human being, are more than 1 in 200 (and are considerably
higher for males).
Scientists have compiled a great body of statistics dealing with homicide, including the ages of
killers and victims, and some of the circumstances in which killing occurs. The most compelling
question, however, has thus far defied a cogent scientific answer: Why do humans kill members of
their own species? This paper presents a new theory of homicide--a theory that departs from prior
explanations of the taking of human life.
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1.1. Homicide Prevalence and Importance
Some authors claim that the United States is an unusually violent culture, and compared to many
other modern technologically advanced cultures, it is. American men aged 15 to 24, for example, kill
at four times the rate of their Scottish counterparts, seven times the rate of their Canadian
counterparts, and 40 times the rate of their Japanese counterparts (Fingerhut & Kleinman, 1990).
During the 100 year period from 1900 through 1999, an estimated 1,056,296 people have been
murdered in the United States alone (White, 1999). But within the larger context of cultures around
the world, including traditional and tribal societies, the United States appears to be an unusually
peaceable culture. In New Guinea, killings account for the deaths of 19.5% of adult men among the
Huli; 25% among the Mae Enga; and 28.5% among the Dugum Dani (Chagnon, 1988). Among the
Yanomamo of Venezuela, nearly 30% of all males die a violent death at the hands of another human
being (Chagnon, 1988). Even among the so-called "peaceful" !Kung San of Botswana, the murder
rate is higher than the most violent cities within the United States (Daly & Wilson, 1988). These socalled "gentle people" recorded 22 murders over a 25 year time span in a population of 1,500--a rate
more than quadruple the rate in a typical year in the United States (Lee, 1984).
These reported within-culture rates of homicide typically do not include killings due to warfare and
genocide. In 1971, for example, the Pakistani army killed an estimated 3 million people in the
Bangladesh conflict (Wrangham & Peterson, 1996). More recently, genocides in Congo, Bosnia,
Kosovo, and Indonesia have produced millions of deaths. Keeley (1996) reported estimates of the
percentage of deaths due to warfare in more than a dozen prestate societies, ranging from the Tiwi
tribe located on Melville and Bathurst islands to the Gebusi in New Guinea. The estimates ranged
from a low of 7% for the Skateholm of Sweden in 4300 B.C. to 40% for the Qadan of Nubia in
10,000 B.C. Keeley (1996) calculates that a typical tribal society lost .5 percent of its membership
per year to combat in warfare. Deaths due to warfare and genocide must be added to within-culture
rates of homicide to gain accurate estimates of the likelihood of dying from homicide.
Killings galvanize human attention and interest like no other phenomenon. Television, movies,
novels, plays, and paintings are filled with dramatic episodes of human killing. No culture with
written laws fails to prohibit homicide. Some argue that laws are often made to prevent people from
doing things that they might otherwise be inclined to do (Daly & Wilson, 1988). Human recorded
history bears this out. It is replete with thousands of accounts of infanticide, fratricide, matricide,
siblicide, duels, blood revenge, jealousy-induced mate killings, intrasexual rivalry killings,
genocides, and organized warfare involving dozens, hundreds, thousands, and occasionally millions
of deaths.
Given the prevalence of homicide and the dramatic nature of its consequences, it may seem
astonishing to note that "we have only the most rudimentary scientific understanding of who is likely
to kill whom and why" (Daly & Wilson, 1988, p. ix). A variety of theories, however, have been
proposed to account for this most drastic form of violence, what has been called "the ultimate
conflict resolution technique" (Daly & Wilson, 1988). These include social learning theories,
theories that invoke "cultures of violence," theories that propose that homicide is a form of
pathology, and theories that invoke principles of evolution and selection (Daly & Wilson, 1989).
We will argue that no existing theory provides an adequate explanation for the known empirical facts
about homicide. A key part of the problem is that homicide has often been treated as a singular
phenomenon, and presumed to be amenable to a single causal explanation. We argue that the
umbrella category of "homicide" subsumes an astonishing variety of different types killing such as
infanticide, step-child killing, intrasexual rivalry homicides, mate killing, and warfare killing. A
comprehensive theory of homicide must account for these different types, including the different
causal conditions that give rise to each.
We present a new theory of murder–Homicide Design Theory. We then show that this new theory
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can more parsimoniously account for the known facts about homicide and can generate a host of
new, testable, empirical predictions not generated by prior theories. Finally, we briefly present
results from several new empirical studies that support predictions from the theory--findings
inexplicable on previous theories of homicide.
2. Prior Theories of Criminality Co -Opted to Explain Homicide
This section briefly describes prior attempts to explain homicide. Each theory is followed by a short
critique that highlights inadequacies in accounting for known homicide data. Several introductory
remarks must be made about these theories. First, many theories offered to explain homicide
originally were not designed specifically for this task. Social learning theory, for example, was first
proposed as a general theory of human behavior (Bandura, 1977), and only subsequently co-opted to
explain particular patterns of homicide (Berkowitz, 1993). Second, most explanations of homicide
deal with extremely delimited aspects of homicide, such as why men kill more than women or why
some cultures have higher homicide rates than others. These theories therefore cannot be used to
provide specific and differentiated explanations for qualitatively different types of homicide, such as
infanticide, uxoricide (i.e., killing one’s wife), intrasexual rivalry homicides, and warfare homicides.
Third, with a few notable exceptions, most extant theories have ignored evolution, natural selection,
sexual selection, and adaptation entirely, focusing instead on local, parochial, and recent patterns.
Finally, all fail to account for documented patterns of homicide.
2.1. Social Learning, Social Role, and Socialization Theories
Social learning theory (Bandura, 1977), social role theory (Eagly, 1995), and socialization theories
(Berkowitz, 1993) share many assumptions, so can be treated together for the purpose of this brief
review. The core premise of these theories is that people acquire social behavior by observing and
imitating others–behavior for which they are rewarded or punished, shaping their subsequent
behavioral repertoire. These social theories have been used to explain two aspects of homicide–(1)
the fact that men kill more than women and (2) a more controversial and less established finding that
seems to point to "contagion" or "imitation" or "modeling effects" of violence (Daly & Wilson,
1989).
Daly and Wilson (1989) provide a cogent critique of explanations of both findings on the grounds
that the debate has occurred in a theoretical "vacuum." No one " . . . has proposed a specific
psychological theory of the alleged ‘imitation’ that can explain what sort of publicized event will
produce what sort of imitative violence, by whom, and after what delay" (Daly & Wilson, 1989, p.
103). Daly and Wilson conclude their critique of social learning theories by suggesting that "the
evidence offered in its support has been shaky, consisting mainly of plausibility arguments,
anecdotes (some of . . . dubious origin and likely to be apocryphal ‘urban myths’), and laboratory
studies of questionable ecological validity" (Daly & Wilson, 1989, p. 103).
Regarding sex differences in homicide rates, most studies find that roughly 86% of all homicides are
perpetrated by men, with more than 70% of the victims also being men (Berkowitz, 1993, p. 274).
Social role theory explains this by arguing that social roles have been "traditionally assigned" to men
and women differently: "Think of all the ways in which modern western society . . . teaches children
that fighting is far more appropriate for men than women. Popular literature and the mass media
consistently show men but not women fighting. Parents buy toy weapons for their sons and dolls for
their daughters. Parents are more likely to approve of and reward aggressive behavior in boys than in
girls. Again and again, directly and indirectly, youngsters learn that males are aggressive and females
are not . . ." (Berkowitz, 1993, p. 395).
Although many of these observations are surely true--even a cursory look at modern movies verifies
that more men than women are portrayed as aggressive killers--the explanation fails on two grounds.
First, it assumes that the causal arrow runs from social environment to sex-linked aggressive
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propensities. It ignores a large body of evidence that suggests that boys preferentially seek out and
respond to weapon-like toys and violent media images, that parents may be responding to already
existing desires in their children, and that popular media merely exploit children’s existing
preferences (Hoyenga & Hoyenga, 1993).
Second, these explanations fail when confronted with the cross-cultural evidence. Cultures that lack
literature, mass media, television, and store-bought toys show precisely the same sex difference in
rates of homicide: ". . . why invoke [features of] North American culture to explain a sex difference
that is universal? Ironically, the sex difference in homicidal violence is smaller in modern North
America than in any other human society yet described . . . [these theorists] have wasted their efforts
positing culturally and historically particular explanations for a general phenomenon" (Daly &
Wilson, 1989, p. 101-102).
In addition to these problems, social learning and social role theories run into a variety of
fundamental difficulties. They offer no specific predictions about homicide in particular that could be
tested and potentially falsified. Boys, girls, men, and women throughout their lives are exposed to an
astonishing variety of "models" and "social roles," from serial killers to transvestites. From this
collage of potential models and social exposures, what determines which models are imitated?
Nothing in the theories of social learning, social roles, or socialization provides answers to this key
question.
These social theories also ignore the fact that people kill despite being bombarded with powerful
social sanctions designed to deter killing, religious exhortations and commandments not to kill,
cultural messages indicating that "crime does not pay," and punishments in the form of life
imprisonment and the death penalty. Finally, these theories fail to explain the origins of homicide to
begin with. How did the modeling of homicidal behavior get started? Why would parents be
motivated to incline their sons rather than their daughters toward killing? Why would popular media
conspire to portray men more than women as killers, and why would subsequent media moguls be
motivated to follow suit? Why does killing exist in the human behavioral repertoire at all?
These social theories, in short, are too general to be useful in explaining known facts about homicide
(e.g., the cross-cultural evidence, the age distribution, the specific patterns of victims and
perpetrators) and fail to generate new predictions about aspects of homicide yet undiscovered.
2.2. Cultural Theories of Homicide
Several cultural theories have been proposed to explain one dimension of homicide--why the rates of
homicide vary across societies and across sub-groups within society. Most prominent are the
"subcultures of violence" theory (Wolfgang & Ferracuti, 1967) and the "cultures of honor" theory
(Nisbett, 1993).
The "subcultures of violence" theory proposes that within some cultures or sub-cultures, such as
inner-city street gangs, individuals valorize violence or make it an obligatory means of defending
and protecting one’s reputation. In other cultures or sub-cultures, such as middle-class academicians,
individuals condemn violence and those who use it suffer damage to their reputation. According to
the "subcultures of violence" theory, these differences explain why homicide rates vary from one
subculture to another.
Daly and Wilson (1989) argue that the "subcultures of violence" theory is a pseudo-explanation,
because it simply describes the behavioral differences it is invoked to explain. No attempt has been
made to measure attitudes toward violence as independent predictors of actual homicide rates. And
even if such tests were conducted, the direction of causality would be unclear. Discovering a link
between attitudes toward violence and homicide rates would still fail to answer the question of why
the subcultures differ to begin with. Furthermore, the "subcultures" theory fails to explain a host of
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known facts about homicide, such as (1) why men kill more than women in every culture for which
we have solid data; (2) why killers are heavily concentrated in the age brackets of 15 — 24 in all
cultures for which data are available, rather than more evenly distributed across the lifespan (Wilson
& Daly, 1985); (3) why most individuals residing in subcultures of violence refrain from killing; (4)
why some individuals not living in such subcultures nonetheless choose to kill; and (5) why
infanticides and stepchild killings occur in predictable circumstances in all known cultures, and
appear to be unlinked to valorization of violence.
More recently, Nisbett (1993) proposed a more penetrating theory to explain variation across cultures
in rates of homicide. He proposed that the economic means of subsistence of a culture affect the
degree to which a group originally develops a "culture of honor." In cultures of honor, insults are
viewed as public challenges that must be met with direct confrontation. The theory proposes that
differences in the degree to which honor becomes a central part of cultures ultimately rests with
economics, and specifically with the manner of subsistence. In herding economies, one’s entire stock
could be lost suddenly at the hands of thieves. Cultivating a reputation as willing to respond with
violent force--for example, by actually responding with violence to a public insult--would
presumably deter thieves who might otherwise encroach on one’s resources with impunity. In more
settled, agricultural communities, in contrast, cultivating such a reputation is less important since
one’s economic means of subsistence cannot be rapidly undermined.
Nisbett (1993) tested his theory using homicide statistics from different regions within the Unites
States and experiments that measured differences in violence proneness in individuals from the
North and South. Interestingly, he found evidence for domain-specificity. Southerners (historically
more characterized by herding) do not endorse more positive attitudes toward the use of violence in
general compared with Northerners (historically more characterized by sedentary agriculture).
Southerners were more inclined, however, to endorse violence specifically for the purposes of
protection and in response to insults.
The "cultures of honor" theory differs from the "subcultures of violence" theory in proposing a
specific causal mechanism for the origins of differences between cultures and subcultures--economic
means of subsistence. It has the further virtue of generating testable predictions about cultures that
have not yet been examined. Nonetheless, the theory does not provide an explanation for the many
patterns of homicide that do not directly involve reputational defense. Nisbett’s theory does not
attempt to explain the existing patterns of infanticide, spousal homicide, or the killing of
stepchildren, so these patterns of homicide are beyond its purview.
The key point, however, is that even if Nisbett’s theory is entirely correct in explaining rate
differences in homicide across cultures, we still need a theory to explain the tremendous variety of
specific and different forms of homicide, many of which appear unconnected with notions of honor.
2.3. Pathology Theories of Criminality
Pathology theories suggest that some individuals kill as a result of a brain defect, caused by factors
such as head injury or alcohol damage. One version hypothesizes seizuring, which invokes damage
to the amygdala, an area of the brain that is implicated in controlling social emotions such as
jealousy and rage, and also has been shown to be the area of the brain most vulnerable to seizures
(Ellis, in press). According to the seizuring idea, convulsive brain seizures may induce an individual
to commit a suddenly violent and impulsive act, sometimes resulting in the death of another person.
The evidence, however suggests that violence resulting from seizures is typically random rather than
purposeful or patterned (Ellis, in press; Gazzaniga, Ivry, & Mangun, 1998).
A related theory proposes that criminality results from damage to the frontal lobe, and specifically
the prefrontal cortex (Ellis, in press). Damage to the prefrontal region of the frontal lobes has been
shown to be linked with flatness of emotion and affect, as well as indifference to the suffering of
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others (Asher, 1982; Gazzaniga, Ivry, & Mangun, 1998). According to the hypothesis implicating
frontal lobe damage, criminal behavior, including homicide, may be caused by frontal lobe damage.
Each of these notions of pathology--seizure theory and frontal lobe damage theory--fail as general
theories to explain known patterns of homicide. First, these forms of brain pathology are typically
inferred from the very behavior that they are invoked to explain, although recent work using CAT
scans and MRI technology may eventually circumvent this problem (Ellis, in press). Second, theories
of pathology suggest that violent aggression, and hence homicide, should be directed randomly
toward others, since the hypotheses contain no specifications about the nature of the victim or the
contexts in which homicide will occur. For these reasons, brain pathology theories of criminality fail
as penetrating explanations of the diverse and specific non-random patterns of homicide documented
around the world.
Another theory of pathology invokes genetic abnormalities. Roughly one male out of every 700 to
1,000 is born with an extra Y-chromosome (XYY rather than the normal XY), and one male out of
every 500 is born with an extra X-chromosome (XXY)(Hoffman, 1977, p. 447). Both genetic
abnormalities result in males who are above average in height, develop more acne problems during
adolescence, and score lower on standard tests of intelligence (Ellis, in press). Persons with these
forms of genetic abnormality show an increased likelihood of criminal behavior, with prison
populations showing five-times the proportion of these individuals compared with the general male
population (Ellis, in press). Nonetheless, these genetic abnormalities are likely to explain only a tiny
fraction of the homicides committed, since males with an extra chromosome only constitute 1 - 2%
of the prison population (Ellis, in press).
2.4. Evolutionary Theories of General Criminality
Several evolutionary hypotheses have been proposed to explain criminality in general, although not
homicide specifically. An application of r/K Theory suggests that criminals are r-strategists
compared with non-criminals who are K-strategists; that is, criminals pursue short -term strategies
involving high reproductive rates that presumably entail more risk-taking than the general population
(Ellis, 1989; Rushton, 1995). The Conditional Adaptation Hypothesis suggests that unstable and
resource-scarce environments during development shunt individuals into an opportunistic
exploitative strategy involving criminal behavior (Harpending & Sobus, 1987; Harpending &
Draper, 1988). Alternative Adaptation Theory suggests that genetic differences that influence the
devotion of effort to mating instead of parenting leads to deception and criminality in the pursuit of
opportunistic copulations (Rowe, 1995). And Expropriative Theory proposes that crime is an evolved
tactic for obtaining resources (Cohen & Machalek, 1988; Vila & Cohen, 1993).
All share common features. All propose that genetic factors contribute to criminality in general. All
propose that these genetic factors exist due to a history of natural selection. And all suggest that
historically there have been adaptive benefits to pursuing strategies linked with criminal behavior
that were sufficiently large to outweigh the costs, at least in certain contexts for certain individuals.
These hypotheses may eventually be able to predict some variance in who becomes a criminal (e.g.,
through molecular genetic techniques) and a few of the contexts that are likely to elicit criminal
behavior (e.g., resource-scarce environments). Nonetheless, all share similar weaknesses as cogent
and comprehensive explanations for the diversity of homicidal phenomena. First, they have not been
developed to account for homicide specifically, and so make no specific predictions about when
homicide, as opposed to some other form of criminal behavior, will occur. Second, these theories fail
to predict most of the specific contexts in which homicide will occur. Third, they do not specify
whether homicide per se (as opposed to criminality in general) has ever been adaptive, nor what the
specific functional benefits of killing might have been. Fourth, they fail to explain why homicide is
often perpetrated by individuals who do not seem to be pursuing a general strategy of criminality.
There is no evidence, for example, that a normally law-abiding man who killed his wife in a jealous
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rage when confronted with her infidelity is pursuing the short-term opportunistic sexual strategy
proposed by theories such as r/K theory and Alternative Adaptation Theory. Similarly, women who
are otherwise "normal" and not pursuing a criminal strategy sometimes kill their infants when such
infants are deformed, diseased, or born prematurely (Daly & Wilson, 1988). Tribal groups regularly
engage in warfare, and this appears to be a "normal" activity in which most young males engage,
rather than a specific subset of men pursuing a particular strategy (Keeley, 1996; Chagnon, 1988).
For all these reasons, the existing evolutionary theories of general criminality are inadequate both for
explaining known facts about homicide and for generating novel predictions about the contexts in
which homicide will occur.
2.5. Killings as Evolutionary Epiphenomena and Maladaptation
Over the past few decades, several evolutionary scientists have offered explanations for homicide.
Almost without exception, however, the evolutionary hypotheses offered posit that killing is
unnatural and not part of human evolved psychology. The ethologist Eibl-Eibesfeldt (1989), for
example, proposed that killing (e.g., in war) is a culturally imposed behavior that overrides an innate
human inhibition to kill: "Our innate inhibitions against killing are also dampened in war. A cultural
norm making killing a duty is superimposed upon the biological one forbidding murder, and this
results in killing actually becoming a virtue. However, a massive indoctrination is required to make
this superimposition . . ." (Eibl-Eibesfeldt, 1989, p. 711). Thus, according to this prominent
ethologist, if natural selection has forged any psychology surrounding homicide, it is a natural
proclivity against it. Killing, according to this view, is a cultural aberration and in no way part of
human evolutionary psychology. This view leads to no detailed predictions about the various forms
of homicide nor about the contexts in which they will occur.
By far the most comprehensive evolutionary explanation specifically advanced to account for
patterns of homicide is that proposed in a series of publications by Daly, Wilson, and their colleagues
(1988, 1995, 1998, 1999). Daly and Wilson’s contributions to the understanding of homicide and the
contexts in which it occurs have been pioneering and seminal. But their explanations, we suggest, are
inadequate. Daly and Wilson advance two distinct arguments about homicide--one may be called the
"agnostic" argument and the other the "slip" or "epiphenomenon" theory. According to the agnostic
argument, " . . . our evolutionary psychological approach in no way depends upon homicide per se
being ‘an adaptation.’ It may or may not be the case that actual killing was a regular component of
the selective events that shaped human passions" (Daly & Wilson, 1988, p. 12, emphasis added).
They have maintained this position in more recent publications: "Using homicides as a sort of ‘assay’
of the evolved psychology of interpersonal conflict does not presuppose that killing per se is or ever
was adaptive" (Wilson, Daly, & Daniele, 1995, emphasis added).
