Homicide Adaptation Theory
Evolved psychological mechanisms to facilitate killing other humans.
Violence has been a recurrent and persistent occurrence throughout the expanse of human history (Daly and Wilson 1988; Pinker 2011). An evolutionary psychological perspective proposes that violence is inherently and inextricably entrenched in human psychology. Accordingly, this perspective suggests that over deep evolutionary history, humans have evolved adaptations for aggressive behavior as means of solving a variety of adaptive problems (Buss and Shackelford 1997). Thus, the capacity for violence disposed by modern humans may be the result of selection favoring aggressive tendencies. In particular, aggressive behavior may have emerged as a pattern of solutions to the many problems presented by group living.
Buss and Shackelford (1997) proposed seven domains for which aggression may serve effective means: (1) co-opting resources from others, such as fertile land, water access, or tools; (2) defending against an attack from conspecifics, which can result from others employing the previous strategy and poses a serious threat to the survival of the victim; (3) inflicting costs on intrasexual rivals, typically within the context of intrasexual competition for mates; (4) negotiating status and power hierarchies in relevant contexts; (5) deterring rivals from future aggression, which can include creating and maintaining a reputation for aggression; (6) deterring mates from sexual infidelity as a means to ensure paternity or paternal investment; and (7) reducing resources expended on unrelated children by means of infanticide. Two of these domains in particular – inflicting costs on intrasexual rivals and reducing resources expended on unrelated children – often include the killing of another human.
Homicide as an Adaptation
In every culture studied, people have been documented killing other people. Although many species kill members of their own species via competition, the sophistication of human killing is observed in only one other species – chimpanzees (Pan troglodytes; e.g., Mitani et al. 2010), the phylogenetically closest living relative to humans. Duntley and Buss (2005, 2011) proposed homicide adaptation theory to explain why people kill other people. Homicide adaptation theory states that humans have evolved adaptations to facilitate killing in specific, and evolutionarily-recurrent, contexts for which the benefits of killing outweigh the costs. The majority of killings perpetrated by humans are believed to be the product of evolved psychological mechanisms designed by natural selection to ensure the death of conspecifics.
Homicide has not been perpetrated indiscriminately over human evolutionary history. Rather, killing of conspecifics occurs in highly specific contents that have posed recurrent adaptive problems over human evolutionary history. Duntley and Buss (2005, 2011) propose specific contexts for which homicide may have been ancestrally advantageous, including: (1) preventing the exploitation of self, kin, mates, or coalitional allies from conspecifics; (2) reputation management to avoid appearing exploitable; (3) protecting resources; (4) eliminating individuals who utilize resources, but who provide no fitness benefits to the aggressor (e.g., step-children); (5) eliminating genetically related individuals who pose a significant fitness cost to the perpetrator; and (6) gaining access to mates. Homicide adaptations could therefore have evolved if, on average, homicide increased the fitness benefits of the perpetrators, relative to the fitness costs.
Homicide adaptation theory does not propose that homicide is the preferred strategy as a solution to any of the above adaptive problems. In most circumstances, the costs of homicide are far too high to make it a viable strategy. Instead, according to homicide adaptation theory, homicidal behavior may have been the best solution for some adaptive problems in some circumstances. Converging lines of evidence provide support homicide adaptation theory as an explanation for the ubiquity of homicide in human history and cultures (Duntley and Buss 2005, 2011).
Homicide adaptation theory proposes that selection has shaped distinct homicide adaptations for men and women (Duntley and Buss 2005, 2011). More generally, men are significantly more likely to engage in violent and potentially lethal behavior than are women. Pioneering work by Daly and Wilson (1988) demonstrated that the proportion of male-male homicides substantially exceeds the proportion of female-female homicides in every culture studied. National statistics reported by Daly and Wilson (1988) are corroborated by a network of evidence demonstrating physical, psychological, and behavioral adaptations for aggression specifically in men (Goldstein 2001; McDonald et al. 2012; Sell et al. 2012). Male-male homicide is committed for a variety of reasons including the escalation of petty disputes, reputation management, and – most often – because of intrasexual competition (Daly and Wilson 1988).
Spousal homicides are overwhelmingly committed by men and are most often fueled by sexual jealousy, with young, reproductive age women disproportionately represented among the victims (Shackelford et al. 2000). Men may benefit from wife-killing if the wife has committed sexual infidelity which disposes risk of investing resources into genetically unrelated offspring. Men with adulterous wives may also incur substantive reputational costs which may be alleviated by uxoricide.
Men and women are both hypothesized to have evolved adaptations for the killing of infants, though for different reasons (Duntley and Buss 2005, 2011). Because internal fertilization and gestation occur in women, ancestral men could not be certain of the genetic relatedness of their offspring. Men with low paternity confidence are at increased risk for killing their offspring. Men that enter a relationship with women who have offspring from a previous mateship(s) may kill the women’s offspring to reduce resource expenditure on genetically unrelated offspring and increase the likelihood of subsequent breeding – a pattern found in nonhuman species, such as lions (Panthera leo; e.g., Packer and Pusey 1983). Women, in contrast are certain of maternity but may kill their offspring they perceive a net cost of continued investment (e.g., severely ill or deformed offspring) or if there are not sufficient resources to adequately care for the offspring (e.g., food scarcity). Infanticide may be considered a particularly powerful force driving its own set of adaptations in males and females (See Infanticide, Infanticide in Non-Humans, Infanticide as a Selective Force for Group Living, Infanticide Risk).
Other lines of evidence converge to support the key proposition that adaptations for killing conspecifics has evolved (reviewed in Duntley and Buss 2011), such as it being more likely that an individual’s killer is someone known to the victim, cross-species documentation of killing conspecifics in similar and predictable contexts to those of humans, evidence on ancient human remains that are consistent with homicide by another human with a weapon, archeological evidence of weapons designed for homicide, extensive cross-cultural evidence of homicide, and consistency of contexts for homicide across diverse cultures. Taken together, existing evidence suggests the longevity of killing conspecifics in human and nonhuman evolutionary history.
Humans have a long evolutionary history of aggression (Pinker 2011). Duntley and Buss’s (2011) homicide adaptation theory explains how homicide has evolved. The theory proposes that humans have evolved psychological mechanisms for killing. Homicide may have emerged as an advantageous strategy in specific contexts when the fitness benefits of committing homicide outweighed the fitness costs. These benefits suggest explanations for the cross-cultural and cross-generational ubiquity of homicide in humans.
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