1. War and Peace Aggression in an Evolutionary Context

2. Aggression (1) May be defined as “behavior that is intended to injure another person who does not want to be injured” (Brehm et al., 1999) May be  Instrumental Harm inflicted as a means to an end  Emotional Harm is inflicted for its own sake

3. Benefits  Co-opting others’ resources  Defense against attack  Intrasexual competition  Status hierarchies  Deter rivals  Deter sexual infidelity in mates Aggression (2)

4. Context-specificity  Spousal violence due to jealousy  Reputation and escalating retaliation  Ability to retaliate Size, strength, skill Proximity of kin Aggression (3)

5. Instinct Theory: Freud Two specific instincts: Life (Eros) and Death (Thanatos)  Death instinct was a post WWI concept, representing an unconscious desire to escape life through death  Aggression is caused by a conflict between Life and Death instincts, targeted towards others

6. In 1966, the ethologist Konrad Lorenz published On Aggression Argued that aggression is adaptive Successful aggression leads to securing of  Food  Territory  Mates Instinct Theory: Lorenz

7. Tautological, untestable hypotheses Due to their inflexibility, they do not effectively account for environmental influences that lead to cultural variation Commit the nominal fallacy  Assume an effect has been explained simply by naming it The problems with instinct theory, however, do not refute the influence of evolution Instinct Theories: Problems

8. Sex Differences in Aggression (1) Once again, we return to minimum investment  Men have greater reproductive variance, which is constrained by access to mates  Thus, men are in direct competition with each other for mates The greater the variance (e.g., effective polygyny), the greater the sexual dimorphism

9. Overwhelmingly greater number of  Murders perpetrated by males  Male homicide victims Females do, however, also engage in aggressive acts  Verbal aggression is common  Simple assault  Serious harm is very rare, though Sex Differences in Aggression (2)

10. Sex Differences in Aggression (3) Campbell (1995)

11. Young Male Syndrome Young men have the greatest degree of intrasexual competition for mates Thus, they employ riskier strategies  Hunting  Combat  Defense Through reputation, these behaviours serve to impress females and deter rivals

12. Young Female Syndrome? Campbell argues that the overall relationship between age and violence holds for females as well  Teen girls and competition for mates Biased sex ratio with fewer males Proportion of resource-rich males

13. Context Effects of Aggression (1) Male-Male  Marital and employment status  Status and reputation  Sexual jealousy and intrasexual rivalry Female-Female  Intrasexual rivalry

14. Male-Female  Sexual jealousy Female-Male  Defense against attack Other effects  Variation in testosterone (T)  Heat effects  Hypoglycemia (e.g., in Qolla) Context Effects of Aggression (2)

15. Circannual Rhythms of T  2 (1) = 10.007, p =.002 Krupp et al. (2002)

16. Heat Effects  2 (3) = 34.44, p <.001 Krupp et al. (2002)

17. Warfare (1) Extremely sexually-dimorphic behaviour Benefits  Increased sexual access  Increased resources  Improved reputations Costs  Death or injury  Lowered reputation

18. Warfare (2) Unique aspect of warfare is its cooperative nature Conditions for its evolution  Average long-term gain in reproductive resources must outweigh costs  Members must believe that success is likely  Risk and contribution of each member must be related to benefits  Veil of ignorance over likelihood of survival

19. Evolutionary Predictions (1) Males will have adaptations for warfare  Historically, only males seek war Sexual access will be primary benefit  For gangs and Yanomamö, this is true Adaptations for defecting might also have evolved when likelihood of death was high

20. Warfare should be more likely when chances of winning are high (e.g., number of soldiers)  WWII and coalitional size Adaptations to enforce “risk contract”  Reputation Males will have adaptations to prefer more able, willing men as coalitional members Evolutionary Predictions (2)

21. Group Processes It is extraordinarily easy to stimulate intergroup competition (Us vs. Them) Robber’s Cave experiments  11 year old boys formed two groups  Escalated competition rapidly over one week  Were only able to deescalate once experimenters gave them a task that they could only perform together

22. The Wrap-Up Benefits to aggressive behaviour Instinct Theories Sex Differences Young male & young female syndromes Context effects Evolutionary perspectives on warfare

23. Things to Come Sexual conflict  Occurrence and timing of sex  Jealousy  Mate retention tactics  Access to resources  Rape