In the same publications, however, they advance the view that humans do not have an evolved
psychology of homicide. Rather, they propose that homicides can be viewed as "dysfunctionally
extreme byproducts," "epiphenomena", or "overreactive mistakes" of evolved mechanisms designed
for other purposes--a kind of "spandrels" argument (see Gould, 1991; Buss, Haselton, Shackelford,
Bleske, & Wakefield, 1998). According to this argument, "It is quite possible that actually killing
one’s antagonist is more often than not an overreactive mistake--an act with negative consequences
both for the killer’s net hedonic utility and for the actual expected fitness of which that utility is an
evolved token" (Daly & Wilson, 1998, p. 438, italics original).
Although Daly and Wilson offer the "mistakes" explanation as a general explanation for homicide,
they most frequently invoke it in the context of spousal homicides: "We propose that such homicides
[killing estranged or unfaithful wives] are the dysfunctionally extreme byproducts of those same
violent inclinations whose much more typical effects are successful deterrence and coercion" (Daly
& Wilson, 1998, p. 449, italics added). It is clear from these quotes that Daly and Wilson are not
proposing that that humans have evolved a distinct psychology of homicide, although they do not
rule out such a possibility completely.
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Subsequent evolutionary psychologists have adopted Daly and Wilson’s position (e.g., Crabb, 2000;
Kenrick & Sheets, 1993). Crabb, for example, suggests that "psychological machinery for aggressive
impulses would have served inclusive fitness well, with the caveat that extreme aggression leading to
homicide may be disastrous for inclusive fitness because it may result in fatal retribution against the
perpetrator" (Crabb, 2000, p. 226).
Although Daly, Wilson, and colleagues do not propose that humans have evolved psychological
mechanisms specialized for killing, they do emphasize the importance and priority of an evolutionary
psychological explanatory account: " . . . what is needed is a Darwinian psychology that uses
evolutionary ideas as a metatheory for the postulation of cognitive/emotional/motivational
mechanisms and strategies" (Daly & Wilson, 1989, p. 108-109). The goal of this article is to propose
precisely such a theory. We briefly outline arguments for the a priori plausibility of evolved
mechanisms specifically designed for conspecific killing, which simultaneously call into question the
Daly and Wilson explanations for homicide.
3. The A Priori Plausibility of Evolved Mechanisms for Conspecific Killing
Killing obviously has many causes, and some theories, as noted above, seek only to provide partial
explanations. Undoubtedly some homicides may be caused by "slips" due to violent brinkmanship or
accidents in a dangerous game of coercion, as Daly and Wilson propose. A variety of sources of
existing evidence, however, call into question the validity of the Daly-Wilson epiphenomenon theory
of homicide as a general explanation for the majority of killings and simultaneously suggest that it is
not unreasonable to propose that mechanisms dedicated to conspecific killing might have evolved.
The first source of evidence is comparative. A large number of species regularly commit conspecific
killings in such predictable contexts that it is reasonable to advance the hypothesis that such species
have adaptations designed to kill. Among insects, a wide variety of species engage in mate killings.
Where mate killing and cannibalism is known to increase the number or viability of offspring
(including mantids, black widow spiders, jumping spiders, and scorpions) males approach females
cautiously and retreat quickly after copulation. Breene and Sweet (1985) showed that in the sexually
cannibalistic black widow spider Latrodectus mactans, when males survive copulation, they often
fertilize multiple females. Males of sexually cannibalistic species use diverse strategies to decrease
their chances of being cannibalized: male scorpions sometimes sting the female after deposition of
the spermatophore (Polis & Farley, 1979); male black widows (Gould 1984) and crab spiders
(Bristowe, 1958) often restrain females in silk prior to copulation. Conspecific killing, as well as
mechanisms to prevent getting killed, appear to be common among insects.
Among the 4,000 species of mammals, many also have well-documented patterns of conspecific
killing. Male tigers, lions, wolves, hyenas, cougars, and cheetahs have been observed to kill the
infants of rival males (Ghiglieri, 1999). These killings often have the effect of hastening the estrus of
the mothers of those infants, at which point they often mate with the killers. Among primate species,
of which humans are one, conspecific killings have now been well documented among langur
monkeys (Hrdy, 1977), chacma baboons (Busse & Hamilton, 1981), red howler monkeys (Crockett
& Sekulic, 1984), savanna baboons (Collins, Busse, & Goodall, 1984), mountain gorillas (Fosse,
1984), chimpanzees (Zuzuki, 1971; Bygott, 1972), blue monkeys (Butynski, 1982), and others
(Hausfater & Hrdy, 1984). The killing of conspecific rival males has also been well-documented
among the chimpanzees of Gombe (Wrangham & Peterson, 1996), perhaps the species genetically
closest to human beings (98.4% of DNA in common), as well as in mountain gorillas, who share
97.5% of their DNA with humans (Fossey, 1984).
Thus, contrary to claims by ethologists such as Konrad Lorenz (1966), conspecific killing is
widespread in the animal world, not limited to humans. The contexts in which many of these
conspecific killings occur, such as males killing intrasexual rivals or the offspring of intrasexual
rivals, provides evidence that some, perhaps many, primate species have evolved specific adaptations
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for killing. The comparative evidence, in short, supports the idea that mechanisms for conspecific
killing may have evolved in a variety of mammalian and primate species. Based on this evidence,
there is no reason to be skeptical about the possibility that mechanisms specifically designed for
conspecific killing may have evolved in Homo sapiens, or may have become evolutionarily
elaborated from earlier designs of ancestral species through natural or sexual selection.
A second source of evidence is the paleontological record, which reveals a highly patterned
distribution of fractures and cranial traumas, and arrow tips and stone projectiles lodged in the rib
cages of ancient humans (Grauer, 1998; Keeley, 1996; Walker, 1995). Fifty-nine human skeletons
were found in a cemetery at Gebel Sahaba in Egyptian Nubia, for example, dating from 12,000 to
14,000 years ago (Late Paleolithic); more than 40% of the skeletons contained stone projectile points
close to, or embedded in, them (Keeley, 1996). The excavator estimated that more than half of the 59
had died violently, many with multiple wounds (the highest number of wounds was 20). Another
study of California prehistory documented that about 5% of the skeletons had arrowheads embedded
in them, obviously only the most transparent evidence of violent intentional death (Keeley, 1996).
The majority of such injuries appeared on male skeletons, and most appeared on the left sides of the
crania and rib cages, suggesting a right-handed attacker (Grauer, 1998). Other evidence includes the
scalping of skulls, which leaves characteristic cut-marks on the cranial bones founds in many regions
in North America, as well as trophy skulls and other body parts such as teeth, hands, arms, or legs
(Keeley, 1996).
There is also evidence of cannibalism, most recently coming from what is now southwest Colorado
(Holden, 2000). Bone fragments show characteristic cuts. Analyses of ancient human feces
(coprolite) revealed the presence of human myoglobin, which could only get there through the
consumption of human muscle or heart tissue. More anecdotal evidence comes from reports from
early missionaries, as illustrated by the following report. A Maori warrior was observed taunting the
preserved head of an enemy chief: "You wanted to run away, did you? But my meri [war club]
overtook you; and after you were cooked, you made food for my mouth. And where is your father?
He is cooked:--and where is your brother? He is eaten:--and where is your wife? There she sits, a
wife for me:--and where are your children? There they are, with loads on their backs, carrying food,
as my slaves" (Vayda, 1960, p. 95). In Oceania as well, enemy captives were eaten. On Fiji, one
chief kept a record of how many enemy bodies he had consumed by placing stones in a line behind
his house, one stone for each victim. At the time of observation, the line contained 872 stones
(Keeley, 1996).
Obviously, these data are fragmentary and not conclusive. But they add one important piece of
circumstantial evidence suggesting that homicide has been a recurrent feature of human ancestral
environments–a critical feature that is the sine qua non of our proposed theory of evolved homicide
mechanisms.
A third source of evidence is archeological, stemming from the existence of weapons. While some
tools have several functions (e.g., the bow and arrow can be used for hunting animals or killing
humans), others seem specialized for homicide. Shock weapons such as maces and clubs serve no
functions except to inflict injury or death on a human. Keeley (1996) argues that maces, lances,
tomahawks, daggers, and swords are excellent for conspecific killing, but have no other apparent
purposes. The graveyard of mass killings discovered at Talheim in Germany from around 5000 B.C.
showed victims with holes in their skulls that correspond exactly to the shape of axes from the Early
Neolithic.
A fourth source of evidence comes from fortifications. Fortifications are designed as defenses against
attackers and invaders. They are seen in concentrations in northwestern Europe from the Early
Neolithic, built as long as 7000 years ago. Palisades, moats, ditches, baffle gates, towers, drawbridge
gates, ditches with sharpened sticks, stone walls, adobe walls, traps at gates, and pitfalls have been
discovered throughout the world (Keeley, 1996). Fortifications and weapons both suggest a long
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human history of homicide.
A fifth source of evidence is pictorial, appearing on rocks and in cave paintings from Europe, the
middle east, and Australia. Cave paintings discovered in Spain and France show men shooting other
men with arrows (Dennen, 1995). In northern Australia, rock paintings dating back as many as
10,000 years depict males in combat, some hurling spears and boomerangs, some dodging these
weapons, and some chasing others with raised weapons (Dennen, 1995). Some show figures pierced
with spears, and some show men bending down to help those felled by spears.
A sixth source of evidence involves the cross-cultural and ethnographic record, which provides
extensive documentation of tribal warfare (Keeley, 1996), genocide (Lee, 1984), blood revenge in
foraging societies (Daly & Wilson, 1988), intrasexual homicide within groups (Chagnon, 1988; Daly
& Wilson, 1988; Hart & Pilling, 1960; Keeley, 1996), infanticide (Daly & Wilson, 1988), and
spousal killings (Daly & Wilson, 1988). Even in cultures held depicted as peaceful, homicide rates
are high. Among the Netsilik Eskimo, for example, who appeared very peaceful to the
anthropologists who studied them, there was a well-documented murder rate four times higher than
in the United States (Keeley, 1996). Although there are obvious difficulties in inferring the
prevalence of homicide in ancestral contexts from the current ethnographic and cross-cultural
evidence, it is nonetheless clear that homicide is neither rare nor trivial from the standpoint of natural
selection.
Finally, we note that homicides appear across cultures in quite predictable contexts and often involve
premeditation, both of which call into question the epiphenomenon explanation of homicide. Spousal
homicide occurs primarily in the context of a female sexual infidelity or defection from the
relationship, male-male homicides occur predictably over issues of status, face-saving, and
reputation, and infanticides occur when babies are deformed or diseased or when the mother lacks an
investing father. All these predicable contexts for homicide, well-documented by Daly and Wilson
(1988), call into question the "accidental" or "slip" or "epiphenomenon" explanation as a general
theory of homicide.
Taken together, the comparative, paleontological, archeological, ethnographic, cross-cultural, and
contemporary evidence all suggest that it is not implausible a priori to hypothesize that humans have
evolved psychological mechanisms that are designed specifically for killing other humans in certain
contexts. The remainder of this paper is devoted to presenting the basic outlines of a new
evolutionary theory of homicide.
4. Précis of Homicide Design Theory
In sharp contrast to the theories reviewed above, we propose that there have been some highly
specific and recurrent contexts in which the fitness benefits of killing outweighed the fitness costs.
We propose that humans have evolved a number of distinct, context-sensitive psychological
mechanisms devoted to homicide. We propose that these mechanisms are triggered by a delimited set
of circumstances, and that they are designed to produce the death of conspecifics. A host of
beneficial consequences, in the currency of fitness, historically flowed to killers as a result of
murdering in some contexts (Buss & Duntley, 1998, 1999; Duntley & Buss, 1998, 1999). Most
murders, according to Homicide Design Theory, are not slips, accidents, incidental byproducts, or
epiphenomena. They are kills by design. Below we outline the major premises of Homicide Design
Theory, enumerate specific propositions about the nature of the proposed cognitive/psychological
mechanisms, and then present preliminary empirical evidence for the theory.
4.1. Premise 1: Over human evolutionary history, there has been recurrent selection in which
the benefits of conspecific killing, on average, outweighed the costs in delimited contexts (i.e.,
rb > c, in the currency of fitness). [1]
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There are many potential benefits to killing another human that could have recurred over human
evolutionary history.[2] The particular benefits reaped as a result of killing undoubtedly varied with
the nature of the person killed, their relationship to the killer, the social context, and personal
circumstances of the killer and victim. According to our theory, there are both lethal and non-lethal
evolved strategies for solving many adaptive problems, which would suggest the possibility of
evolved decision rules that guide which strategies to deploy in which circumstances. The purpose of
this section is merely to present the argument that, over human evolutionary history, there would
have been many potential benefits to killing another human being. We illustrate the 10 clearest and
most likely fitness benefits that would have accrued to killers in ancestral conditions.
4.1.1. Benefit 1: Eliminating an intrasexual competitor. Because selection operates on genes for
existing designs relative to genes for other designs around at a given time, eliminating an alternative
design by killing a rival can provide direct reproductive advantages, even when the killer accrues no
additional benefits from murder. A rival’s death is the killer’s gain, just by virtue of the fact that the
rival is dead. A conspecific killer terminates the rival’s future reproduction, and begins a cascading
set of fitness-reducing consequences for the rival’s kin, leading to a decrease in the frequency of the
rival’s genes in subsequent generations. The killer simultaneously eliminates the direct costs that the
rival otherwise would have inflicted if he or she were not killed, such as the costs of a the rival
monopolizing reproductively relevant resources, potentially contributing to an increase in the
frequency of the killer’s genes in subsequent generations.
4.1.2. Benefit 2: Gaining access to a rival’s material resources. By killing a rival, the killer can gain
direct access to the victim’s material resources. These include the victim’s food, territory, tools,
weapons, cookware, and shelter--resources that can aid in the killer’s survival and future
reproduction.
4.1.3. Benefit 3: Gaining access to a rival’s fertile mates. Men who kill potentially can gain access to
a rival’s reproductively valuable women, either for short-term sexual access or long-term mating.
Increased sexual access to fertile women in ancestral environments would have constituted a large
reproductive benefit that alone could have provided the selective impetus for the evolution of
psychological mechanisms for homicide.
4.1.4. Benefit 4: Cultivating a reputation that deters exploitation and reduces the likelihood of costs
inflicted by other potential rivals. Humans live in groups where status and reputation can boost
access to survival and reproductively relevant resources, or consign a person to reproductive
oblivion. By killing, and showing a willingness to kill, men can successfully deter other competitors
who might encroach on one’s resources if such deterrence did not exist. Men unwilling to kill may
cede their resources more willingly to men with credible reputations as killers.
4.1.5. Benefit 5: Protecting oneself from exploitation, injury, rape, or death at the hands of others. In
some social contexts, humans face a choice of killing or being killed. Over human evolutionary
history, those who failed to kill when their lives were threatened, or when some massive cost such as
severe injury or rape was about to be inflicted, were likely to have been out-reproduced by those
willing to kill to defend themselves. Indeed, if there were one context above all others that might
prompt even the most pacific individual to commit homicide, it would be when there is a life-ordeath zero-sum outcome between a would-be victim and their would-be killer.
4.1.6. Benefit 6: Protecting children and kin from exploitation, injury, rape, or death at the hands of
others. Children and other kin are the vital "vehicles" necessary for transporting genes into the next
generation. Protecting the survival of such vehicles would have been essential for successful
reproduction. Fitness benefits would accrue to those who killed when their key vehicles were
threatened with exploitation, debilitating injury, or death. Based on the logic of inclusive fitness
theory, we predict a gradient of a willingness to kill to defend one’s vehicles, depending on genetic
relatedness.
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4.1.7. Benefit 7: Protecting one’s mate from exploitation, injury, rape, or death at hands of others.
Valuable mates are always in short supply compared to the large numbers who desire access to them
(Buss, 1994). A man who possesses a reproductively valuable mate incurs the envy of others and
draws efforts at mate poaching (Schmitt & Buss, in press). Men in some contexts may have had to
kill in order to protect their mates (e.g., Valero, 1970). The fitness benefits of killing in this context
include retaining access to a valuable mate and deterring the mate poaching efforts of others in the
future.
4.1.8. Benefit 8: Protecting existing resources such as territory, shelter, and food sources. The
protection of valuable resources by killing extends to the resources needed for survival and
reproduction, including shelter, food, tools, weapons, and choice territories. The recurrent fitness
benefits of killing in some ancestral contexts in order to retain access to these reproductively
valuable resources could have selected for a killer psychology.
4.1.9. Benefit 9: Eliminating resource-absorbing or costly conspecifics that do not contain copies of
one’s genes. Step-children can absorb time, energy, attention, food, resources, and investments from
a step-parent, but the survival and proliferation of step-children would not generally have increased
the fitness of the stepparent. The diversion of a man’s or a woman’s resources to stepchildren-resources that alternatively could have been invested in one’s own genetic children--would have
been costly in the currency of fitness. Killing stepchildren delimited contexts could have freed up
these resources for more profitable avenues of investment. A male lion that kills a rival’s cubs prior
to inseminating his new mate frees up her reproductive and parental resources for his offspring.
Analogously, humans who killed stepchildren prevented the diversion of their new mate’s somatic
and parental resources to a rival’s genetic vehicles instead of their own.
4.1.10. Benefit 10: Eliminating costly children who would otherwise interfere with investments of
resources in vehicles better able to translate the investment into fitness. Evolution by selection
creates decision rules that favor investing in vehicles that yield high fitness returns relative to other
forms of investment. Children are sometimes born with defects, deformities, or diseases that make
them poor vehicles for parental investment because of the poor return on investment (Daly &
Wilson, 1988). Parents who killed such infants in certain circumstances would have avoided wasting
parental effort on poor fitness vehicles, freeing up effort for children offering a better return on the
limited resource of parental investment.
These are a sample of the most obvious and direct fitness benefits that could have flowed to ancestral
human killers. Clearly they do not exhaust the list of specific potential fitness benefits of murder,
which is longer and more varied. The key point is not that these benefits would always or necessarily
have flowed to killers as a consequence of killing. The key point is that the potential benefits of
conspecific killing are so numerous and substantial in magnitude that there is no a priori reason to be
skeptical about the possibility that homicidal mechanisms could have evolved. Whether or not such
benefits recurrently flowed to killers would have depended heavily on a variety of key contextual
conditions, including the costs of pursuing a killer strategy--a topic to which we now turn.
4.2. Premise 2: Over human evolutionary history, selection has fashioned specific psychological
mechanisms for killing in contexts in which the costs were low.
The costs of killing can be large and even catastrophic in the currency of fitness. By attempting to
kill someone, the would-be killer risks getting injured or killed himself. Indeed, we propose that
because killing has been a recurrent selective force, humans have evolved a number of anti-homicide
mechanisms designed to prevent getting killed (see Premise 8 below). Successful killings may
provoke costly retaliation by the victim’s friends, mates, or kin, inflicting damage or death on the
would-be killer. People who perpetrate certain types of kills in some contexts may suffer ostracism,
reputational damage, status loss, or decrements in mate value that interfere with the successful
accomplishment of reproductively relevant tasks such as surviving, mating, and investing in children.
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These substantial costs historically would have deterred, and perhaps continue to deter, many
homicides that otherwise might have occurred.
Premise 2 hypothesizes that selection has fashioned assessment mechanisms that gauge the
likelihood of incurring such costs, as well as their magnitude, and subsequently compare these costs
with representations of the cumulative benefits of killing, in comparison with alternative behavioral
strategies that might be available. For example, the physical formidability of one’s rival compared
with one’s own physical formidability undoubtedly affects risk assessment (Gilbert, 1989), and there
is evidence that men frequently perform these comparative assessments (Fox, 1997). These
assessments should affect decisions about whether to attempt a kill or instead pursue an alternative
strategy. Evolved decision rules would favor attempts to kill rivals only when there is a reasonable
probability of success, or when the risks were worth taking because of a bounty of benefits that the
killer could accrue. Similarly, decision rules may favor killing in circumstances where the costs of
failing to kill loom so large that killing may be attempted even under risky conditions where success
is uncertain.
Decision rules about going to war, to take another example, will undoubtedly incorporate
information about the relative size of the potentially warring tribes, with positive decisions more
likely when one’s own group outnumbers the rival group (Tooby & Cosmides, 1993; Wrangham,
1999). Killing in contexts of anonymity (e.g., at night, or alone in the woods) might offer lower risk
of retaliation than killing in front of a tribe of witnesses. The frequent use of "surprise raids" in tribal
warfare involves selecting contexts in which the costs of killing are reduced (Wrangham, 1999).
We propose that uncertainty assessment is an important design feature of the cost-benefit analyses
conducted by our killer psychology. Uncertainty resides in at least two sources of potential error. A
first source of uncertainty lies with lack of complete information (e.g. lack of knowledge about the
numbers, experience, and formidability of a rival group). A second source of uncertainty lies with the
fact that some relevant variables may be difficult to predict with confidence, such as the
psychological states of the intended victims (e.g., the degree to which the attempted kill is
anticipated by the victim).
