Humans: Within-Group Conflicts
Aggressive conflict between individuals within a human group or culture, involving violent interactions, in certain circumstances leading to death of one party and fitness benefits to aggressors.
Conflicts of interest and competition are a common feature of social life within groups. Many interindividual altercations lead to escalation of aggression and ultimately to killing. To have an evolutionary understanding of intragroup aggression and killing and assess evidences of adaptive functions, it is important to empirically examine (1) how ubiquitous these phenomena are among human cultures; (2) what social and biological variables explain rates of intragroup aggression and homicide, and if they do so in a predictable way in line with theory; and (3) what consequences these behaviors have.
Registered homicide rates vary greatly among cultures and across time, although they it is a widespread phenomenon in humans (Knauft 1987). While in countries such as Iceland and Japan they are below 0.5 per 100,000 persons per year in recent years, they are more than 100 times more prevalent in the Virgin Islands and Honduras and up to 1000 more prevalent in certain prestate societies. Homicide rates are estimated to have reduced by approximately 20 times in Europe since medieval times (Spierenburg 2008). Some homicides are consequences of individual-level conflicts and altercations and another considerable part are due to gang violence and organized crime. It is important to consider the individual characteristics that may lead to adaptive benefits in lethal aggression, examining the sociodemographics of killing across different sectors of the human population.
Aggression and murder are observed to be a more prevalent behavioral feature of young, unmarried individuals and in those with few resources (such as low socioeconomic status, in modern industrialized societies), especially populations having high levels of inequality, as they have comparatively little to lose by engaging in high-risk behavior such as physical altercations over conflicts of interest (Daly and Wilson 1988). For individuals in such life contexts, expected returns from safe courses of action may be negligible, and relying on delayed benefits may be suboptimal considering socioecological uncertainty; as such, future discounting coupled with aggressive, coercive tactics and elimination of competition may be an adaptive response.
A common reported motive for aggressive and homicidal desires and actions is jealousy and distress over sexual competition (Buss 2000). Even though various behavioral strategies for intrasexual competition exist (involving, e.g., self-promotion, derogation of competitors, mate manipulation, and mate guarding) and various anatomical and physiological features can be argued to be adaptations for intrasexual competition (e.g., penile anatomical characteristics which may facilitate sperm displacement in the female reproductive tract, thus reducing other males’ chances of fertilization), aggressive behavior towards sexual competitors is hypothesized to be an effective strategy for competition. Young men are more likely than individuals in other demographic categories to act on such aggressive desires, suggesting that sexual selection has maximized male competitive prowess during young adulthood. Successfully confronting and attacking other males may not only fend off sexual competition and facilitate mate guarding, but may also permit mate poaching, and increasing status by demonstrating dominance, willingness to take risks, and physical formidability, thus increasing attractiveness.
As such, generally speaking, men are more prone to violence than women. There are various possible nonmutually exclusive explanations for this (Tanha et al. 2010): (1) men are sexually selected to be about one-fifth larger in total body size than women, conferring a greater capacity to deal damage, making violence a more effective means of obtaining goals; (2) men are sexually selected to be anatomically more able to sustain damage in combat (having more muscle and skeletal mass), lowering the costs of violent encounters; and (3) men are sexually selected to possess more cognitive and affective psychological adaptations for violent behavior, making them both more proficient in violent tactics and more predisposed to deploy them.
In a study of six human cultures, 91% of men and 84% of women admitted to having had at least one fantasy of murder (Buss 2000), figures which are more similar than is usually speculated. Although many studies have shown that the frequencies of violent acts are also roughly comparable for male aggressing against female intimate partners and female aggressing against male intimate partners, the rates at which these assaults translate into injuries are highly discrepant. This could be because men are more capable of dealing and sustaining damage in physical interpersonal altercations. Thus, such violent interactions more often lead to the death of the female than the male intimate partner. Many special theories have been proposed to explain sex differences in death following intimate partner violence as a result of male sexual jealousy or of other strategic conflicts of interests between the sexes. However, recent evidence has indicated that the major predictors of between-sex and within-sex interpersonal aggression are virtually identical, rendering these special evolutionary theories somewhat unparsimonious (Figueredo et al. 2018).
