Evolutionary Explanations

Evolutionary Explanations of Human Aggression

Evolutionary psychology proposes that human aggression — rather than being pathological — represents adaptations shaped by natural and sexual selection in the EEA. Specific forms of aggression are proposed to have solved specific adaptive problems: acquiring or defending resources, competing for mates, protecting offspring, and deterring competitors. Understanding aggression as evolved behaviour generates predictions about when and why it should occur.

Sexual Selection and Male Aggression

Males are substantially more aggressive than females in virtually all human societies studied — a cross-cultural consistency that invites evolutionary explanation. Sexual selection theory (Darwin, 1871) proposes two mechanisms: intrasexual selection (competition between members of the same sex) and intersexual selection (mate choice by the opposite sex). Male-male competition for mates produces selection pressure for aggressive traits: males who could out-compete rivals had greater reproductive success, selecting for larger body size, risk-taking, and aggressive disposition in males.

Evidence: the male homicide rate is dramatically higher than the female rate in all societies studied (Daly and Wilson, 1988). Male-male homicide peaks in young adulthood (18–30) — precisely the period of maximal mate competition. This pattern holds across cultures, times, and social conditions — suggesting an evolved male competitive psychology rather than a cultural construct.

Mate Retention and Jealousy-Induced Aggression

Daly and Wilson (1988) proposed that sexual jealousy is an evolved mechanism in males to deter female infidelity. For males, uncertain paternity means that unwittingly investing in a competitor's offspring would be a catastrophic fitness cost. Sexual jealousy — the aversive emotional response to actual or suspected partner infidelity — motivates mate-guarding behaviours. When mate-guarding through monitoring and restricting partner freedom fails, some males resort to violence. Daly and Wilson's analysis of homicide data showed that a substantial proportion of female homicide victims are killed by intimate partners, predominantly associated with jealousy over suspected infidelity or attempts to leave the relationship. Men killing female partners is the 'extreme manifestation' of mate-guarding evolved psychology.

Violence and Status: The Young Male Syndrome

Wilson and Daly (1985) proposed the young male syndrome: young, unmarried, low-status males are the group most likely to engage in dangerous risk-taking and aggressive competition, precisely because they have little accumulated reproductive investment and everything to gain from successfully raising their status through competitive success. Violence is a high-risk, high-reward strategy for young males who have no other route to status and resources. This explains why homicide rates peak in young adult males and are elevated in contexts of inequality (where status differences are large and legitimate routes to status are limited).

Kin Selection and Group Aggression

Hamilton's (1964) kin selection theory proposes that organisms act to maximise their inclusive fitness — the transmission of genes they share with relatives. Aggression towards out-group members who threaten shared genetic relatives (intergroup aggression in defence of kin) would be favoured by selection. Chagnon (1988) studied the Yanomamö of South America and found that men who had killed had significantly more wives and children than those who had not — direct evidence that violent success can increase reproductive fitness in certain cultural contexts.

Evaluation

Evolutionary explanations generate specific, falsifiable predictions that have been largely confirmed across diverse cultures (Daly and Wilson, 1988). However, they face criticism: they may commit the naturalistic fallacy (deriving what should be from what is), provide post-hoc explanations that cannot be directly tested by examining ancestral conditions, and ignore the substantial within-species variation and cultural malleability of aggression. Sociocultural accounts (Bandura) provide equally plausible alternative explanations for the same gender differences — male socialisation into aggression may account for the observed cross-cultural patterns without invoking evolution.

 Key Takeaways

  • Sexual selection: male-male intrasexual competition for mates selects for aggressive traits in males. Male homicide rate dramatically exceeds female rate in all human societies (Daly and Wilson, 1988).
  • Young male syndrome (Wilson and Daly, 1985): young, low-status, unmarried males most aggressive — little to lose; high-risk violence is a rational strategy for status acquisition when legitimate routes are unavailable.
  • Mate retention and sexual jealousy: evolved mechanism to deter infidelity and rival males. Intimate partner violence linked to jealousy over suspected infidelity or partner's attempt to leave (Daly and Wilson, 1988).
  • Kin selection (Hamilton, 1964): aggression protects shared genes in relatives. Intergroup aggression defends kin. Chagnon (1988) Yanomamö: killers had more wives and children — direct fitness benefit from violence.
  • Peak homicide age 18–30 in males across cultures — consistent with mate competition theory; inconsistent with purely cultural explanation.
  • Limitation: naturalistic fallacy risk — evolution explains why behaviour occurs, not why it is inevitable or acceptable. Substantial within-culture variation suggests social/cultural factors are also important. Post-hoc storytelling is difficult to test.