We hypothesize that evolved decision rules evaluate all of these assessments as input to decision
rules that determine whether or not a homicidal strategy is implemented. These are not "blind
instincts" or urges to kill regardless of context. They are sensitive psychological procedures that
weigh the costs and benefits that historically have been linked with the strategic use of homicide as
one solution among several potential solutions to particular classes of adaptive problems.
4.3. Premise 3: The recurrent contexts in which the fitness benefits of killing outweighed the
fitness costs, on average, were many in number and distinct in nature.
This premise is essential for Homicide Design Theory. Without it, a number of distinct homicide
mechanisms could not have evolved. Furthermore, the contexts in which the benefits of killing
outweighed the costs had to be sufficiently distinctive that they could not be handled by a single,
more general homicide mechanism.
Consider the following contexts: Giving birth to an infant who is highly deformed, experiencing an
impending attack from a neighboring tribe intent on killing, and walking in on a spouse who is
having sex in flagrante delicto with someone else. The benefits and costs of killing in each of these
contexts are highly distinctive. In the case of the deformed infant, the parent who commits
infanticide can staunch the reproductive losses incurred by continuing to invest in a poorly equipped
vehicle, while freeing up valuable parental resources for existing or future children who afford a
better return on investment. Infanticide may also come with costs--the infant killer may suffer
reputational damage, the loss of their mate, or punishments from their group. Killing in defense
against a hostile tribe provides benefits in survival of self and family and preservation of land, food
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resources, and mates against rival encroachment. It also puts the would-be killer’s life in jeopardy
and could produce a deluge of fitness costs if untimely death leaves the remaining family
defenseless. Killing an unfaithful mate or a mate-poaching rival may be a last-ditch defense against
paternity uncertainty and all of the "misdirected" paternal effort that might be expended on vehicles
containing the genes of a rival male. But killing in response to sexual infidelity could also lead to
reputational damage, difficulty securing mates in the future, social sanctions, and the loss of a
caregiver for shared children. The benefits and costs of killing in each of these three contexts are
qualitatively distinct. We propose that there will be a number of distinct contexts in which the
benefits of killing, on average, outweighed the costs and provided a greater overall payoff than other
strategies.
4.4. Premise 4: Evolved homicide mechanisms are many in number and specific in nature, each
corresponding to the selective pressures created by each distinctive and historically recurrent
benefit/cost context.
A single, simple psychological mechanism would be extraordinarily unlikely to govern information
processing in all contexts of killing. We hypothesize that each context would require different
assessment procedures for determining benefits, costs, and alternative courses of action. The
personal risks to one’s life as a result of killing a deformed infant, for example, are typically trivial
compared to the risks associated with killing a spouse’s lover, who may fight back and kill in selfdefense against the aggrieved husband. The alternative strategies available in response to an attack
from a neighboring village (e.g., attempt to kill, flee, submit) are substantively different from
strategies available to a cuckolded husband who discovers a wife naked with another man (e.g., get a
divorce, seek a retaliatory affair).
The decision rules that govern the alternative courses of action in these contexts would not be
generalizable across contexts, any more than "mate choices" and "food choices" could both be
governed by a single general set of preferences (Symons, 1987; Buss, 1999). According to our
theory, evolved homicide mechanisms will be numerous and distinct to the degree that the selective
contexts in which killing was historically beneficial were numerous and distinct.
4.5. Premise 5: Each evolved homicide mechanism has distinct functions.
Each homicide mechanism is proposed to have a distinct set of functions, corresponding to the
unique benefits that killers would have accrued in each context. The distinct functions of a man’s
step-child homicide mechanism, for example, would include eliminating his investment in a child
containing an intrasexual rival’s genes and curtailing the investment of his current mate in a child
produced by a rival male. One function of the warfare mechanisms hypothesized by Homicide
Design Theory, in contrast, is to gain direct sexual access to new fertile mates. One function of the
intrasexual rivalry homicide mechanisms is to eliminate a key source of strategic interference and to
cultivate a reputation as someone to be deferred to and not interfered with. Each of the evolved
homicide mechanisms, in short, is proposed to have unique functions.
4.6. Premise 6: Consciously articulated homicidal premeditations are central functional
components of design for murder.
Although the human brain comprises only 2% of the average human’s body weight, it consumes
roughly 20% of the body’s calories (Leonard & Robertson, 1992; 1994). Cognitive activity, simply
put, is metabolically costly. Metabolic energy devoted to processing information about one adaptive
problem precludes metabolic energy available for other adaptive problems. The costs of information
processing are not merely metabolic. They include opportunity costs. Time allocated to processing
information about one adaptive problem preempts opportunities for cognitive work on other adaptive
problems. The nature, content, and duration of particular kinds of thought, according to this view,
reveal important clues about the problems the human mind evolved to solve.
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According to Homicide Design Theory, homicidal fantasies function in some ways that are
analogous to sexual fantasies. As Symons notes in the latter context, "the mind is adapted to cope
with the rare, the complex, and the future . . . the function of the mind is to cause behavior; even if
only one impulse in a thousand is consummated, the function of lust nonetheless is to motivate
sexual intercourse" (Symons, 1979, p. 206-207). Clearly, humans experience hundreds or thousands
of sexual thoughts and fantasies for every one that is actually acted upon. But this ratio of thought to
deed should not be interpreted as calling into question the important functions of those thoughts.
Precisely the opposite. The rare and occasional reproductive payoff associated with one of those
thoughts is likely to have been so large (e.g., motivating actual sexual access to a new fertile partner)
that it outweighed all of the metabolic and cognitive opportunity costs of the thousands of un-actedupon thoughts.
According to our theory, the first step in the emergence of a homicidal thought is the recognition of
an adaptive problem that historically has been potentially solvable by killing, such as having one’s
life threatened by a conspecific or being burdened by a deformed offspring. This does not imply that
homicide historically has been the only solution to the adaptive problem under consideration, or even
that it has been the most common solution to that adaptive problem. Indeed, one hypothesized
function of scenario building is to evaluate the utility of alternative potential solutions, of which
homicide is only one (see below).
Homicide Design Theory proposes that homicidal thoughts, fantasies, and premeditations are key
components of the evolved design of homicide mechanisms. The theory proposes that homicidal
thoughts or premeditations will occur universally or nearly universally in humans around the globe
(i.e., will be species-typical) when they are evoked by predictable circumstances for which homicide
historically was a potentially effective solution to an adaptive problem. At the most general level, we
propose the following features of homicidal premeditations:
4.6.1. Context Specificity Assumption. Homicidal thoughts will occur in specific and predictable
contexts. Selection is hypothesized to have fashioned cognitive mechanisms designed to recognize
contextual cues to adaptive problems that were potentially solvable through conspecific killing.
4.6.2. Attention Mobilization Assumption. Homicidal thoughts function to direct attention to
particular features of the adaptive problem, such as future costs if potential victim continues to live,
the different reputational consequences if the potential victim is or is not killed, potential
vulnerabilities of the victim, and the formidability of the victim and the victim’s kin group.
4.6.3. Scenario Building or Cognitive Simulation Assumption. Homicidal thoughts include scenario
building, a cognitive simulation of carrying out the homicide. Following the creation of images and
cognitive simulations, further scenario building involves calculating the costs and benefits,
associated with actually carrying out the kill. Evaluating the costs include the risk of being caught,
injured, ostracized, or killed, as well as evaluations of the impact of possible errors in assessment.
Scenario building will often entail considering alternative strategies, including non-lethal ones,
capable solving the adaptive problem. Finally, scenario building entails evaluating the multiple
cascading consequences, including reputational ones, for self, kin, and group members of carrying
out a particular strategy. In the vast majority of instances, these cost/benefit calculations will lead to
the decision not to carry out a homicide, just as in the vast majority of sexual fantasies, the
cost/benefit calculations lead to the decision not to carry out the sex act. Ironically, one of the
proposed functions of homicidal scenario building is to inhibit acting on a homicidal impulse when
the costs are too high.
4.6.4. Emotional or Affective Assumption: Affect, in our theory, serves several key functions. First,
it can serve as a tangible trigger to a homicidal thought. Different evolved homicide mechanisms
contain different affective triggers. The fear that serves as a trigger for self-defense homicide, for
example, will differ from the jealous rage that triggers a sexual rival homicide. And both of these
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differ from the relatively affectless state that accompanies certain types of infanticide. Second, affect
functions to tag particular persons and events for storage in memory, facilitating subsequent
retrieval. Third, affect is hypothesized to guide decisions about whether to enact a particular
homicidal scenario (e.g., a homicidal scenario which, when played out, leads to the emotional
experience of fear will generally inhibit carrying out the kill; a major exception occurs with antihomicide mechanisms, in which a fear of being killed might facilitate carrying out a preemptive
homicide). These distinct affective states are hypothesized to function as tags used in the comparison
of alternative strategies.
4.6.5. Simulation Rehearsal Assumption: Cognitive simulations of homicide will often be repetitive,
with various components getting repeated, rehearsed, and refined over the temporal course of the
premeditation (see Prentky et al., 1989).
4.6.6. Motivational Assumption: Affects linked to thoughts propel deeds. Analogous to Symons’s
logic with sexual fantasies, even if one homicidal thought in a thousand is acted upon, one critical
function of homicidal premeditation is to motivate carrying out an actual kill. This does not require,
however, that an individual have the conscious intention or desire to kill a conspecific. Only the
motivation to engage in behaviors whose function is to cause the death of a conspecific need be
present for the operation of a psychology of homicide.
The features of homicidal thoughts just described, of course, are not independent of one another. We
hypothesize that they are tightly integrated components of human information processing that makes
up the psychology of homicide. It is also not our contention that the entirety of evolved killer
psychology is manifest in conscious homicidal thoughts. Much of the information processing
machinery involved in homicide undoubtedly operates without the killer’s awareness, just as most of
human digestive machinery operates without the eater’s awareness. Furthermore, some homicides
often occur on the spur of the moment, perhaps even in the absence of conscious premeditations.
These presumptive facts do not negate the hypothesis that homicides are the designed output of
psychological mechanisms. We hypothesize that the majority of homicides, despite the fact that
some occur in the absence of assiduously elaborated homicidal premeditations, result from the
implementation of an evolved killer psychology. Such implementation requires thought, but not
necessarily conscious thought. Nonetheless, conscious homicidal thoughts provide one window into
the psychology of homicide, and according to our theory, are functional components mechanisms
that evolved to regulate decisions to kill or not to kill in response to particular adaptive problems.
A number of direct empirical predictions follow from these assumptions. First, the contexts in which
homicidal thoughts occur should exhibit a predictable pattern across cultures worldwide, that is,
exhibit universality. This does not imply that every person in every culture will have already
experienced homicidal premeditations. Rather, the same contexts that trigger homicidal thoughts in
Japanese or Jamaicans, for example, should trigger homicidal thoughts in Zambians or Zulus.
Second, even though the vast majority of homicidal thoughts will not lead to an actual homicide,
homicidal thoughts will typically precede carrying out an actual homicide. Third, the contexts in
which homicidal thoughts occur should exhibit the same patterns as the contexts in which actually
homicides occur. Fourth, the frequencies of particular kinds of homicidal thoughts should correlate
positively with the frequencies of those same kinds of actual homicides.
In summary, homicidal premeditations are both windows into the psychology of killing and key
functional components of the evolved homicide mechanisms. They serve a variety of functions, from
scenario building to calculating cascades of consequences. One critical function of homicidal
premeditations is to inhibit the translation of a homicidal impulse into actual behavior. Most
homicidal thoughts do not lead to actual killing, because the cost-benefit appraisals suggest that the
costs are too high, the benefits too low, or more effective solutions are available for solving the
adaptive problem. Nonetheless, one of the critical functions of homicidal thoughts is motivational,
preceding and prompting murder.
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Finally, we propose that conspecific death is a key designed output of evolved homicidal
mechanisms. The existence of recurrent cognitively-costly homicidal thoughts in specific
circumstances are difficult to explain on theories that propose that strategies of violence have
evolved solely for non-lethal deployment.
4.7. Premise 7: Men and Women Have Evolved Different Homicide Mechanisms
Selection has not forged identical homicide mechanisms in men and women. In its most general
form, this assertion is unlikely to be controversial. The vast majority of actual homicides are
committed by men, not women, and so it would not surprise evolutionarily informed scholars that
men have evolved bodies and minds designed to kill more than women. But our theory goes well
beyond the premise of a general sex difference in the likelihood of killing. We propose that men have
evolved some homicidal mechanisms that are entirely lacking in women, such as those designed for
warfare (see also, Tooby & Cosmides, 1988; Wrangham, 1999). We propose that women have
evolved some homicidal mechanisms entirely lacking in men, such as certain types of infanticide.
And we propose that even when men and women both possess similar homicide mechanisms at one
level of abstraction (e.g., both sexes have evolved mechanisms to kill infants), those mechanisms
will contain design features that differ substantially in women and men. In the case of infanticide, for
example, we propose that only men have evolved a psychology of infanticide sensitive to the context
of lack of certainty in their paternity, whereas only women have evolved a psychology of infanticide
sensitive to the context of the lack of an investing father. Although both sexes have evolved
infanticide mechanisms, the specific design features of these mechanisms are proposed to differ
profoundly for the sexes.
Some of these hypothesized sex differences have a straightforward evolutionary logic that has
essentially been articulated by others in the context of violence (e.g., Daly & Wilson, 1988; Ellison,
1985). Men historically had a higher ceiling on reproduction than women, since a man’s limit is
imposed primarily by the number of women he can successfully fertilize (e.g., through polygyny or
opportunistic short-term mating). At the same time, a man has a greater likelihood than a woman of
dying with zero descendants. These recurrent historical differences in fitness variances have been
hypothesized to lead to the evolution in men of a wide variety of risk-taking mechanisms. Risk
taking, including the sort of violent risk taking involved in attempted homicide, would have paid
greater reproductive dividends to men than to women, both in gaining great reproductive bounties
and in avoiding reproductive oblivion.
This evolutionary reasoning can partially explain why men kill more than women, and perhaps even
why men might have evolved more numerous or more frequently acted-upon homicide mechanisms.
But it stops short of explaining the specific homicide mechanisms men possess, as well as their
component design features. And it fails to explain why women have evolved homicide mechanisms
or why there are contexts in which women are more likely to kill than men. Our theory, which
includes many sex-linked premises, is meant to fill these theoretical gaps.
4.8. Premise 8: Victims incur extreme and heretofore unrecognized cascading fitness costs by
being killed.
Although it would certainly not make headlines to announce that it’s costly to be killed, being a
homicide victim has fitness consequences that no prior theories of homicide or human survival have
enumerated or illuminated. To start with, being killed cuts off all avenues to the victim’s future
reproduction. The victim loses access to his or her mate’s residual reproductive value, which may
instead be coopted by a rival. Furthermore, additional mating opportunities through affairs, serial
mateships, and multiple simultaneous spouses are lost, with all the attendant reproductive losses. The
earlier the reproductive age of the victim, ceteris paribus, the larger the average reproductive costs
will be to the victim if he is killed. A barely post-pubescent female victim, for example, would suffer
a greater cost of being killed than her post-menopausal counterpart. Being killed, in short, curtails a
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victim’s future reproductive possibilities, which inflicts a massive fitness cost.
Second, being killed can damage the victim’s children in countless ways. Being dead, the victim is
no longer present to invest in his or her children. The children become more vulnerable to
exploitation or injury by others without the protection of the investing parent (Hill & Hurtado, 1996).
The homicide victim loses the ability to affect the children’s future mating or any of the children’s
future prospects, strategies through which reproductive success historically may have been increased.
By being killed, the victim’s children become more vulnerable to losing the remaining spouse’s
investment, which may get re-channeled to the children produced with a new mate. Finally, the
victim’s children risk becoming step-children if the victim’s mate remarries. Since step-children
suffer physical abuse and homicide at rates 40 to 100 times higher than children residing with two
natural parents (Daly & Wilson, 1988), becoming a stepchild would have impaired the potential
success of key fitness vehicles of the homicide victim.
A third set of detrimental fitness consequences of being murdered centers on damage to the victim’s
extended kin group. By being dead prematurely, the victim loses the ability to protect or invest in
kin. The entire collection of extended kin–brothers, sisters, aunts, uncles, nephews, nieces –become
more vulnerable or exploitable as a result of the victim’s death. Without the victim’s presence and
influence, specific kin members could lose mate value, impairing their future mating prospects. And
the victim’s death could sever key ties between two different extended families, which originally
forged an alliance through the marriage of the victim.
Last but certainly not least, all of the victim’s losses become potential gains for the rivals of the
victims, and hence the rival’s genes. The victim’s mate, for example, becomes available to mate with
one of the victim’s intrasexual rivals. The elimination of the victim from the status hierarchy opens a
niche for a rival to ascend, a rise that previously may have been blocked by the presence of the
victim. Children of a rival will thrive in competition with the victim’s children, who will be
hampered by their parent’s death. In short, the costs of getting killed cascade to one’s children,
grandchildren, great-grandchildren, and the entire collection of extended kin descendants or wouldbe descendants of the homicide victim.
Although human intuition tells us that it’s undesirable to be killed, no prior theories of human
survival or homicide have articulated the large, specific, and multifaceted fitness costs. We propose
that these multiple and far-reaching costs are critical to the evolution of homicide mechanisms. They
describe a key portion of the selective benefits that would accrue to killers–fitness costs inflicted on
intrasexual rivals can become fitness gains for the killers. Killers, in short, historically could have
increased their fitness by inflicting these multiple and multifacted costs on conspecific competitors.
At the same time, it is precisely these same costs that historically selected for anti-homicide
mechanisms, leading to the next premise.
4.9. Premise 9: Because of the tremendous fitness costs associated with getting killed, a complex
suite of anti-homicide mechanisms has evolved that function to prevent premature death.
If killing has been a recurrent feature of human evolutionary history and the costs of getting killed
are as catastrophic as Premise 8 indicates, then it would be exceedingly unlikely that selection will
have failed to forge mechanisms designed to prevent being killed. Just as humans appear to have
evolved specific fears of snakes, spiders, and heights to prevent poisonous bites and fatal falls, we
propose that humans have evolved specific anti-homicide mechanisms to prevent being killed by
other human beings.
According to this premise, anti-homicide mechanisms would be sensitive to the precise contexts in
which one’s life is at risk from a conspecific. One set of anti-homicide mechanisms may get
activated in a fetus to combat evolved abortion mechanisms in mothers (Haig, 1993). Haig presents
evidence that mothers have evolved abortion mechanisms designed to terminate investing in fetuses
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under certain conditions such as poor health of genetic abnormalities. Since the fetus has only one
shot at life, it’s interests can depart from the mother’s. One adaptation that appears to have evolved
for this function is the fetal production of human chorionic gonadotropin (hCG), a hormone the fetus
secretes into the mother’s bloodstream. The hormone has the effect of preventing the mother from
menstruating, and thus allows the fetus to remain implanted. Producing elevated levels of hCG,
therefore, appears to be an adaptation in the fetus to subvert the mother’s attempts to spontaneously
abort it. The female body appears to "interpret" high levels of hCG as a sign that a fetus is healthy
and viable, and so does not spontaneously abort. Fetal anti-abortion devices, if future work confirms
their existence, constitute evolved anti-homicide mechanisms designed to combat the very first lethal
threat to existence.
A second hypothesized anti-homicide mechanism is "stranger anxiety," which appears nearly
universally in infants around six to nine months, precisely the time when they become capable of
crawling away from caregivers (Hrdy, 1999). Infants are not taught to fear strangers, and the
seemingly irrational panic "derives from a built-in prejudice so deep it persists despite every
reassurance the parent offers" (Hrdy, 1999, p. 416). It is reasonable to hypothesize that the intense
fear of strangers exhibited so reliably by infants across cultures is an anti-homicide device, designed
to evoke parental protection, that evolved in response to the likelihood that "infanticide may have
been a chronic threat during hominid evolution" (Hrdy, 1999, p. 416).
We defer a full treatment of the evolution of anti-homicide mechanisms (see Duntley & Buss, in
prep., a), but note here that adult anti-homicide mechanisms are hypothesized to have a number of
design features. These include sensitivity to the contexts in which one’s life is in danger, such as
trespassing on an enemy’s territory or being discovered having sex with an intrasexual rival’s wife.
They include detecting a homicidal rage in another person or anticipating revenge or retribution from
a person or group whom one has previously violated, and so invoke specific mind-reading abilities.