It has also been found that in societies where critically important subsistence resources are difficult to defend (such as privately owned livestock) and populations densities are low (which is also typical of herding ecologies), leading to low efficacy of law enforcement (where state authority even exists in principle), a “Culture of Honor” evolves where individuals are selected to take justice into their own hands (Nisbett and Cohen 1996). This system depends upon the maintenance of individual reputations for retributive justice, as this is the only proactive way that vindictive behavior can deter raiding of resources by potential enemies. The maintenance of personal or family “Honor” therefore becomes the psychological mechanism for implementing the implicit threat of violent retribution, by the intentional cultivation of an informal public record for revenge.
The question of how comparable interindividual aggression and killing are within human populations is also an important one from an evolutionary perspective. Homicide is in no way exclusive to only specific kinds of human cultures or to specific subsistence types (Knauft 1987). Homicide rates in bands and tribal societies exceed those of cities and states. Societies such as the Gebusi in New Guinea, and the Yanomamo in Venezuela, exhibited estimated homicide rates of 683 and 165.9 per 100,000 per year, respectively. These estimates are considerably higher when compared to 2.2 homicides per 100,000 per year in Japan, during 1951–1956 and to 15.1 in the city of Kent, during the thirteenth century. Similarly, societies traditionally considered to exhibit low levels of intragroup conflict such as the! Kung had annual homicide rates of 41.9 between 1920 and 1955. However, it has been contented that these cases may not adequately distinguish between intra- and intergroup killings despite the fact that they have been classified as homicide.
Circumstances and reasons for infanticide are strikingly comparable among human cultures (Daly and Wilson 1988). Although not observed in high rates, filicide is more commonly committed by parents in life contexts that are not favorable to child rearing and in cases of children illness or deformations, both of which are situations prone to jeopardize parental fitness. Unrelated men most commonly commit infanticide if adulterous conception is suspected. Thus, although extremely distressing, killing of children appears to largely conform to theoretical expectations that would facilitate gene-copying replication, showing signs of adaptation.
The challenges of investigating the archaeological record for evidence of interpersonal violence are greatly reduced compared to those of studying intergroup conflict. Interpersonal violence can be inferred through the evidence of trauma, and injury on skeletal remains such as embedded lithic points, cut marks, and bone fractures (Walker 2001). Additionally, it can be established by a single case exhibiting this evidence. However, a considerable challenge in establishing interpersonal violence is encountered in attempting to correctly distinguish between accidental injury and intentional harm. Another nuance of importance is the recognition that many forms of behavior can be subsumed under the label of interpersonal violence, such as ritualized fighting as a form of conflict resolution, domestic violence, and social execution.
The sizeable quantity and relative completeness of known Neanderthal remains has facilitated analysis into the nature of their numerous injuries. In comparison to a sample of hospital records on fracture injuries, Neanderthals were found to exhibit considerably high incidences of trauma indicative of interindividual aggression (Walker 2001), although part are due to accidents and hunting-related injuries. Similarly, a smaller sample of early human skeletal remains from Africa and the Middle East seems to exhibit a consistent pattern of elevated rates of head trauma and injury (McCall and Shields 2008), like those observed in Neanderthals and modern human assault victims. Further archaeological evidence supports the occurrence of lethal violence during prehistory. For example, in sites such as Grimaldi (27kya-24kya) and San Teodoro (14kya) in Italy, embedded projectiles were found within the remains of juvenile victims.
Finally, ethnographic descriptions have also considered how societies respond after the occurrence of homicide, with responses ranging from indifference to a collective disapproval accompanied by punishment (Boehm 1999). As such, even though it seems that the existence of within-group killing is ubiquitous, societal attitudes toward it may vary somewhat, although they are in no culture positive, as they are in the case of between-group conflicts and warfare. Based on the type of social sanction, killers sometimes prefer to desert the group rather than facing any form of social sanction. Blood money, for example, is the financial restitution to the family of the victim and has been a common practice across prestate societies, such as in Norse cultures during the eighth to tenth centuries. It is worth noting, however, that under some circumstances, societies such as the Hadza, the Alaskan Eskimos, and the Kung resort to executions when other regulatory mechanisms are deemed ineffective. However, due to the risk of retaliation by either kin or allies of the offender, the decision of eliminating the offender is taken collectively, and under some conditions the execution is conducted by the kin of the attacker.
Although cross-cultural variations among modern human societies are observed in terms of homicide rates, sociodemographic predictors and attitudes towards within-group aggression and killing are less variable and more in line with expectations based on evolutionary theory. It is clear that such forms of social interactions are not a product of civilization and are not limited to only a part of human societies, and the evidence strongly supports the notion that within-group killing can be, although it need not always be, an adaptive strategy for competition for reproductive maximization, rather than a dysfunctional or exaggerated outcome of aggression.
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