They include emotions such as specialized terror of death and battlefield panic. And they include a
variety of behavioral responses, such as emitting a piercing death scream to alert kin and allies,
fleeing in terror, or furiously attacking a would-be killer.
Since an individual shares many genes with his kin, we predict that the psychology of anti-homicide
will be activated by cues to the impending homicide of both an individual and his kin. We predict
that the degree to which anti-homicide psychology will be activated will depend partly on the
number of common genes, as well as coalitional allies, that an individual stands to lose as a result of
the homicide. In effect, our psychology of anti-homicide evolved to protect our genes, wherever they
may reside.
One central design feature hypothesized to characterize most or all anti-homicide mechanisms is a
seemingly irrational overreaction to threats, which evolved according to the logic of Error
Management Theory (Haselton & Buss, 2000). In making inferences in an uncertain world, there are
two types of errors. One can falsely infer a problem or signal that does not exist, such as thinking
that one’s life is in danger when it is not. Or one can fail to perceive a signal that is present, such as
believing that one is safe when in fact one’s life is in danger. According to Error Management
Theory, there have been many recurrent cost asymmetries associated with the two types of errors. In
this example, it will generally be more costly to fail to infer real threats than to err on the side of
over-inferring threats that are in fact non-existent. Recurrent cost asymmetries, according to this
theory, result in the evolution of cognitive biases to err in the direction of less costly error (Haselton
& Buss, 2000). Since getting killed is so catastrophically costly, we expect that human anti-homicide
mechanisms contain cognitive biases that result in overestimating the likelihood of getting killed in
certain circumstances.
This premise leads to a raft of testable empirical predictions about he design of anti-homicide
mechanisms. First, humans are predicted to routinely exhibit biases in overestimating the likelihood
that their lives are in danger across a variety of circumstances. Second, this bias should extend to
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close kin, and especially children, given their value as genetic vehicles. Fourth, the bias should
extend to mates, and especially to the extent that they are reproductively valuable to the individual.
Fourth, the degree of bias should decrease as a function of decreasing genetic relatedness of the
person to the potential victim.
In summary, according to our theory, anti-homicide mechanisms include: (a) fear of strangers in
infancy; (b) specialized mind-reading abilities to infer homicidal intent in others, (c) adaptively
biased inference mechanisms that result in protective overestimates of the likelihood of getting
killed, (d) selective preferences for habitats that afford protection and refuge (e.g., seeing without
being seen), (e) anticipatory defensive maneuvers including development of arms, fortifications, and
protective postures, (f) panic reaction to motivate fleeing when confronted by another human
displaying homicidal intent, and (g) homicidal mechanisms designed to impel killing to prevent
getting killed.
The discovery of anti-homicide mechanisms affords a powerful and fresh source of potential
empirical evidence to test the central tenets of Homicide Design Theory. The key to whether an
evolved psychology of anti-homicide provides compelling evidence for the existence of an evolved
killer psychology rests largely on the specificity of psychological design features for anti-homicide.
The greater the number and degree of elaboration apparent in the design of psychological
mechanisms for anti-homicide, the greater the likelihood that they evolved in response to the
presence of an equally specific and elaborated evolved homicidal psychology. Discoveries of
specificity in one will lead to investigation for corresponding specificity in the other. This leads to
the final premise, involving a co-evolutionary arms race between killers and victims.
4.10. Premise 10: The evolution of anti-homicide mechanisms makes killing less beneficial and
more costly, resulting in selection for increasingly refined homicide mechanisms and
increasingly elaborate defense mechanisms, producing a within -species co-evolutionary arms
race between homicidal strategies and defensive anti-homicide strategies.
The principle of co-evolution typically has been used to describe reciprocal evolutionary changes in
interacting species, such as predators and prey or parasites and hosts. It has been used less often to
explain reciprocal evolutionary changes within a single species, although there are important
exceptions to this generalization (e.g., Buss, 2000; Haig, 1993; Holland & Rice, 1998). Premise 10
invokes the principle of co-evolution to describe the reciprocal evolutionary changes set into motion
when killing became an important part of human selective environments and anti-homicide
mechanisms appeared on the scene.
Anti-homicide mechanisms lower the on-average benefits that otherwise would accrue to would-be
killers in two ways. First, the evolution of anti-homicide mechanisms reduces the average success of
attempted homicides. Potential infanticide victims who cry to alert parents, male rivals who
anticipate an attack, and women who secure the protection of close kin in order to defend against an
enraged husband intent on uxoricide all are acting to reduce the average success of a killer strategy.
Second, anti-homicide mechanisms can inflict heavy costs on would-be killers. Alerted parents,
defensive rivals, and protective kin can injure or ostracize a prospective killer. In the extreme case,
when people kill in self-defense in order to prevent getting killed (or to prevent kin from being
killed), the cost inflicted on the would-be killer are grave indeed. For both reasons–reduced success
and the risk of costs--the emergence of anti-homicide mechanisms has the effect of lowering the onaverage benefits that otherwise would have been obtained by pursuing a homicidal strategy.
The evolution of anti-homicide mechanisms, in turn, would prompt reciprocal evolutionary changes
in killer psychology. First, when it becomes more costly to attempt a kill, the detection and
evaluation of costs become increasingly important inputs to decision rules about whether or not to
attempt a kill. Ceteris paribus, this would make killing a less frequently chosen solution to a variety
of adaptive problems, thus reducing overall homicide rates or holding them temporarily in check.
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Second, there would be further selection for refined homicidal strategies designed to circumvent a
victim’s anti-homicide defenses. These refinements may include the evolution of deceptive tactics,
such as feigning friendliness or concealing homicidal intent so as not to arouse a would-be victim’s
anti-homicide mechanisms, and then surprising the victim with a sudden attack. Another refinement
would be an assessment of the comparative formidability of self and intended victim to identify
circumstances in which the victim’s anti-homicide defenses will be ineffective. The co-evolution of
these and other design features in killers would have had the effect of lowering the costs to killers
inflicted by the anti-homicide mechanisms of victims, increasing the average success of homicidal
attempts, and thus increasing the average success of homicide as a strategy. If there were no further
co-evolution, the homicide rate would then increase over time as killers evolved to circumvent the
anti-homicide defense mechanisms of victims.
Given how costly it is to be killed, however, selection would immediately favor refinements in antihomicide psychology. Consequently, defense mechanisms would become increasingly sophisticated
and elaborate. People would evolved heightened sensitivity to a larger array of subtle signals of
homicidal intent. Thresholds for detecting homicidal intent would decrease. People would become
increasingly suspicious to counteract being deceived. Behaviorally, people would avoid
circumstances in which their lives were at risk and more prone to resort to lethal violence as a
defense. Adaptive anti-homicide biases, evolving according to the logic of Error Management
Theory, would be heavily favored.
In summary, a core premise of Homicide Design Theory is that homicide and anti-homicide
mechanisms co-evolved, each imposing selective pressure on the other, resulting in a coevolutionary
arms race throughout human evolutionary history. Like predators and prey, conspecific killer
mechanisms and victim defense mechanisms evolve to become increasingly elaborate, responding to
the constantly evolving contrapuntal adaptive problems imposed by each.
5. Hypothesized Evolved Homicide Mechanisms
In this section, we outline our theory of specific homicide mechanisms, noting selection pressures
hypothesized to have given rise to each mechanism, the proposed design features of each
mechanism, the sex-differentiated components of each mechanism, and specific empirical
predictions that follow from each proposed mechanism. We begin with infanticide because it is the
most well documented form of conspecific killing.
5.1. The Evolution of Infanticide
Early in her research on langur monkeys, Hrdy (1977) noted that the infant monkeys she was
observing started to vanish. Langurs live in matrilineal groups consisting of several adult females
and their infants, with only a single adult male permitted within each. They subsist primarily on
leaves, and are not carnivorous. Hrdy witnessed at separate times four adult males enter a matrilineal
group and defeat or scare away the resident male. In each case, the victorious male proceeded to kill
the youngest infants in the group, averaging three infanticides per male. The new male then
proceeded in the ensuing months to mate with the mothers of the infants they had killed. A new male
who failed to kill genetically unrelated infants would typically have had to wait for several years
before the female became fertile because of lactation-produced amenorrhea. Since males reign on
average for two years, a male who failed to commit infanticide would fail to reproduce. The
infanticide came at a tremendous cost to the mothers, who often attempted to defend their young. But
after the killings, they showed no hesitation in mating with the killer males.
Many animal biologists and primate anthropologists grant that such brutal infant murders make
eminent reproductive sense (Ghiglieri, 1999). Hrdy, for example, titled her 1977 article on lemur
infanticide "Infanticide as a Primate Reproductive Strategy." More recently, Ghiglieri concluded that
"Overall new males who did not kill infants but instead waited sired fewer infants than males who
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did kill them. This is all sexual selection needs to install infanticide as a male reproductive
strategy" (Ghiglieri, 1999, p. 131). Similar observations have been found for gorillas. One out of 7
gorillas born ends up a victim of infanticide. And 40% of infant morality in the Virunga Volcano
gorilla groups has been attributed to infanticide by Diane Fossey (Goldsmith & Zimmerman, in
press, chap. 13, p. 12). Similar observations have been made among lions, tigers, cheetahs, and
cougars (Ghiglieri, 1999).
When it comes to humans, we observe a predictable set of circumstances in which people commit
infanticide, but surprisingly, most scientists stop short of proposing that humans have a specific
evolved infanticidal psychology. Daly and Wilson note that "There is no reason to suppose that an
evolved parental psychology should be such as to value every offspring equally and
indiscriminately" (Daly & Wilson, 1988, p. 42). They continue: "parental psychology has been
shaped by selection to make adaptive decisions about the magnitude of parental commitments" (p.
44). Then they say that "We thus expect child-specific parental love to be variable in its strength" (p.
73). Even Hrdy, who originally discovered infanticide among Langurs, notes strikingly analogous
patterns in humans, but fails to propose the evolution of specific mechanisms for infanticide. Instead,
she argues that "what evolved was a high threshold for responding in a solicitous way toward an
offspring not likely to be genetically related–the equivalent of emotional earplugs" (Hrdy, 1999, p.
237). The metaphor of "emotional earplugs," however, does not offer a satisfactory explanation of
the highly predictable contexts in which human infanticides occur. Nor can "a high threshold for
responding in a solicitous way" explain the premeditated and deliberate manner in which infanticides
are carried out, the specific contexts in which they occur, or the brutal battering that some of the
infants suffer at the hands of their killers.
Daly and Wilson present an impressive array of evidence suggesting that parents kill their infants
under very predictable circumstances. These include: (1) When there is paternal uncertainty, such
that the infant might not be the putative father’s child; (2) when the infant is deformed, diseased,
severely underweight, or of otherwise questionable quality such that it might not survive or thrive;
and (3) when external circumstances, such as food scarcity or lack of an investing father render this
not a propitious time for a woman to reproduce.
Among the Ayoreo women residing in Bolivia, for example, women kill their infants when they lack
a mate who could provide for her and her offspring, when the infant is born deformed, when twins
are born (typically one is killed), and when an infant is born too soon after a previous birth, which
jeopardizes the survival of the older child (Bugos & McCarthy, 1984). Similar patterns have been
documented among the Ache of Paraguay (Hill & Kaplan, 1988), among the Yanomamo (Chagnon,
1983), and in fact all over the world (Scrimshaw, 1984).
The key point is that over the long course of human evolutionary history, there would have been
tremendous fitness costs associated with continuing to invest in an infant that was deformed,
diseased, unlikely to survive, or unlikely to reproduce.
After much discussion of the evidence, several theorists have come to the conclusion that humans
have evolved mechanisms for murdering infants, just as lemurs, cougars, and lions have evolved
mechanisms for murdering conspecific infants. Ghiglieri notes that "Infanticide seems written into
our genes, but it is generally manifested in young, unwed, desperate mothers who murder
neonates" (Ghiglieri, 1999, p. 135). He notes that "most infanticide has been decided by people
trying to produce more children in the long run by sacrificing an infant now" (Ghiglieri, 1999, p.
136). He argues that "Men kill stepchildren for the same reasons other primate males kill infants: to
increase their reproductive success by sweeping the reproductive arena clean of the offspring of male
competitors" (Ghiglieri, 1999, p. 137). Even Daly and Wilson, authors of the "byproduct
hypothesis," come quite close to postulating an evolved infanticide mechanism: "an evolved
assessment mechanism should be such as to terminate any hopeless reproductive episode as early as
possible, rather than to squander parental effort in an enterprise that will eventually be
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abandoned" (Daly & Wilson, 1988, p. 75).
We build on the conceptual claims by Ghiglieri, Daly, and Wilson by specifying additional design
features in the evolved psychology of infanticide. Specifically, it is important to cleanly separate
hypotheses about male and female infanticide mechanisms, since the proposed evolved design of
infanticide psychology differs sharply for the sexes.
5.1.1. Female infanticide mechanisms. Women differ from men in the magnitude of their obligatory
parental investment (Trivers, 1972). Women carry a nine month gestational obligation, which
involves heavy metabolic costs, decreased mobility, increased physical vulnerability, and substantial
mating opportunity costs. Compared to men, women's fertile years are more sharply compressed into
a smaller fraction of their overall lifetime. Given the small number of genetic "vehicles" a woman
can produce, it would be unlikely that selection would have failed to forge mechanisms to terminate
maternal investment in some infants in order to preserve her finite time and resources for others.
We propose that infanticide solved five key adaptive problems for ancestral women: (1) avoiding
wasting reproductive resources on phenotypically or genotypically sub-par offspring (e.g., those who
were deformed, diseased); (2) avoiding investing in offspring when external conditions substantially
imperil the infant’s survival (e.g., food scarcity; lack of an investing father, harsh season); (3)
preventing competition with older or more viable offspring (e.g., twin births or when birth spacing is
too close); (4) increasing the mate value of the mother, for whom children either interfered with an
existing or emerging mateship or lowered her odds of successfully attracting a desired new mate; and
(5) maintaining an existing long-term relationship (e.g. a child that obviously does not look like the
woman’s long-term mate, a cue to infidelity).
The fitness benefits of committing infanticide in all five of these conditions are predicted to decrease
as a function of the age of the mother. Young mothers, because of their greater residual reproductive
value, can better afford to wait for more auspicious resource and mating circumstances to replace the
dead offspring. Older mothers, having lower residual reproductive value, cannot wait, and so are
predicted to be more reluctant to kill an infant under each of the five conditions.
We predict that mothers faced with these circumstances will entertain homicidal premeditations,
conducting cognitive simulations of behaviors leading to killing, with the attendant cost-benefit and
affective evaluations described earlier. Our theory predicts that younger women confronted with
these adaptive problems will be more likely than older women to entertain such infanticidal
premeditations. Other theories of infanticide, such as that infanticide is merely an incidental
byproduct of more general mechanisms of parental solicitude, do not make these predictions and
would have difficulty explaining their existence and their context-specific nature.
5.1.2. Male infanticide mechanisms. The adaptive problems solved by male-perpetrated infanticide,
according to our theory, are in some ways dramatically different from those of females, and hence
the contexts in which men and women kill infants should differ correspondingly. A man’s mate
value, for example, does not decline as much as a woman’s as a result of having existing children by
a former mate (Buss, 1994). Furthermore, since a smaller portion of a man’s than a woman’s total
effort is typically allocated toward parenting, the costs of investing in sub-par offspring are
commensurately lower for men than for women.
Conversely, there are several adaptive problems solved by male-perpetrated infanticide that we
hypothesize were recurrent for men, while being weaker or entirely absent in women. These include:
(1) avoiding investing in an infant known to be sired by a rival male (e.g., mate was already pregnant
prior to the initiation of the current mateship); (2) terminating investment in an infant for whom there
is paternity uncertainty (e.g., appraisal of possible infidelity by the woman); (3) preserving the
mate’s resources for the man’s current or future offspring; and (4) hastening the commencement of
the mate’s ovulation and hence conceptability.
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In sum, we predict that the contexts triggering both infanticidal premeditations and actual
infanticides will be sharply sex-differentiated. Non-evolutionary theories of infanticide would have
great difficulty explaining the existence of such context-specific premeditations and their
hypothesized sex-linked occurrence. Similarly, evolutionary theories that hypothesize that infanticide
is a byproduct of more general mechanisms such as "differential parental solicitude" or "a high
threshold for responding in a solicitous way" contain no premises that can explain the contextspecific and sex-specific design features of infanticide.
We have begun our discussion of evolved homicide psychology with infanticide for several
important reasons. First, infanticide is well documented in the mammalian world, and those who
have studied species ranging from lemurs to lions appear to have no doubt that these species have
evolved specific infanticide mechanisms (e.g., Hrdy, 1999). The adaptation logic for such nonhuman infanticides is apparently so clear and compelling, whether it’s a male killing an infant he did
not sire or a female killing an intrasexual rival’s infant, that the hypothesis of evolved infanticidal
mechanisms is apparently uncontroversial.
Second, many of the same patterns are observed in human infanticide, and some theorists, even those
who are skeptical of the existence of evolved homicide mechanisms in other contexts, have alluded
to the possibility that humans have evolved mechanisms for murdering infants. In fact, of all the
scientists with whom we’ve discussed our theory, the infanticide component appears to be the
evolved homicide mechanism that most seem ready to accept as plausible (e.g., Margo Wilson,
personal communication).
We mention these points for a specific reason: Infanticides represent a relatively trivial proportion of
all homicides--merely 2-4% of all human killings. If it’s reasonable to hypothesize that selection has
forged a specific psychology of infant murder given the relative rarity of the event, then it is
reasonable to hypothesize that selection has forged specific mechanisms for murder in other contexts
that appear to be far more frequent. Spousal homicides in the United States, for example, are roughly
five times more common than infanticides.
Selection operates on the recurrent frequency of selective events in conjunction with the fitness
consequences of those events. Homicides have particularly dramatic fitness consequences, as we
argued in Premise 1 (potential fitness benefits to killers) and Premise 8 (potential fitness costs to
victims). Therefore, if selection can forge specific evolved mechanisms of infanticide given the
relative rarity of infant killing, then it is reasonable to hypothesize that selection could have operated
even more powerfully to forge mechanisms for murderous contexts that are many times more
common. One of the most common is intrasexual rivalry homicide, a topic to which we now turn.
5.2. Killer Males: Intrasexual Rivalry Homicide
By far the most frequent type of homicide worldwide is male-on-male intrasexual killing. In Daly
and Wilson’s study of Canadian killers, 69% of the killings fell into the male-male quadrant (Daly &
Wilson, 1988). Similar statistics have been found among the !Kung Bushmen (Lee, 1979; Shostak,
1981), the Yanomamo of Venezuela (Chagnon, 1983), the Gebusi of New Guinea (Knauft, 1985),
and the Ache of Paraguay (Hill & Hurtado, 1996).
The !Kung Bushmen, often described as "the harmless people," provide a good illustration of the
prevalence of male-on-male homicides. A key ethnography noted of the !Kung that "It is not in their
nature to fight . . . Bushmen cannot afford to fight with one another and almost never do because
their only real [lethal] weapon is their arrow poison, for which there is no antidote. But even were
they to disregard this danger, Bushmen would try not to fight because they have no mechanism in
their culture for dealing with disagreements other than to remove the causes of their disagreements . .
. The !Kung call themselves zhu twa si, the harmless people" (Thomas, 1959, p. 21-24).
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Richard Lee recorded 22 homicides among the !Kung over a 35 year period (Lee, 1984), which
corresponds to a homicide rate of 29.3 killings per 100,000 per year (Ghiglieri, 1999). This homicide
rate exceeds that for modern day Los Angeles or New York City, which have homicide rates of 24.5
and 16.1, respectively (Ghiglieri, 1999, p. 116). All the killers were men, as were 86% (19) of their
victims. Nearly all the !Kung killers used poisoned arrows to carry out the deed. The homicides
appear to have been intentional, not slips or accidents. When the ethnographer interviewed the !Kung
killers about why they chose to use poisoned arrows rather than a less lethal weapon to harm their
rival, a common response was "We shoot poisoned arrows because our hearts are hot and we really
want to kill somebody with them" (Lee, 1984, p. 95).
Daly and Wilson (1988) compiled same-sex homicide statistics from 35 different studies
representing a broad span of cultures from Miami to the BaLuyia of Kenya. Although homicide rates
vary widely from culture to culture, the proportion of same-sex homicides that involve male killers
and male victims are remarkably consistent, ranging from a low of 85% in Denmark from 1933
through 1961 to a high of 100% in Iceland (1946-1970), among the Tzeltal Mayans in Mexico
(1961-1965), and among the Munda of India. The average across samples is approximately 95%; 19
out of twenty same-sex killings are committed by men. Even this figure underestimates the
magnitude of the adult-on-adult homicide sex difference, since infanticides are included. When
infanticides are excluded, the ratio of male-to-female same-sex killers jumps to almost 40 to 1
(Ghiglieri, 1999, p. 146).
In every culture for which there are data, the rate at which men kill other men vastly exceeds the rate
at which women kill other women: "there is no evidence that women in any society have ever
approached the level of violent conflict prevailing among men in the same society" (Daly & Wilson,
1988, p. 149; emphasis original). Any comprehensive theory of homicide must account for both
facts–why men kill so much more often than women worldwide and why other men comprise the
majority of their victims.
Daly and Wilson (1988) explain this large sex difference by invoking sexual selection for risky male
tactics, originally traceable to the greater reproductive variance between males and females. For
example, "If status has persistently contributed to reproductive success, and a capacity for controlled
violence has regularly contributed to status, then the selective advantage of violent skills cannot be
gainsaid" (Daly & Wilson, 1988, p. 133). It is important to stress that Daly and Wilson are not
proposing an evolved psychology of homicide. Instead, they argue that homicides "have to be
understood as the rare, fatal outcomes of a ubiquitous competitive struggle among men for status and
respect" (Daly & Wilson, 1988, p. 146). In a recent publication, Daly and Wilson are even more
explicit in arguing that intrasexual homicides are epiphenomenal byproducts and over-reactive
mistakes (Daly & Wilson, 1998).
Beyond general statements about "the selective advantage of violent skills," the evolution of "risky
competitive tactics," and the expressed view that killing antagonists may be, and may always have
been, an over-reactive "mistake," Daly and Wilson say little about the precise nature of the
psychological mechanisms involved in same-sex killings. Nor do they enumerate the specific
reproductive benefits that would have accrued to intrasexual killers. They do document, however, a
range of contexts in which men kill other men, interpreted within a general theory of conflicts in
reproductive interests. They argue that male-male killing is reducible to a fairly small number of
underlying motives such as redressing a public insult or other blows to reputation, acquiring material
resources (e.g., robbery murders), and sexual rivalry (e.g., killing a rival who has had sex with one’s
wife).
According to Homicide Design Theory, the underlying psychological mechanisms involved in malemale killings are specific evolved homicide mechanisms that are not well described in general
phrases such as "risky competitive tactics" or "over-reactive mistakes." Our theory proposes that
most intrasexual homicides are not mistakes, but rather are kills by design. Mechanisms for male
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intrasexual murder, according to our theory, have evolved to solve at least five distinct adaptive
problems–protection, reputation management, resource acquisition, mate encroachment, and mate
acquisition.
5.2.1. Self and kin protection: Killing in self-defense. Although we lack a videotape of the past four
million years, much available evidence (reviewed earlier) suggests that conspecific killing has been a
recurrent feature of human ancestral environments. This inference is endorsed even by scientists who
do not propose that humans have evolved specific homicidal mechanisms. Daly and Wilson, for
example, note after reviewing the then-available paleontological evidence that "What is certain is
that men have fought and killed one another for tens of thousands of years" (Daly & Wilson, 1988, p.
144). Since that time, substantially more paleontological evidence has been gathered, all pointing to
the same conclusion (Grauer, 1995; Grauer & Stuart-MacAdams, 1998).
If conspecific killing has been a recurrent feature of human ancestral environments, it reasonable to
infer that the risk of being killed formed a significant adaptive problem. Given the devastating fitness
costs of being killed, selection would favor the evolution of defenses designed to lower the
likelihood. Clearly, there are may potential solutions. Potential victims could offer to cede all of their
mates, resources, territory, and goods to the would-be killer, a strategy that might lower the odds of
being killed. They could run or hide to escape from would-be killers. They could migrate to a distant
territory. Or they could attempt to gather kin and non-kin allies in sufficient numbers to deter a
would-be killer. According to Homicide Design Theory, these and other anti -homicide strategies
have evolved in humans (Duntley & Buss, in prep., a).
Another potential anti-homicide strategy is murder. Although there are costs associated with this
strategy, including risking retaliation from the kin of the dead victim, the benefits in certain
circumstances could easily have outweighed the costs. By killing his attacker, the self-defense killer
prevents getting killed, thereby preventing all the costs to self, children, and collateral kin described
earlier. The self-defense killer also preserves, rather than cedes, resources, territory, and mates, all of
which could have been lost to an intrasexual rival. He or she can eliminates future sources of
"strategic interference," including costs that the would-be killer and his children would inflict in the
near and distant future. And by killing an intrasexual competitor, one inflicts a severe cost on that
rival, including damage to the rival’s children and extended kin group. For all these reasons, we
hypothesize that humans have evolved homicidal mechanisms to solve the adaptive problem of lethal
threats from conspecific rivals.
Several specific, although perhaps not surprising, predictions follow from this theory: (1)
Humans worldwide will entertain thoughts of killing another human being in order to prevent being
killed themselves, when faced with a life-threatening rival; (2) Humans worldwide will entertain
thoughts of killing another human being in order to prevent their children from being killed; (3)
There will be a gradient of willingness to kill to prevent the death of a kin member, falling off as the
degree of genetic relatedness decreases (e.g., given the risks of attempting to prevent a kin member
from being killed, the benefits of killing must conform to Hamilton’s c < br rule, whereby the fitness
costs must be less than the fitness benefits, weighted by degree of genetic relatedness); (4) Among
cultures, subgroups, or specific individuals within a group that recurrently face the adaptive problem
of getting killed, intrasexual homicidal premeditations will be more common than among those who
have not faced this adaptive problem; (5) among those who have never before entertained the
thought of killing, homicidal premeditations can be easily evoked by confronting an individual with
the credible threat of being killed (or close kin being killed).
All these predictions are testable. No other theory of homicide, to our knowledge, generates these
specific predictions, nor would any previous theory of homicide be able to explain them if they are
confirmed.
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Although both sexes historically have faced the adaptive problem of being killed by a conspecific,
death at the hands of rivals has been a more frequent adaptive problem for males than for females.
Furthermore, the contexts in which one’s life and one’s children’s lives were at risk from other
humans are and were sharply sex differentiated. The person most likely to kill a man, for example, is
an intrasexual rival. In contrast, the person most likely to kill a woman is a current or former
romantic partner. Men across cultures face the problem of getting killed in a variety of contexts by
rivals, including being the victim of a jealous husband or boyfriend who becomes enraged, having
honor and reputation challenged, and being attacked in a surprise raid by a neighboring tribe.
Women are rarely at risk of being killed by intrasexual rivals, even from women whose husbands
they are caught sleeping with.
For these reasons, two additional predictions, both sex-linked, can be derived from our theory: (6)
men will be more likely than women to have homicidal premeditations about killing in self-defense;
(7) men will have these homicidal self-defense premeditations in a wider variety of contexts than
will women (e.g., when being caught in flagrante delicto with another man’s wife).
5.2.2. Killing to protect resources. The direct threat of being killed is merely one end of a continuum
of reproduction-impairing costs that can be inflicted by rivals. Rival encroachment on one’s
resources, in some circumstances, would have been disastrous to survival and reproductive success.
Food resources stolen in ancestral times would sometimes have pushed a person to the brink of
starvation. Stolen stone tools, spears, bows, or arrows could leave a person stripped of the means for
food acquisition. Weapons stolen could leave a person bereft of the means to defend self and family.
Expropriating a shelter could render the victim vulnerable to the hostile forces of nature (e.g.,
predators, cold, heat) as well as more exposed to hostile conspecifics. Expropriated land could
deprive the victim of the water to survive or the game to thrive. Having one’s resources encroached
upon with impunity could have led to a ruinous reputation as someone who could be exploited
without fear of retaliation–a selective force so powerful in itself that we consider it separately below.
Clearly, there are many possible solutions to the adaptive problems posed by rival resource
infringement other than killing the rival. Non-lethal violence, assembling a kin coalition in defense,
retaliatory thefts, and migration away from hostile humans, for example, undoubtedly were and are
common solutions. Nonetheless, we hypothesize that males have evolved decision rules that include
killing as one solution to the problem of rival resource encroachment. Although attempting to kill a
resource poacher obviously carries risks, it also can bring benefits such as (1) preserving
reproductively vital food, tools, weapons, shelter, and habitat for oneself and one’s kin; (2) depriving
an intrasexual competitor of access to those resources; (3) eliminating future strategic interference
that the rival would otherwise inflict if he were alive; and (4) cultivating a reputation that deters
other conspecifics from attempting to encroach on one’s resources.
Specific empirical predictions follow from the hypothesized resource protection killing mechanism:
(1) Humans will have homicidal fantasies about someone who has robbed or plundered a significant
personal resource; (2) Males will have homicidal thoughts in these contexts more than females; (3)
The probability of carrying out the kill will depend on a variety of cost-benefit calculations such as
the risk of being killed, anticipated reputational consequences, costs to kin, alternative sources
available for replacing lost resources, and the viability of non-lethal alternative solutions such as
migration. The higher the perceived costs of killing, the lower the odds that a man will attempt to
enact the homicidal premeditation.
5.3.3. Killing for reputation management. The importance of reputation, honor, face-saving, and
humiliation in male-male killings has been noted by a number of authors (e.g., Chagnon, 1988;
Coon, 1971; Daly & Wilson, 1988; Ghiglieri, 1999; Wolfgang, 1958). Roughly 37% of one sample
of 560 cases of murder were classified into the label of "Altercation of relatively trivial origin; insult,
curse, jostling, etc." (Wolfgang, 1958; cited in Daly & Wilson, 1988, p. 125).
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Daly and Wilson provide the most detailed evolutionary explanation of the importance of reputation:
"the emphasis on ‘triviality’ obscures a still more important point. A seemingly minor affront is not
merely a ‘stimulus’ to action, isolated in time and space. It must be understood within a larger
context of reputations, face, relative social status, and enduring relationships. Men are known by
their fellows as ‘the sort who can be pushed around’ or ‘the sort who won’t take any shit,’ as people
whose word means action and people who are full of hot air, as guys whose girlfriends you can chat
up with impunity or guys you don’t want to mess with . . . In most social milieus, a man’s reputation
depends in part upon the maintenance of a credible threat of violence . . . one’s interests are likely to
be violated by competitors unless those competitors are deterred. Effective deterrence is a matter of
convincing our rivals that any attempt to advance their interests at our expense will lead to such
severe penalties that the competitive gambit will end up a net loss which should never have been
undertaken" (Daly & Wilson, 1988, p. 128). Ghiglieri concurs with Daly and Wilson, arguing that
"men who win in this murderous game of face-saving do not simply win a single contest. By killing
once, men gain or enhance a reputation of ferocity, which later helps them wrest resources from
other men without conflict" (Ghiglieri, 1999, p. 143).
Homicide Design Theory is in alignment with these ideas, and proposes that human males have
evolved features of rival homicide mechanisms designed specifically for status and reputation
management. There are at least three distinct adaptive problems that are solved by killing in this
context: (1) establishing a reputation that serves a deterrent function, preventing rival males from
resource encroachment, rape of kin, or other forms of abuse; (2) establishing a reputation for ferocity
that facilitates the submission of other men, and hence the ceding of their resources; (3) causing
competitors to migrate, or at least give wide berth, thereby reducing the number of active
competitors in the area.
Several specific predictions follow: (1) being humiliated publicly will catalyze homicidal fantasies;
(2) the more public the humiliation and the more severe the reputational consequences, the more
likely the homicidal fantasies; (3) homicidal fantasies as a consequence of public humiliation by a
rival will more frequently occur in men than in women; (4) creating a reputation as a killer will have
the effect of inducing others to cede resources and territory. As with other hypothesized evolved
homicide mechanisms, translating a homicidal fantasy into a homicide attempt will often hinge on
specific cognitive simulations, the magnitude of rage and humiliation suffered, the viability of
alternative solutions to reputational damage, and specific cost-benefit calculations associated with
the competing scenarios.
Most people will not act on their homicidal fantasies, especially in modern societies that contain
police forces, a legal system that has largely usurped the role of retribution from individuals, and jail
sentences that might act as deterrents. Even in traditional cultures lacking police forces, an individual
may choose an alternative strategy for dealing with public humiliation, or might lack the physical
prowess or coalitional backing to combat a rival who has delivered the insult. Chagnon, for example,
notes the following case, about a man named Rerebawa, who has had an affair with his older
brother’s wife. As Rerebawa was giving Chagnon a tour around the village, he passed the older
brother, "grabbed him by the wrist, and dragged him to the ground: ‘This is the brother whose wife I
screwed when he wasn’t around! ’ A deadly insult, one that would usually provoke a bloody club
fight among more valiant Yanomamo. The man did nothing. He slunk sheepishly back into his
hammock, shamed, but relieved to have Rerebawa release his grip."
The relevant background is that several months earlier, when the older brother discovered the affair,
he attacked Rerebawa with a club. But Rerebawa, being substantially stronger, beat his brother with
the blunt side of an ax. The brother was so intimidated by the thrashing and threat of further violence
that he kept away from the village for several days. The key point is that humiliation, insults, and
other forms of reputational assault do not invariably result in homicide. But they frequently trigger
homicidal thoughts, with all the attendant cost-benefit calculus, some of which lead to attempts at
murder.
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This homicide mechanism is proposed to be highly sex-linked, present in men but largely absent in
women. The rationale for this sex linkage involves the more severe cascading costs to men than to
women of reputational assaults. A woman’s mate value might be lowered as a result of public insult,
but a humiliated man might lose entirely his ability to attract or hold onto a mate, given the
importance women place on status in selecting a mate (Buss, 1994).
5.2.4. Killing to acquire resources. Those who hold valuable reproductive resources rarely relinquish
them willingly. Precisely the opposite. The more valuable the resource, the more ferociously people
guard it. This principle is most easily seen in the context of human mate guarding. The intensity of
effort men devote to guarding their mates and repelling potential rivals has been shown empirically
to be a function of the mate’s youth and physical attractiveness, both cues to reproductive value
(Buss, 1988; Buss & Shackelford, 1997). The intensity of guarding as a function of resource value
relevant to survival and reproduction can be expected to apply to a wide variety of items, including
prime habitats, water sources, places to see without being seen, edible animals, precious plants, tools,
weapons, and social status. The benefits to those who guard resources intensely come at a cost to
those who lack these resources. Humans have evolved mechanisms for gaining access to
reproductively relevant resources held by others.
There exist several potential solutions to this suite of adaptive problems. One could migrate to
different places that are less saturated with human competitors. One could attempt to covertly steal
the resources. One could issue threats in the hope that those holding resources will cede them
without further violence. But an additional strategy of resource acquisition is to kill those holding the
critical resource. According to Homicide Design Theory, intrasexual homicide evolved as one
context-contingent strategy for gaining access to reproductively relevant resources possessed by
others.
The conditions under which this strategy would be deployed hinge on a variety of contextual
variables, including the intensity with which the rival, kin, and coalition members are guarding the
resources; the formidability of the prospective killer and his kin and coalition; the viability of
alternative methods of resource acquisition, including migration, stealing, and threats; and the
prospective killer’s ability to successfully guard or hold onto the resources so acquired after the
killing has occurred (e.g., vulnerability to incursion by others).
Specific predictions follow: (1) thoughts of killing will occur in contexts where an intrasexual rival is
monopolizing a critical resource and hence preventing someone else from gaining access to it; (2)
thoughts of killing intrasexual rivals will increase as a function of the value of the resource
monopolized; (3) thoughts of killing will be more common in contexts where alternative routes to
resource acquisition are not available (e.g., if migration, stealing, or bluffs are ineffective); (4) when
resources are scarce, imposing the threat of pushing an individual below the threshold of survival,
thoughts of killing intrasexual rivals will increase.
5.2.5. Killing to solve the problem of mate encroachment. From our review of infanticide, it is
apparent that for many species there have been substantial fitness benefits of killing the infants of
intrasexual rivals. Killing a rival’s infant creates a temporally extended cascade fitness benefits for
murderers, including eliminating future competition for one’s own children. Homicide Design
Theory proposes that infanticide is held in check primarily by the coevolution of anti-infanticide
mechanisms, both in infants and in their parents (Duntley & Buss, in prep., a). But if the selective
benefits of infanticide are apparent, wouldn’t it be more beneficial to kill intrasexual rivals directly
prior to their production of any or additional offspring?
Since sexual access to fertile females defines the limiting resource for males in their production of
offspring, rivalry over sexual access to mates becomes a key adaptive problem. This can be
partitioned into at least two distinct adaptive problems–mate encroachment and mate acquisition.
The problem of mate encroachment occurs primarily in long-term mating contexts. When a person
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has expended effort to acquire a long-term mate, and a rival attempts to lure that mate away (either
temporarily or permanently), the adaptive problem becomes especially severe. For a man, a sexual
encroachment on his mate jeopardizes his certainty in paternity, while an encroachment causing a
permanent mate defection causes the total loss of access to the woman’s reproductive value. For a
woman, a sexual encroachment by a rival on her mate per se causes relatively little damage, whereas
an encroachment causing a permanent mate defection can cause the total loss of access to her
husband’s resources (Buss et al., 1992). For a variety of reasons, these losses are generally more
reproductively costly for males than for females, and hence our theory predicts sex differences in
evolution of homicide as a solution to the problem of mate encroachment.
Humans appear to have evolved a host of adaptive solutions to the problem of mate encroachment,
ranging from vigilance to violence (see Buss, 2000). One evolved solution, according to Homicide
Design Theory, is killing rivals who threaten to encroach, or who succeed in encroaching, upon one’s
mate. The use of murder as a solution to the mate encroachment problem is hypothesized to depend
on a variety of circumstances. These include one’s personal formidability, formidability of one’s kin
and coalition, the formidability of the rival’s kin and coalition, the reproductive value of one’s mate,
opportunities for mate replacement, extent of reputational damage caused by the encroachment, and
others.
Successfully killing an intrasexual rival would have produced many benefits tributary to fitness.
These include, in principle, maintaining access to one’s mate, preserving social status and reputation,
eliminating the rival’s future encroachment on one’s mates, curtailing the rival’s reproduction, and
eliminating a source of competition for one’s children in the next generation.
Intrasexual rivalry homicide strategies are hypothesized to have the following design features: (1) it
will be triggered by attempts by another male to lure, attract, or "hit on" one’s long-term more often
than one’s short-term mate; (2) the more severe the encroachment, the higher the likelihood of
homicidal premeditations; (3) sexual encroachment will be more likely to trigger men’s than
women’s intrasexual rivalry homicidal thoughts, since paternity uncertainty, but not maternity
certainty, is threatened by sexual infidelity; (4) encroachment on a mate of higher reproductive value
will evoke more homicidal thoughts than encroachment on a mate of lower reproductive value (e.g.,
one who is post-menopausal); (5) jealousy will be a primary emotion triggering, and accompanying
intrasexual homicidal thoughts due to mate encroachment. These are all testable predictions based on
our theory of an evolved rival homicide psychology–predictions not generated by any previous
theory of homicide.
5.2.6. Killing to solve the problem of mate acquisition. Males compete to attract and retain
reproductively valuable females (Darwin, 1871; Symons, 1979; Trivers, 1972). At the most abstract
level, there are two general methods of successful competition–(1) besting a rival through mate
attraction tactics designed to embody or fulfill what the potential mate desires, and (2) inflicting
costs on a rival that eliminates him from successful contention (Buss & Dedden, 1990). The tactics
for inflicting costs on rival males are highly variable, and in humans can range from verbal
derogation tactics designed to damage reputation to inflicting physical damage through violence. We
propose that one strategy of mate acquisition within men’s evolved arsenal is the outright killing of
the intrasexual rival. This hypothesis was anticipated by Charles Darwin in his description of the
process of sexual selection. He noted that "the sexual struggle . . . is between individuals of the same
sex, generally the males, in order to drive away or kill their rivals . . . " (Darwin, 1871, emphasis
added).
Killing as a mate acquisition strategy is more likely to occur in the context of between-group warfare
than within-group intrasexual rivalry (see below). Nonetheless, the problem of mate acquisition can
also be solved by killing intrasexual rivals within one’s group. This might happen in several
contexts: (1) when a married woman conspires with her lover to kill her husband; (2) when a man’s
status within the group is sufficiently enhanced by killing a rival, resulting in increased success at
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attracting potential mates (Chagnon, 1988); and (3) when lethal violence by a man against a rival
serves as a deterrent to his mate or mates against defecting.
Killing an intrasexual rival within one’s group, of course, can carry considerable costs, such as the
risk of injury death as a result of the operation of the intended victim’s anti-homicide mechanisms.
Killing a within-group rival also could lower the formidability of one’s coalition for the purposes of
defense against, or attack on, outside coalitions. And like many other forms of killing, retribution
from kin, again stemming from coevolved anti-homicide mechanisms, can lower the average benefits
associated with killing a rival in the service of mate acquisition. For these reasons, we suggest that
intrasexual rivalry mechanisms have evolved to assess these costs, and decision rules have evolved
to attempt a kill only when these costs are unlikely to be incurred.
5.2.7. Killing to reduce a rival’s strategic interference. At one level of abstraction, the functions of
rival homicide can be reduced to acquiring resources or defending against their incursion. There are
many more forms of incursion, or "strategic interference," than those outlined above. First, a man
might interfere with another man by blocking his ascension within a status hierarchy. Stunted status
ascension could have the effect of precluding access to mates and other key resources that would
otherwise have been obtained. Second, a rival could inflict strategic interference by causing him to
be ostracized from a desired coalition. Third, a rival could usurp dyadic alliances. Male intrasexual
rivalry mechanisms are hypothesized to be sensitive to these key sources of strategic interference,
triggering homicidal premeditations, leading to actual killings in some circumstances.
5.3. Spousal Homicide
Within the United States, roughly 1,500 women are murdered each year by their mates or former
mates, a category that includes husbands and boyfriends and ex-husbands and ex-boyfriends
(Crowell & Burgess, 1996). This represents approximately 30% of all women who are killed, a stable
percentage from year to year, as contrasted with only 3% of male-victim homicides being committed
by wives or girlfriends (Ghiglieri, 1999). Canadians show a similar rate of male-perpetrated mate
murder, despite the relative absence of handguns, but Canadian women murder their mates only half
as often as American women do (Ghiglieri, 1999). Why do men sometimes kill their mates? Why are
nearly a third of all women victims of homicide killed by the ones who profess to love them?
The most frequently endorsed explanation is that proposed by Daly and Wilson (1988), who argue
that mate killings are "slips," or maladaptive byproducts of evolved male mechanisms of coercive
control: "Men . . . strive to control women . . . women struggle to resist coercion and to maintain
their choices. There is brinksmanship and risk of disaster in any such contest, and homicides by
spouses of either sex may be considered slips in this dangerous game" (Daly & Wilson, 1988, p. 205,
italics added). Recently, they elaborate: "We propose that such homicides [killing estranged or
unfaithful wives] are the dysfunctionally extreme byproducts of those same violent inclinations
whose much more typical effects are successful deterrence and coercion" (Daly & Wilson, 1998, p.
449, italics added).
Homicide Design Theory proposes instead that men have evolved a specific mate-killing mechanism
designed to (1) inflict heavy costs on intrasexual rivals by depriving them of sexual access to a
reproductively valuable mate whom the killer has lost irrevocably; and (2) staunch the fitness costs
associated with cuckoldry. The costs to a man of his mate’s infidelity and defection are multiple and
severe. They include losing access to the woman’s reproductive value, risking the devotion of
personal resources to a rival’s offspring, having the mate devote all of her parental efforts to a rival’s
offspring, and incurring reputational damage. The reputational damage alone could cripple a man’s
ability to attract future mates or hold on to other current mates, impair his future ascension in status
hierarchies, and increase his risk of being exploited injured, or killed.
The fitness costs a man incurs as a result of his mate’s infidelity or desertion do not come merely
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from the direct costs associated with the loss, such as the magnitude of future effort that will have to
be expended to attract and retain a replacement mate. They also come from the fact that one man’s
loss of a desirable mate constitutes an intrasexual rival’s gain. Since selection acts through the
relative replicative success of competing designs, this "double-whammy," we hypothesize,
constituted a powerful selection pressure that forged a male psychology of context-contingent mate
killing. Below we consider this selective logic in greater detail, and offer specific predictions that
contrast our theory of mate killing with the "slip" or "epiphenomenal byproduct" theory.
5.3.1. Paternity uncertainty. A woman’s sexual infidelity creates uncertainty in her husband’s
paternity of children. The risk of genetic cuckoldry is not merely hypothetical. Estimates from
modern populations based on blood and DNA fingerprinting studies conducted over the past 30 years
estimate the rates to be between 9% and 13% (Baker & Bellis, 1995). Mistaken paternity historically
would have inflicted multiple fitness costs on a male. First, all of the effort the man devoted to
selecting, courting, and attracting his spouse goes down the fitness drain. Second, all the effort he has
expended to date in maintaining and retaining her, including the vigilance costs, direct guarding,
provisioning, and fending off rivals similarly become wasted. Third, the cuckolded man suffers the
opportunity costs of foregone mating opportunities, both short-term and long-term, as a result of
investing in an unfaithful wife. Fourth, he risks his parental effort becoming channeled to a rival’s
offspring in the mistaken belief that they are his own. Fifth, his mate’s parental effort becomes
invested in a rival’s offspring rather than his own. Sixth, the new genetically unrelated child becomes
a half-sibling rather than full sibling of whatever existing children he has or future children he will
have, which results in his children experiencing greater competition and conflict than would
otherwise be the case.
Seventh, the cuckolded man can suffer large reputational damage, hindering his current position and
future ascension in the status hierarchy and impairing his ability to attract future mates. Men will be
more likely to scoff at him. Women will assume that he lacks the ability to prevent other men from
encroaching, and so lower their perceptions of his value as mate. An inability to prevent rival
encroachment, for example, may signal an inability to protect the woman physically and lower his
social status in the eyes of other men as well as women--key components of men’s mate value.
Adding all these factors together results in formidable fitness costs associated with being cuckolded.
The hypothesis that mate killing has evolved as one among several solutions to the cuckoldry
problem confronts two obvious challenges. First, killing a mate does not solve the problem of sunk
costs. The initial courting costs a man incurs, for example, are not recouped by killing an unfaithful
mate. Nonetheless, killing her can curtail some costs, such reputational damage. More importantly, it
can deprive intrasexual rivals of access to a her reproductive value and simultaneously terminate a
rival’s prenatal child. A second challenge is more formidable: If primary fitness benefits to killing an
unfaithful mate include depriving rivals of access to a reproductively valuable resource and
terminating prenatal offspring, why don’t men just go out and kill the mates or children of their
rivals?
Our hypothesis is that the cost-benefit calculus of killing a rival’s mate and killing one’s own mate
are likely to have been dramatically different. First, any woman who has not rejected a man as her
mate is a potential future mate. Just because a rival currently has sexual access to her does not
necessarily mean that the rival will maintain access to her. Injury, change in status, or death of the
rival all could result in the woman becoming available to re-mate. Second, men fiercely guard their
mates from rivals and inflict grave costs on those who attempt to destroy their reproductively
valuable resources. This leads to a specific prediction: A man will be more likely to kill a mate who
has cuckolded him or defected from the relationship when he believes that he has lost her
irrevocably, but before the woman has firmly established a new long-term mateship with another
man who would be willing to protect her. Killing under these circumstances would have been
considerably less costly than attempting to kill a woman who is already in, or who has already
established, a long-term mateship with another man.
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Much evidence already exists that selection has fashioned male defenses designed to guard against
paternity uncertainty. These include psychological mechanisms such as male sexual jealousy (Buss,
2000; Buss et al., 1992; Daly, Wilson, & Weghorst, 1982; Symons, 1979), behavioral tactics such as
mate guarding and retention (Buss, 1988; Buss & Shackelford, 1997), and physiological mechanisms
such as increased sperm volume, increased sperm insemination in contexts in which cuckoldry might
have occurred, and possibly different sperm morphs designed to combat rival male sperm (Baker &
Bellis, 1995). According to Homicide Design Theory, men have also evolved mate killing as one
extreme strategy for solving the cuckoldry problem, and specifically for inflicting costs on rival
males by effectively killing their prenatal children.
Killing a mate who has sexually defected carries several fitness benefits. It would prevent his
existing or future children from having to compete with genetically unrelated children or partially
related children. In some social circumstances, the killer reduces the reputational damage as a result
of the social derision incurred apparently universally by cuckolds, and hence maintains his ability to
attract a replacement mate. But most important, killing an unfaithful mate inflicts costs on the man’s
intrasexual rivals. It deprives the rival’s children of what would have been her maternal effort, thus
seriously hampering the rival’s children’s chances of surviving and thriving. It deprives the rival of
current sexual access as well access to her residual reproductive value. It inflicts costs on the rival by
causing him to have wasted all the time and effort he expended to lure her into an affair. And perhaps
most of all, killing the unfaithful mate, in some cases, results in literally killing the rival’s prenatal
children–essentially a form of rival infanticide.
Killing an unfaithful mate, of course, can be an extremely costly solution in many circumstances. It
could deprive his existing children of maternal investment. It could deprive the killer of future sexual
access he might have achieved with his partner. It could evoke retribution from her kin. According to
our theory of mate homicide, human evolved psychology contains design features that assess and
weigh these costs. And in most circumstances, the costs of killing a mate would have far outweighed
the benefits, especially when killing is compared to other possible solutions to the adaptive problem
of cuckoldry. In fact, most men worldwide do not kill mates who are discovered to have been
sexually unfaithful. Our theory proposes that mate killing is a low base-rate solution, one strategic
outcome of decision rules that get activated only in highly particular circumstances--where the costs
of killing are evaluated to be comparatively low and the benefits high, relative to alternative courses
of action.
Our theory offers several specific predictions that contrast sharply with those of the "slip" or
"epiphenomenal byproduct" theory of mate killing:
(1) most killings of unfaithful wives will be intentional and designed, not accidents or "slips";
(2) most men who kill unfaithful wives will not be evaluated to be "psychotic" or legally insane;
(3) men whose mates are discovered to be sexually unfaithful will have thoughts, fantasies, or
premeditations of killing them, not merely beating or coercing them (the "byproduct" theory cannot
explain why the terminus of a mate killing fantasy is the spouse’s death, not her control);
(4) sexual infidelity will be a sex-linked trigger of mate homicide thoughts, occurring more
frequently in men than in women;
(5) men who have no existing children with an unfaithful spouse will be more likely to experience
homicidal premeditations than men who do have existing children with her (due to the increased
fitness costs of depriving his existing children of her maternal investments);
(6) as the likelihood increases that a man’s existing children with an unfaithful spouse can survive
without her, the likelihood that men will experience homicidal premeditations will increase;
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(7) men whose wives have existing children sired by another man will be more likely to kill an
unfaithful wife, since killing her thus inflicts greater costs on intrasexual rivals;
(8) men residing in cultures or subcultures where cuckolds suffer greater reputational damage will be
more likely to think about, and carry out, killing a sexually errant mate;
(9) size and proximity of the wife’s kin group will affect decision rules to kill an unfaithful mate,
with increases in the likelihood of kin retribution deterring would-be mate killers due to the heavy
costs of killing under these circumstances (this prediction also holds for many of the homicide
hypotheses, since the number of kin linked to the victim will often enter into the cost-benefit
calculus).
All these are testable predictions. The empirical outcome of would adjudicate the two central
competing theories of mate killing. If the majority of these empirical predictions are confirmed, the
balance of evidence would shift to Homicide Design Theory and away from the "killing as incidental
byproduct" theory.
5.3.2. Mate defection. Many costs associated with being cuckolded apply to being abandoned. For
several reasons, the outright defection of a mate from a marriage could inflict a greater fitness cost
on the husband than a sexual infidelity. If a woman remains with a man she has cuckolded, her
husband may continue to gain access to her reproductive value, whereas with a total defection, he
loses that access entirely. Whereas a wife’s infidelity gives a husband’s intrasexual rival partial
access to her current reproductive value, abandonment may grant the rival total access to her residual
reproductive value. For these reasons, we expect that thoughts of mate homicide would be even more
strongly evoked in men who are abandoned than those who are cuckolded.
Nonetheless, not all selective forces suggest that mate homicide will be more common after
abandonment than infidelity. Men whose partners abandon them do not incur at least one cost that
may be incurred as a result of cuckoldry–the mistaken diversion of the man’s own parental
investment toward a rival’s offspring. A cuckolded husband, in contrast, could devote years or
decades of parental effort toward genetically unrelated children, a possibly catastrophic cost the
abandoned man does not incur. Furthermore, being abandoned, as opposed to being cuckolded, frees
up time and resources to pursue alternative mates, albeit with possible reputational damage as a
result of public knowledge of the abandonment.
For some costs, of course, there may be no a priori rationale for determining whether being
cuckolded or being abandoned is worse. In both cases, men suffer reputational damage. Being
abandoned can create harmful social inferences--that the deserted partner is lower in mate value than
his partner, that he has some fundamental flaw that became revealed over the course of the mateship,
or that he has declined in mate value sufficiently for her to trade up incrementally on the mating
market. Being cuckolded can create these damaging social inferences as well, with perhaps the added
mockery that goes along with the judgment that the man is a fool for allowing a rival into his wife’s
naked embrace.
We hypothesize that there are two primary adaptive problems that are solved by killing a wife who
has permanently defected. First and most important, killing a wife who has permanently defected can
inflict a heavy costs on an intrasexual rival. It deprives a rival entirely of access to a mate whom the
man failed to retain. He also effectively aborts any of the rival’s prenatal children his former mate
may be carrying. The fitness benefits of killing a mate who has defected, of course, would have been
substantially greater in the small group living contexts in which humans evolved than in modern
urban contexts, since the number of "effective" rivals of the killer would have been small.
Several other ancillary costs are inflicted on rivals by murdering a wife who has defected. A
defecting wife can reveal private and potentially damaging information to others, including hidden
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flaws, hidden weaknesses or strengths, and specific vulnerabilities. She can reveal specialized
knowledge the abandoned husband has accrued, including secret plans, tactics, and strategies
harbored by the husband, as well as scandalous or shameful information about the man or his kin. All
this private information could be revealed to a rival, giving him a potentially large strategic
advantage in intrasexual competition. Killing the wife essentially deprives intrasexual rivals of
access to this damaging information, above and beyond depriving the rival of access to the woman’s
reproductive value and parental effort.
The second key adaptive problem solved is curtailing the reputational damage that occurs as a
consequence of being abandoned. One of the key concerns of O.J. Simpson when his wife, Nicole
Brown Simpson left him, was the effect on his reputation. According to some evidence, he killed her
when it became clear that she was not going to come back to him combined with evidence
suggesting that she was having sex with another man. One story does not a case make, of course, but
Simpson is not unique in his concern with the reputational damage linked with being abandoned. By
killing a defecting woman, in some contexts, a man can restore his reputation as someone who
cannot be trifled with, and for whom any kind of social defection comes at a steep price, deterring
friends, coalition members, other wives (if he’s polygynous), and future wives from defecting.
Killing a wife who has defected should be more likely to occur in certain contexts and to be carried
out by specific men. When the defecting woman has not yet formed a new mateship, for example, the
costs to the killer are lower since she may lack the physical protection that might be offered by a new
mate (her kin, of course, can also deter a would-be killer). Moreover, the higher the mate value of the
defecting woman, the greater the costs that will be inflicted on rivals as a consequence of killing her.
Hence, women higher in mate value (e.g., younger, more physically attractive) should be more
vulnerable to getting killed than women lower in mate value (e.g., older, less physically attractive).
One countervailing force, however, is the evolution of anti-homicide mechanisms. The kin of a
young woman, for example, should show special alacrity in deterring potentially murderous mates,
due to her reproductive value to the each member of the extended kin network.
Empirically, reproductive-aged women are indeed more likely to be killed than post-reproductive
aged women (e.g., Daly & Wilson, 1988; Shackelford, in press a, b). Proponents of the "byproduct"
theory offers this explanation: "we propose that such homicides are the dysfunctionally extreme
products of violent inclinations whose lesser manifestations are effective in coercion . . . Uxoricide
risk is indeed maximal for the youngest . . . This finding may strike the reader as evidence against the
proposition that men ‘value’ young wives maximally, but the paradox disappears when one views
uxoricides as the dysfunctional extremes of ‘normal,’ nonlethal coercive violence" (Wilson, Daly, &
Scheib, 1997, p. 448-450). "This pattern [men killing young wives] may seem to belie the
proposition that male minds place high ‘value’ on young wives, but again, as with the estranged
husband who pursues and kills a woman he can’t abide losing, violent inclinations seem best
understood as coercive tools for controlling wives about whom they feel proprietary–and the lethality
is a rare and dysfunctional outcome of the most extreme feelings" (Wilson & Daly, 1998).
We suggest a simpler explanation--men have evolved adaptations that are designed to incite murder
of mates in particular circumstances, especially those mates who are high in reproductive value. The
greater the reproductive value of the defecting wife, the greater the reproductive benefits flow to an
intrasexual rival, and hence the greater fitness benefits that accrue to the killer. Killing a young wife
who has irrevocably defected, simply put, confers more fitness benefits on the killer than does killing
an older wife, since the cost inflicted on the relevant intrasexual rivals are commensurately greater.
Human beings are notoriously difficult to kill. According to Homicide Design Theory, humans have
evolved a large collection of anti-homicide mechanisms designed to avoid getting killed (Duntley &
Buss, in prep., a). Although the "accidental death" argument is sometimes used by killers as a
defense ("I just wanted to coerce her, I didn’t mean to kill her"), and some mate killings may be
byproducts of mechanisms designed for coercive control, this argument strains plausibility when
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faced with the evidence of premeditated, well-planned, intricately designed uxoricides. Even in socalled impulse killings that are not premeditated, one must explain why the man chose to use "deadly
force" rather than non-lethal coercive force. The evolved mate killing hypothesis provides a more
parsimonious explanation for the available data, including the greater rates of killing young wives
who have defected, without resorting to special "dysfunctional" explanation for a very predictable
and patterned form of homicide. Our theory also offers a suite of novel testable predictions that, if
confirmed, could not be explained by the byproduct theory of mate killing.
None of these arguments, of course, deny the plausibility of the hypothesis that men have evolved
mechanisms, including the use of non-lethal violence, that function to deter mates from infidelity or
defection. Indeed, there is abundant evidence for precisely such mechanisms (e.g., Buss, 2000; Daly,
Wilson, & Weghorst, 1982). Rather, our argument is that the mechanisms that result in mate
homicide differ from those involved in mate coercion, control, and deterrence.
5.3.3. Which rivals suffer from a man killing a mate who has defected? All of the multiple costs
inflicted on rivals by killing a mate who has deserted are not incurred by all intrasexual competitors
equally. They are inflicted most heavily on the rivals with whom the killer is in most direct
competition. To understand the logic of this hypothesis, the logic of assortative mating for mate
value must be introduced.
As a general rule, men and women assortatively mate on the basis of overall mate value (Buss, 1994;
Frank, 1988; Symons, 1979; and others). The "10’s" tend to pair off with other "10s," the "8’s" with
other "8’s," and "4’s" with other "4 ’s." There are discrepancies in mate value, of course, although
these are linked with a higher likelihood of infidelity and defection (see Buss, 2000, for a review of
this evidence). Nonetheless, strong positive assortment is the rule. As a consequence, in intrasexual
mate competition, men are primarily competing with men who are closest to them in mate value. In
the modern world, for example, the burger flipper and gas station attendant (low status, low
resources, and hence low in mate-value) are not in direct competition with successful movie stars or
rock stars for mating with attractive models. They compete, instead, with those in their local mating
pool who are most similar to them in mate value.
Selection, according to this logic, has designed psychological mechanisms to inflict costs on those
rivals with whom one is in most direct competition. The evolved mechanisms are designed for "local
skirmishing," not for competing "in general" with all conspecifics (see Frank, 1985, for similar
arguments in the context of status and hierarchy negotiation). These phenomena are witnessed in
many domains. Finding out that a member of one’s department who is similar in rank and reputation
has received a higher pay raise, for example, may cause more psychological anguish than finding out
that someone in another department or another university has received a higher pay raise.
As a consequence, calculating the fitness benefits that would have accrued to a killer from murdering
a mate who has defected does not involve dividing the benefits over the entire population of other
males. The benefits that accrue stem from inflicting costs on the intrasexual rivals with whom the
killer is more directly and immediately in competition–those in the local pool who are most similar
in mate value. These more concentrated benefits essentially increase the selection pressure for the
evolution of mate killing mechanisms.
5.4. When Women Kill Their Mates
According to Homicide Design Theory proposes, women have evolved mate killing mechanisms that
differ profoundly in design from men’s mate killing mechanisms. To start with, the fitness benefits to
women of killing a partner who is sexually unfaithful would historically have been relatively trivial,
even without considering costs. A sexually unfaithful husband does not jeopardize a woman’s
certainty that she’s the mother of her children. Nor does a sexually unfaithful husband cause the
potential misdirection of a woman’s own parental efforts toward children who are genetically not her
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own. Sexual infidelity per se, therefore, should be less likely to trigger homicidal thoughts or deeds
in women.
Nonetheless, there are some delimited circumstances in which it might historically have been
advantageous for a woman to kill a partner: (1) when the mate is physically abusing her to an extent
that it threatens her survival, and as a consequence, her children’s well-being and survival (i.e., antihomicide mechanism); or (2) when the mate is inflicting severe costs on her children, such as
through physical abuse or sexual abuse.
In these delimited contexts, killing a mate could solve key survival and reproductive problems by
curtailing the massive costs being inflicted on the woman or her children by the mate. Killing a mate
in some circumstances also could inflict a cost on female intrasexual rivals by depriving them of
access to the husband and his investments, although we expect that selection has been weak for
killing under these circumstances. Reproductive variance is lower among women than among men,
so the benefits of depriving a rival female of a particular male are commensurately lower. Because
sperm are cheap and eggs are expensive, men are rarely the limiting reproductive resource for
women that women are for men.
In sum, selection pressure is hypothesized to have been considerably weaker on women than on men
for the evolution of a psychology of mate killing. Men are proposed to have evolved specific mate
killing mechanisms that are absent in women. Women, in turn, are hypothesized to have evolved
mechanisms that lead to killing in the very delimited contexts in which their lives and the lives and
well-being of their children are in serious danger from a mate.
5.5. Evidence Bearing on Hypothesized Mate Killing Mechanisms
A variety of sources of evidence can be used to test predictions from the theory of evolved mate
killing mechanisms and to differentiate it from the predictions generated by the Daly-Wilson
byproduct theory of mate killing. Before examining this evidence, we note that that some (we think a
relatively small) proportion of mate homicides may result from accidents, whereas others (we think
the majority) are caused by evolved mechanisms designed for murder. If the competing theories are
to have any conceptual and explanatory power, however, they must generate specific predictions that
do not flow from the other. Below we outline several sources of existing evidence that distinguish
our theory from the byproduct theory.
One of the key differences pertains to the motivational processes underlying violence and homicide
proposed by the two theories. The byproduct theory clearly states that "lethal and nonlethal wife
assault are motivationally similar" (Wilson & Daly, 1998, p. 300); both are presumed to be
motivated by the goal of coercion. In sharp contrast, according to Homicide Design Theory, mate
killing and the nonlethal assault of a mate are motivationally distinct. We concur with Wilson and
Daly that the primary motivation for nonlethal spousal violence is coercive control. But we differ in
arguing that the central motive for lethal mate killing is not control or coercion, but rather the literal
death of the partner. Below we summarize evidence that bears on these fundamentally different
theoretical positions.
If men kill their mates accidentally as a byproduct of using violence as a means of coercive control,
as predicted by the byproduct theory, then the majority of mate killings should occur in mateships
that show a substantial amount of male violence. That is, among those relationships in which a wife
or partner gets killed, there should be prior evidence that the males have been using violence as a
means of coercive control that somehow has escalated. A large-scale study appears to refute this
prediction from the byproduct theory. A study of 15,537 cases of domestic battery, which included
both spousal beatings and spousal homicide, found that only one in 33 spousal homicides showed a
prior history of domestic violence (Staff, 1990). The study also found that, of the 110 cases in which
one partner made threats to kill the other partner, not a single one later led to death. As Ghiglieri
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concludes, "men who decide to kill their wives simply go ahead and kill them without giving as
many advanced signals as one might expect" (Ghiglieri, 1999, p. 153). If the byproduct theory were
correct, then mate homicide should typically occur in the context of a history of marital violence. It
does not.
A second source of evidence comes from a study of Canadian wife killings conducted by Daly et al.
(1997). They found that when the wife had one or more co-residing children who were sired by a
previous partner, and the present partner had not sired any children by the current wife, the rate of
wife killing was more than ten times that which occurred when co-residing children were entirely
sired by the present partner. When the household contains at least one child of each (one from
current and one from previous partner), the rates of uxoricide are only slightly higher than when all
co-residing children are from the present partner. Daly and Wilson do not provide an explanation for
this important and dramatic set of findings. Instead they conclude that "marital violence may be
elevated in stepfamilies," a domain-general claim that, while true, fails to explain the dramatic
context effects within different types of stepfamilies(Wilson & Daly, 1998, p. 226).
More specifically, the Daly-Wilson explanations for wife beatings and wife killings are identical–
male sexual proprietariness that uses violence for coercive control, which sometimes reaches
"dysfunctional" extremes of uxoricide: " . . . most men who coerce, pursue, and threaten women do
not go so far as to kill them, and those who do may be considered dysfunctional over-reactors in a
game of brinkmanship" (Wilson & Daly, 1998, p. 302. The empirical findings, however, are not
compatible with a single explanation for both phenomena. The rates of women seeking shelter from
assaulting husbands is virtually identical if they have children solely from a previous partner and if
they have at least one child each from the current and previous partner, and both are equally elevated
compared with women who have children only with the present partner (see Wilson & Daly, 1998,
Figure 8.5, p. 227). In sharp contrast, the presence of a single child by the present partner, even when
the wife has one or more from a previous partner, dramatically reduces the uxoricide risk compared
to when the only children present are from a previous partner. This pattern of findings supports the
notion that non-lethal violence against a mate and uxoricide are fundamentally different phenomena.
The data contradict the idea that a single, general theory of coercive control can simultaneously
explain spouse beatings and spouse killings. By killing a wife who has produced children only by a
previous partner, a man inflicts costs primarily on his intrasexual rivals. In contrast, when she has at
least one child sired by him, a would-be wife killer would be inflicting a heavy cost on his own
fitness as a result of killing her, since he would be depriving his own child of the woman’s maternal
efforts.
This result was predicted by our theory of evolved mate killing mechanisms prior to our discovery of
this finding by Daly and Wilson (Buss & Duntley, 1998). Killing a wife who has existing children
sired by an intrasexual rival, yet none sired by the current mate, inflicts a great cost on previous and
future intrasexual rivals. It deprives the prior rival of the maternal effort that she would otherwise
direct toward his children. And it deprives the future rival of a valuable reproductive resource. In
summary, existing evidence suggests that a single theory of coercive control cannot explain both
non-lethal spousal violence and spousal homicide.
5.6. Warfare and Genocide
Warfare may be defined as "a conflict between social groups that is resolved by individuals on one or
both sides killing those on the opposite side" (Ghiglieri, 1999, p. 160-161). Homicide Design Theory
includes detailed hypotheses about the evolution of killing in the context of warfare. A number of
other evolutionary theories of warfare have been proposed, notably by Alexander (1979), Chagnon
(1988), Hamilton (1975), Ghiglieri (1999), Tooby & Cosmides (1988, 1994), and Wrangham
(Wrangham, 1999a, b; Wrangham & Peterson, 1996). Our theory coordinates with many of the
components of these theories, but a detailed analysis of these theories and our own theory of warfare
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is beyond the scope of this paper (see Duntley and Buss, in prep., b).
Here we merely mention some fundamental premises of our theory of war, since many homicides
throughout human history have been committed in the context of war. Keeley (1996) estimated that
0.5% death rate per year occurred from raiding alone, based on 21 societies for which data on death
were available. Combined with the historical record, paleontological evidence, archeological
evidence, and ethnographic evidence, it is reasonable to hypothesize that warfare has been a
powerful and pervasive selective force over human evolutionary history. Indeed, Wrangham (1999b)
hypothesizes that lethal raiding evolved in both humans and chimpanzees, sharing a common
evolutionary origin around five to six million years ago. Our theory, like those of Alexander,
Wrangham, Ghiglieri, and others, also assumes that group against group killing has been a recurrent
feature of human evolutionary history that has selected for features of psychology that are specific to
warfare. The theory rests on five key premises.
5.6.1. Premise 1: Men have evolved warfare mechanisms that are lacking in women. Men, but not
women, have benefited in some contexts from forming coalitions with the express purpose of killing
members of other groups. Recurrent benefits of coalitional killing have resulted in the evolution of a
warfare-specific psychology in men that is lacking in women (Tooby & Cosmides, 1988, 1994). As
Tooby and Cosmides note, "females have more to lose, and less to gain" from engaging in coalitional
warfare (Tooby & Cosmides, 1988, p. 8).
5.6.2. Premise 2: Primary fitness benefits of warfare are multiple, including the acquisition of new
mates, resources, and territory, as well as eliminating key competitors. As several prior authors have
suggested (e.g., Dennen, 1995; Tooby & Cosmides, 1988, 1994; Wrangham, 1999a, b), the primary
fitness benefits that recurrently accrued from warfare included the (1) acquisition of new women as
mates, (2) immediate short-term sexual access to fertile women, possibly through rape, (3)
acquisition of resource-rich territory, (4) cultivating a reputation that deters exploitation or
encroachment, and (5) eliminating sources of intrasexual competition for these and other scarce
resources. Wrangham (1999a) notes additional benefits of lethal raiding, including (6) inspiring fear
in neighboring groups, perhaps one mechanism by which deterrence occurs, (7) protection from
territorial incursion, and (8) expanding one’s borders and hence "safe zones." To this list one may
add (9) securing honor, status, or glory within one’s own group, which could lead to a cascade of
reproductive benefits and (10) inflicting costs on rival groups in revenge for having sustained costs.
Wrangham (1999a) makes the important point that warriors need not be required to identify the
specific benefits that they might accrue through warfare for the evolution and maintenance of
warfare mechanisms (the same is obviously true of all of the evolved homicide design features
proposed by Homicide Design Theory). It is only necessary that the one or several of these benefits
flow to warriors, and that the benefits of this strategy, on average, outweigh the costs over the
relevant instances.
5.6.3. Premise 3: Psychological design features for warfare. The psychological design features of
male warfare psychology include:
(1) strategies for forming cooperative warfare coalitions (Tooby & Cosmides, 1988; Alexander,
1979, 1987);
(2) methods for increasing coalitional number, since numerical superiority is an excellent predictor
of success (cf. Wrangham, 1999a, b);
(3) preferential selection of coalition partners depending on specific characteristics (e.g., physical
formidability, bravery)(DeKay, Buss, & Stone, 1998);
(4) procedures to create and enforce coalitional solidarity (e.g., status rewards for bravery and
sacrifice for the group; reputational damage from cowardice);
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(5) procedures to minimize within-coalition conflict (e.g., eliminating sources of sexual conflict and
jealousy);
(6) procedures for enforcing individual compliance with coalitional contract, such as inducing
obedience and conformity (Dennen, 1995) and punishing cheaters (Tooby & Cosmides, 1988);
(7) self-assessments, other assessments, and comparative assessments of group size and
formidability, with sensitivity to numerical superiority (Wrangham, 1999a, b);
(8) motivational mechanisms to induce attack (e.g., working group into an emotional frenzy); and
(9) ethnocentrism, xenophobia, and pseudospeciation that lead to derogation, demonization,
diabolization of opposing group (e.g., that they are sub-human animals) (Dennen, 1995).
5.6.4. Premise 4: Evolution of anti-homicide mechanisms to combat coalitional warfare from other
groups. Being a member of the group that becomes the victim of a coalitional attack would have
inflicted severe fitness costs. This created a selective force that led to the evolution of anti-homicide
warfare mechanisms. Indeed, Alexander (1979, 1987) has proposed that the primary function of
group formation in humans, in terms of their fitness significance for the individuals within them, was
protection from other aggressive human groups–a prime candidate for an anti-homicide mechanism.
He notes that "in no other species do social groups have as their main jeopardy other social groups of
the same species" (Alexander, 1987).
In addition to coalition formation as an important anti-homicide device, other hypothesized antihomicide mechanisms to combat other groups include:
(1) heightened psychological sensitivity and vigilance to the contexts in which one’s group is
vulnerable to attack from other groups (e.g., being numerically weaker);
(2) adaptive cognitive "errors" that produce overestimates of the likelihood of enemy attack (see
earlier discussion of Error Management Theory);
(3) specialized fear or paranoia about being attacked by a hostile group;
(4) preemptive protective strategies, such as migration, ceding resources;
(5) promoting strategic alliances through trade or intergroup marriage (Wrangham, 1999b);
(6) coordinating surprise attacks to minimize costs of being victimized by opposing coalition;
(7) reputation management as non-exploitable (e.g., willing to suffer large costs in order to inflict
damage on an attacking coalition);
(8) fleeing in response to immediate onslaught; and
(9) a psychology of vengeance that functions simultaneously to inflict costs on others and to deter
rival groups from future attacks.
Warfare can simultaneously be a manifestation of homicide and anti-homicide mechanisms. When
the chiefs of headhunting tribes of Borneo were interviewed about why they continued on "this
senseless practice of going on the warpath, the best reason he can give is that unless he does so his
neighbors will not respect him and his people, and will fall upon them and exterminate
them" (McDougall, 1915). Coevolved anti-warfare mechanisms, according to Homicide Design
Theory, led to the further coevolution of warfare mechanisms designed to circumvent the antifile://C:\Documents%20and%20Settings\Tom%20Simpson\Desktop\Project%20Webs... 12/04/2002
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warfare psychology, as follows.
5.6.5. Premise 5: The co-evolution of warfare psychology in response to anti -warfare mechanisms.
As anti-warfare psychology evolved, it created further selection for the evolution of increasingly
context-specific warfare psychology. These design features include: Surprise raids to circumvent
anti-warfare mechanisms; exaggerated displays of threat and ferocity to extort resources without
having to incur the risks of going to war; and heightened vigilance to contexts in which potential
victim might engage in a preemptive strike.
The theory that humans have evolved elaborate suites of warfare and anti-warfare mechanisms
obviously does not imply that warfare is inevitable or occurs rigidly or invariantly across contexts
and cultures. The engagement of warfare mechanisms, like those of other hypothesized evolved
homicide mechanisms, are predicted to be highly context-dependent. Identifying the specific design
features and contexts in which they are activated remains the most viable and hopeful means of
reducing their engagement. Interested readers are referred to Duntley and Buss (in prep., b) for a
detailed exposition of this branch of Homicide Design Theory, along with specific empirical
predictions and preliminary empirical tests.
5.7. Empirical Evidence Relevant to Homicide Design Theory
In this paper, we have enumerated more than four dozen specific testable empirical predictions that
follow from Homicide Design Theory and cited dozens of empirical findings that are consistent with
the theory and inconsistent with competing theories. Our research program over the past three years
has been devoted to testing the novel predictions about which there exist no relevant data. We have
conducted more than a dozen empirical studies of homicidal premeditations (N = 2,900), antihomicide premeditations (N = 500), hypothetical scenarios in which people estimate the likelihood
that they would kill in certain circumstances (N = 1,000), and free listings of the reasons that would
impel people to resort to murder (N = 300). We have also conducted a study of 800 people actually
charged with murder and analyzed FBI homicide statistics involving thousands of convicted killers.
A detailed report of the dozens of new findings is beyond the scope of this paper, which has focused
on explicating the premises and logic of the Homicide Design Theory. The research is being
prepared for publication in the primary empirical journals. Nonetheless, we note that the results thus
far provide strong support for a number of the specific empirical predictions. We find that more than
82% of women and 91% of men (in a sample close to 3,000) report having had vivid homicidal
premeditations. Similar percentages occur in our cross-cultural sample from Singapore. Men tend to
have more frequent, more detailed, and more recurring homicidal thoughts than women, sex
differences also found by Kenrick and Sheets (1993) and by Crabb (2000). Homicidal thoughts are
often recurring over periods of days, weeks, months, and sometimes years. They involve detailed
scenarios about means, motives, and opportunities.
Fantasies about killing intrasexual rivals far outnumber fantasies about any other category of victim,
corresponding to the empirical fact that most homicides involve intrasexual rivalry (Daly & Wilson,
1988). Men are far more likely to have these fantasies than women, also corresponding to the actual
rates of homicide. Indeed, the percentages of homicidal fantasies in the two-by-two matrix of sex of
killer and sex of victim show a remarkable correspondence with the known percentages in that
matrix. The order in both cases is male killer/male victim (65%, 59%), male killer/female victim
(22%, 18%), female killer/male victim (10%, 17%), and female killer/female victim (3%, 6%),
where the first number in parentheses refers to actual homicides and the second to our data on
homicidal fantasies.
The catalysts of the homicidal fantasies are also highly sex-linked. Men are more likely than women
to cognitively rehearse killing a rival who has sex with their mate, a rival who has humiliated them in
public, a rival who has physically attacked them, or a rival who has physically dominated them.
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Women are more likely to fantasize about killing a rival who is more physically attractive,
particularly if their mates have indicated an attraction to those rivals.
The second most common victims of homicidal fantasies are killing romantic partner, again
corresponding to the second most common category of actual homicides. The causes of homicidal
fantasies are sharply sex-differentiated in ways that correspond precisely to the predictions. Among
those who report thoughts of killing a romantic partner, more than twice as many men as women
report a partner’s sexual infidelity as being a catalyst and more than three times as many men as
women report getting irrevocably dumped as a catalyst. These findings accord with findings from
studies of uxoricide. In one study of all uxoricides committed in a Canadian city over 22 years, for
example, female-initiated separation emerged as a motive in the testimonial evidence in 63% of the
cases (Daly, Wiseman, & Wilson, 1997). Women, in contrast, are far more likely to report a partner’s
emotional infidelity and physical abuse at the hands of a partner as a cause of their homicidal
thoughts.
Among those who reported thoughts of killing a family member who was not a romantic partner,
reports of killing a non-genetic relative were nearly four times as frequent as thoughts of killing a
genetic relative. The most common victims in these premeditations were stepparents. The most
common causes cited involved the stepparent inflicting costs on the person (or the person’s
genetically related siblings) and absorbing all of the resources of their genetic parent. One woman,
for example, reported thoughts of killing her stepmother because "She is an evil repugnant witch
who only wants my father’s money and she despises me because I seem to be the only one who
realizes this."
We have documented a cognitive bias suggesting that people overestimate the likelihood of getting
killed in a variety of circumstances (e.g., being discovered in bed with a rival’s mate), as predicted
by the error management component of the theory. For example, men give far higher estimates of the
likelihood that a rival who finds them in bed with the rival’s partner will try to kill them than to the
likelihood that they will try to kill a rival they find in bed with their partner. These and related
findings were predicted in advance by applying Error Management Theory logic to the evolution of
anti-homicide psychology. To our knowledge, no competing theories of homicide predict cognitive
biases of this sort. Our studies of 800 killers, homicide scenarios, free listings of reasons for killing,
and FBI statistics on homicide also reveals support for several specific predictions.
The ultimate merits of Homicide Design Theory in contrast to competing theories of homicide will
ultimately rest with the weight of evidence collected by the community of scientists who study
homicide. Although the authors of any theory are obviously not unbiased, we suggest that unbiased
reviewers of the evidence already existing in the published literature will come to the conclusion that
prior theories of homicide are inadequate to explain what is already known.
6. Objections to Homicide Design Theory
In the context of presenting our theory to colleagues both formally at professional conferences and
informally, we have encountered a number of objections. Below we address the more substantive of
these objections.
6.1. Objection 1: The costs of killing would have been too high for a psychology of homicide to
evolve. The costs of attempting to kill another human being are obviously substantial, including the
risk of being injured or killed while attempting murder and the risk of retribution from the victim’s
kin, friends, or coalitional allies. The fact that it is so costly to kill, ironically, is actually evidence
for, rather than against, the idea that homicide has been a recurrent selective force over human
evolutionary history. The tremendous costs associated with pursuing a killer strategy did not, and
could not, have arisen in a vacuum. The heavy fitness costs to victims of homicide provided
powerful selective forces leading to the evolution of anti-homicide mechanisms–both mechanisms
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designed to promote peace (e.g., de Waal, 1989) and mechanisms that make homicide an extremely
hazardous strategy to carry out. According to our theory, these costs have held homicide in check,
resulting in a lower frequency of homicide than would have occurred without these co-evolved antihomicide mechanisms. They led additionally to the coevolution of additional complex design
features for a homicidal strategy, such as (a) preferences for exploitable victims, and (b) preferences
for contexts where costs will be least likely to be incurred (e.g., when victim is less formidable and
victim’s kin less numerous).
Selection operates according to net benefits relative to costs in the currency of fitness averaged over
the relevant instances. All evolved strategies carry costs. According to Homicide Design Theory, the
benefits of killing in certain contexts provided access to fitness benefits, centrally the elimination of
intrasexual competitors and the resources that would flow to those competitors (including potential
mates), to a far greater extent than has been previously recognized. The benefits of killing a rival’s
prenatal or not-yet-conceived children, for example, produce a cascade of benefits for the killer’s
children–a benefit not recognized in prior analyses of homicide. As Tooby and Cosmides (1994)
point out in the more delimited context of warfare, even when there is a substantial risk of death, a
strategy of killing can evolve if the benefits get distributed to those who succeed in the strategy (and
obviously if the fitness benefits outweigh the fitness costs of the strategy, on average, recurrently
over the relevant events).
We are not attempting to minimize the costs of killing; precisely the opposite. The costs of pursuing
a killer strategy are so high precisely because killing has been so beneficial to killers over human
evolutionary history. If it had not been so beneficial, we would not see today the formidable array of
evolved anti-homicide mechanisms that keep killing in check. Demonstrating that anti-homicide
mechanisms inflict heavy costs on potential killers provide evidence for, not against, the central
assertion that homicide has been a recurrent feature of human ancestral environments.
6.2. Objection 2: Homicide occurs too infrequently to be acted on by selection.
This objection can be countered with two points. First, even if homicide has been a low base-rate
event in the lives of our ancestors (e.g., a 1% probability of getting killed), the fitness consequences
nonetheless would have been tremendous. The cascading fitness costs incurred by the victim and the
victim’s kin and the cascading fitness benefits accrued by the killer and the killer’s kin, for reasons
outlined earlier, would have provided more than enough selective impetus for mechanisms designed
for killing conspecifics to have evolved. Indeed, a design feature that provides a mere 1% fitness
advantage over competing designs can spread from 1% to 99% of the population in a mere 285
generations. It is not an extravagant claim to suggest that design features that terminate rival designs
could confer at least a 1% fitness advantage over those competing designs.
Second, homicides may not have been rare, and the fitness advantage may have been considerably
higher than 1%. To use one Amazonian group as an example, among the Yanomamo, as many as
30% of all males are killed by someone else (Chagnon, 1988; see also Valero, 1970). The
Yanomamo are not alone in these astonishing rates, nor are they the highest. Keeley’s (1996)
analysis suggests that the percentages of death due to warfare alone in various prestate societies
reached as high as 1.5% per year, which would add up to a cumulative probability of dying at the
hands of another human of higher than 30%. These rates of homicide, given the associated fitness
consequences to individuals and their rivals suggests that selection easily could have operated to
fashion psychological adaptations devoted to killing. Furthermore, high rates of conspecific killings
have also been documented in chimpanzees, our closest primate relative. Wrangham (1999) notes
that adult chimpanzee killings have been reported from four independent study sites. Goodall (1986)
estimates 30-40% adult male mortality from intraspecific coalitionary aggression. Although this is
likely to be an overestimate, Wrangham comes to the conclusion that "the killings, in relation to total
observed adult deaths . . . appear to be demographically significant" (p. 10).
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A final rate issue pertains to the type of homicide. In our professional presentations of Homicide
Design Theory, many have found our analysis of the evolution of a psychology of infanticide highly
plausible, but express greater skepticism about the evolution of some of the other proposed homicide
mechanisms (e.g., Margo Wilson, personal communication, June 1998). However, infanticide
represents a tiny fraction of all homicides, less than 2 percent. Mate homicides are than five times
more frequent, and male intrasexual rivalry homicides are more than thirty times more frequent. If it
is plausible that selection can fashion infanticide mechanisms in humans, given their low base-rate,
there is no reason to believe that selection could not have fashioned other homicide mechanisms,
given that these other homicides occur at far greater rates.
6.3. Objection 3: For each adaptive problem to which homicide is a hypothesized solution,
there are many obvious, more effective, less costly non-lethal alternative strategies.
For most of the adaptive problems discussed in this paper, we are not arguing that homicide is the
only evolved solution; precisely the opposite. Our theory is that homicide is one designated option
within several evolved psychological decision trees. Consider the adaptive problem confronted by a
young unmarried woman who gives birth, but who has failed to induce the father of her child to
commit or invest. Clearly, there would have been several potential solutions to this adaptive
problem–attempt to secure another mate who might be willing to invest, solicit help from kin, or
attempt to raise the child alone. A fourth solution would be to kill the infant and to wait until
circumstances are more favorable to bear and rear a child. The hypothesis that infanticide is one
evolved solution to the adaptive problem this young woman is facing does not in any way contradict
the hypothesis that there are alternative evolved solutions.
Whether the evolved decision rules produce in infanticide or some alternative course of action in any
particular instance, of course, depends heavily on context. Specifically, what is the woman’s
appraisal of her ability to attract a mate willing to invest in a child who is not his own? How
extensive, resource-rich, and willing-to-invest are the members of the woman’s extended kin
network? These and other contexts are hypothesized to be central components of the evolved
psychological apparatus and therefore will affect the likelihood of choosing infanticide rather than
some other alternative as the behavioral solution to the relevant adaptive problem.
The same logic applies to each of evolved homicide mechanisms proposed Homicide Design Theory.
Even with the self-defense homicide hypothesis, there are alternative solutions to the adaptive
problem of being confronted by a person who shows intent to kill you. A potential victim can run,
hide, migrate, attempt to placate the killer, enlist allies, or attempt to murder the attacker. Which
solution a person adopts depends heavily on contextual factors that influence cost-benefit appraisals
(not necessarily conscious) of the possible solutions–factors such as comparative physical
formidability, relative weaponry, availability of escape routes, and so on. The key point is that
existence of non-homicidal solutions to particular adaptive problems is not a cogent argument
against homicide being one of the evolved solutions.
6.4. Objection 4: Homicide merely represents the end of a continuum of violence, "the tip of
the iceberg of coercive control rather than a motivationally distinct phenomenon" (Daly &
Wilson, 1999, p. 67).
According to our theory, and in sharp contrast to the Daly and Wilson position, most homicides are
manifestations of evolved psychological mechanisms that are distinct from mechanisms designed for
coercive control and other nonlethal methods of solving adaptive problems. Like the rain, the burden
of proof for these competing theories falls on both alike. We suggest that there are already sufficient
grounds for doubting the theory that homicide is merely an extreme manifestation of the same
mechanisms responsible for coercive control.
Before enumerating these grounds, it is important to keep distinct several issues. We fully
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acknowledge that some of the same circumstances that provoke non-lethal violence also can provoke
homicide. Finding a rival in bed with one’s partner, for example, may for one man evoke a non-lethal
beating and for another man evoke a homicidal rage. Being a stepchild, to take another example, is a
circumstance that evokes both an elevated risk of being physical abused and an elevated risk of being
killed (Daly & Wilson, 1988). But similarity or of some of the circumstances that provoke homicide
and non-lethal violence does not warrant the conclusion that the underlying psychological
mechanisms of the two phenomena are identical, motivationally or cognitively.
Let’s first consider motivation, and use mate killing as the example to illustrate the logic of our
argument. According to our theory of uxoricide, one key motive for murder is the destruction of a
mate who has defected, resulting in depriving intrasexual rivals of access to what would be a
extremely valuable reproductive commodity. A common phrase spontaneously reported by both
killers (Daly & Wilson, 1988) and in our studies of spontaneous homicidal premeditations is "If I
can’t have her, no one can." In contrast, the motive for non-lethal beatings is to try to coerce the
woman back into the relationship. Thus, the two phenomena are motivationally distinct, even though
one central circumstance is the same, namely that the mate has departed.
Obviously, the decision by a deserted man to kill the woman, beat her, try to damage her reputation,
or leave her alone entirely will depend on a host of other circumstances–the perceived likelihood that
she can be coerced back into the relationship, whether she has produced children with him, extensity
and strength of her kin group (Figueredo, 1995), and so on.
Competitive tests of our theory against alternative theories that argue that homicide and non-lethal
violence are generated by the same psychological mechanisms (continuity theories) must rest with
identifying whether or not there are psychological mechanisms unique to homicide. According to
Homicide Design Theory, the existence and pervasiveness of homicidal premeditations provides one
direct source of evidence bearing on the alternative theories. The continuity theories have difficulty
accounting for these findings: (1) men have homicidal premeditations of killing their mates when
their mates have left them irrevocably; (2) in our studies of homicidal scenario building, men think
of killing their partner, not merely injuring her; (3) homicidal thoughts are recurrent and pervasive,
and can consume large quantities of cognitive capacity. All of these findings are well explained by
Homicide Design Theory, but are entirely puzzling on the coercive control theory of mate killing.
In sum, although some of the same circumstances that are causally connected with non-lethal
violence are also causally connected with homicide, there is already compelling evidence to suggest
that the two are motivationally and psychologically distinct.
6.5. Objection 5: Distinct Objections to the Theory of Evolved Mate Killing.
During professional presentations, critics have raised two objections to the mate killing component
of our theory. First, why it ever would have been advantageous to kill a mate, who after all has
positive reproductive value? Second, wouldn’t the costs associated with the risk of retribution from
the dead wife’s kin have imposed have outweighed whatever benefits might otherwise have accrued
to the killer?
Our theory, combined with ancillary information, provides a powerful refutation of these objections.
The multiple fitness costs one inflicts on an intrasexual rival, including the effective infanticide of
the rival’s prenatal children, alone could confer in principle the necessary fitness benefits for mate
killing mechanisms to have evolved. And since the best prediction of future sexual behavior is past
sexual behavior (Buss, 1994), a wife found to be unfaithful is both likely to have been so in the past
and likely to be so again in the future. Thus, killing her in some circumstances solves a raft of
adaptive problems, confers fitness benefits on the killer and his future children, and inflicts large
costs on the rival and his would-be or future children (including reducing the number of fitness
vehicles his rival’s children have to benefit from and invest in).
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Regarding the deterrent effect of retaliatory costs inflicted by the mate’s kin (Figueredo, 1995), we
hypothesize that this was indeed a selective force that co-evolved in response to the adaptive
problem of having a woman kin member killed by her mate! Without this evolved anti-homicide
mechanism, mate killing would have been far more beneficial than it was. But unless one supposes
that people lack evolved kin-protection mechanisms and just "figure it out," then the very existence
of a context-contingent retaliatory mechanism in people to inflict severe costs on men who kill their
mates constitutes evidence for the recurrence of mate killing over human evolutionary history.
It is likely that the majority of ancestral social groups were composed of a core of men who were
related, and females who were imported from other groups. Most females would have lived among a
group of people to whom, on average, she was less genetically related than they were to one another.
In such an environment, males who killed their partners would have been much less likely to suffer
the costs of retaliation from the woman's kin.
Furthermore, there is no reason to believe that the "kin retaliation" argument against the theory of
evolved mate killing would not apply with equal force to other forms of killing. Why doesn’t the
prospect of kin retaliation deter men, for example, from killing their intrasexual rivals? Again, it’s
important to stress that we think it likely that kin retaliation has acted indeed as one deterrent to mate
homicide. But we propose that it has evolved as one among many anti-homicide mechanisms, and
has co-evolved along with mate killing psychology. The existence of these anti-homicide
mechanisms impose co-evolutionary selection pressure on the evolution of killer sensitivity to
circumstances in which the benefits of mate killing will be unusually high and the costs unusually
low (e.g., if perpetrator detection can be evaded). We suggest that precisely such context-sensitivity
is part of the co-evolved mate killing mechanism.
7. Conclusions
Evolution by selection operates through the relative replicative success of competing designs. At the
highest level of abstraction, there are two general strategies that can be favored by selection (Buss &
Dedden, 1990). One is to acquire reproductively relevant resources more adeptly than competitors.
The second is to inflict reproductive costs on competitors. Homicide is a remarkably effective
strategy for inflicting costs on competitors. Its consequences cascade down generations, affecting
entire kin lineages. In circumstances where a killer acquires the resources plundered from a
conspecific competitor, murder provides the double-benefit of eradicating a rival entirely while
gaining access to his prized reproductive resources. Because killing conspecifics is so extraordinarily
beneficial, it is simultaneously extraordinarily costly to victims. Therefore, when conspecific killing
recurs, selection will rapidly favor the evolution of anti-homicide mechanisms, including killing to
prevent getting killed, resulting in a coevolutionary arms race between homicide and anti-homicide
strategies.
People have killed other people in all known cultures and have done so throughout human recorded
history and prehistory. Humans kill in a wide variety of distinct circumstances, from a young mother
who kills a deformed infant to a tribe that conducts a retaliatory war raid to avenge a prior attack.
The known empirical facts provide a complex mosaic that must be explained by any adequate theory
of homicide.
Prior theories of homicide fail at this task. Part of the failure may be attributed to the fact that
homicide is not a singular phenomenon, but rather a diverse set of different phenomena. Beating a
stepchild to death, for example, is so different from declaring war that we should not expect the
explanation for one to successfully account for the other. Nonetheless, theories of homicide have
tended to focus on single-cause explanations, which then collapse when the broader empirical picture
is considered. Theories that invoke culture-specific causes such as media violence, for example,
cannot explain why identical patterns of homicide are documented in cultures lacking these causes.
Theories that invoke sex-neutral causes, as most do, cannot explain the substantial sex differences in
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both the rates and the circumstances in which men and women kill.
Prior efforts to explain killing by invoking evolutionary factors have been hastily dismissed by
many, sometimes for good reasons, but often based on profound misunderstandings. In an otherwise
brilliant book on war before civilization, Keeley (1996) dismisses "a biological explanation for
warfare" because of "the incredible plasticity of human conduct" (p. 158). As evidence, he cites the
fact that some peoples have changed from peaceable to warlike, or vice-versa, within a few
generations. Examples of human mutability are abundant. The mistaken assumption behind Keeley’s
analysis is that if warfare is "biological," then somehow it must be invariant, universally expressed,
difficult to suppress, and impossible to modify. Because human behavior is so obviously variable and
changeable with context, Keeley and others have concluded that killing can’t be "biological," so it
must be "shaped by learning and decision making" (Keeley, 1996, p. 158).
As Wrangham points out, "This error seems remarkable, because behavioral ecologists [and
evolutionary psychologists] have long stressed that that psychological adaptations are expected to
respond in a contingent way to appropriate contexts" (Wrangham, 1999b, p. 21). Nonetheless, this
"remarkable error" appears to be widespread among social scientists. In a commentary on an article
by Steven Pinker suggesting that infanticide could have evolved, a well known sociologist wrote:
"Your article on infanticide illustrates how silly evolutionary explanations of human behavior have
become. When mothers protect their newborns . . . it’s because that behavior is evolutionarily
adaptive. And now, when a few mothers kill their newborns, that’s evolutionarily adaptive too. Any
behavior is ‘explained’ by evolutionary selection: homicide, love, suicide, hate, nurturance,
aggression–you name it. Thus, nothing is explained" (C.S. Fischer, Nov. 2, 1997, New York Times
Magazine). Obviously, nothing is "explained" merely by pointing to a behavior and slapping the
label "adaptive" on it, but that fails "remarkably" to capture the logic of the articulation of precise
testable evolutionary psychological hypotheses such as those proposed in this paper. The notion that
humans have evolved a complex collection of psychological mechanisms that are selectively
engaged according to the exigencies of circumstances, and that these different mechanisms could
give rise to behaviors as diverse as helping and homicide, is logically coherent and internally
consistent.
Homicide Design Theory discards the false dichotomy of "biological" versus "learned." It proposes
that evolution by selection has produced a mosaic of psychological mechanisms that are designed to
be highly sensitive to social circumstances. The proposal that humans have evolved infanticide
mechanisms, for example, does not imply that humans blindly kill babies. They are hypothesized to
do so only under delimited conditions (e.g., when infant is deformed) that differ for men and women
(e.g., paternity uncertain is a context unique to men) and according to circumstance (e.g., available
resources). The proposal that humans have evolved warfare mechanisms does not imply that humans
blindly go into combat or invariantly express an "instinct" for battle. Only some humans do it (e.g.,
men, not women), and only under delimited social conditions (e.g., in self-defense) or ecological
conditions (e.g., nearing starvation). The variability and specificity of contexts in which killing does
and does not occur suggests the need for a theory that can explain that variability and specificity.
Clearly, a "biological" theory that proposed an invariant killer instinct cannot succeed. Neither can
vague hand-waving about "learning" or "decision making."
The paleontological, archeological, and ethnographic evidence is strong that homicide has been a
recurrent feature of human evolutionary history. Given the dramatic fitness costs inflicted on victims
of homicide, it would be surprising if selection had failed to fashion mechanisms designed to prevent
getting killed. Indeed, the logic of the evolution of anti-homicide mechanisms may have been
presaged by Charles Darwin: "as . . . there is a frequently recurring struggle for existence, it follows
that any being, if it vary however slightly in any manner profitable to itself . . . will have a better
chance of surviving, and thus be naturally selected" (Darwin, 1859). Homicide Design Theory
suggests that the survival strategies that have evolved in humans to combat the "hostile force of
nature" include defenses against realistic threats of being murdered by other humans.
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According to our theory, these include mechanisms such as fear of strangers in infancy; specialized
mind-reading abilities to infer homicidal intent in others; adaptively biased inference mechanisms
that result in protective overestimates of the likelihood of getting killed; selective preferences for
habitats that afford protection and refuge (e.g., seeing without being seen); anticipatory defensive
maneuvers including development of arms, fortifications, and protective postures; panic reaction to
motivate fleeing when confronted by another human displaying homicidal intent, and homicidal
mechanisms designed to impel killing to prevent getting killed.
No prior theories predict the existence of these well-designed anti-homicide mechanisms, in part
because no prior theory of homicide has identified such a large range of evolved homicide
mechanisms that have imposed specific and recurrent problems of survival.
Once anti-homicide mechanisms started to evolve, they would immediately have imposed selective
pressures on the further evolution of killer psychology. The evolution of anti-homicide mechanisms
increases the costs of pursuing a killer strategy, holding the homicide rate in check. At the same time,
anti-homicide mechanisms impose selection pressure that favors the further refinement of killer
psychology. These refinements include sensitivity to the specific contexts in which the immediate
and future costs of killing might be low and as well as meta-mind reading abilities such as
concealing homicidal intentions from victims to surprise them or to avoid activating their antihomicide mechanisms. From Biblical account of Trojan Horses to modern accounts of sneak tribal
raids, deception has been used to prevent the activation of intended victim’s anti-homicide
mechanisms.
Because killing has been a recurrent feature of human living for so long, we expect the evolved
psychology of killing and of avoiding getting killed have coevolved, each become highly elaborated,
each containing many specific design features not explicable on any other theory. One of the most
important collections of psychological design features center on homicidal premeditations.
Cognitive space is a precious and limited human resource. Information processing capacities,
including attention, memory, retrieval, judgment, and decision-making, are finite and metabolically
expensive. Cognitive effort allocated toward one adaptive problem creates opportunity costs that
preclude effort allocated to other adaptive problems. Our research and that of others on homicidal
fantasies has found that the vast majority of men and women have experienced a number of thoughts
about killing. While some are fleeting, many are detailed, elaborate, and recurring (e.g., Crabb, 2000;
Kenrick & Sheets, 1993; Duntley & Buss, in prep.). These premeditations are highly patterned,
sharply sex-differentiated, and pervasively occurring in precisely the contexts in which humans have
faced grave adaptive problems.
The existence of highly patterned, sharply sex-differentiated, pervasively occurring, cognitively
taxing, apparently-designed homicidal thoughts cannot be explained by prior theories of killing. The
available evidence for highly context-specific, sex-linked, premeditated forms of actual murder that
recur across cultures and over time cannot be explained by prior theories of homicide. The apparent
existence of well articulated anti-homicide mechanisms, corresponding in specificity to the contexts
in which human life is in danger, cannot be explained by prior theories of homicide. When combined
with the paleontological, archeological, ethnographic, psychological, and behavioral evidence, it is
not unreasonable to suggest that selection over human evolutionary history has produced specific
mechanisms whose function is to produce the death of other humans.
Acknowledgments:
Special thanks go to Victoria Beckner, Randy Diehl, Sam Gosling, Martie Haselton, Bill Swann, and
Andy Thompson for providing insightful suggestions on earlier drafts.
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Notes
[1] This equation is usually called "Hamilton’s rule," which outlines the conditions under which a
design feature can evolve. The benefits (b) in reproduction conferred by design feature , multiplied
by the coefficient of relatedness (r), must exceed the reproductive costs (c) in order for that design
feature to evolve. Back
[2] The use of the term "benefit" in this context refers solely to benefits in fitness from the standpoint
of natural selection, and does not correspond to any personal, intuitive, or societal construals of
benefit. Back
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