War and PeaceWar and Peace

Aggression in an Evolutionary Context

Aggression (1)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

Benefits Co-opting others’ resources Defense against attack Intrasexual competition Status hierarchies Deter rivals Deter sexual infidelity in mates

Aggression (2)Aggression (2)

Context-specificity Spousal violence due to jealousy Reputation and escalating retaliation Ability to retaliate

Size, strength, skillProximity of kin

Aggression (3)Aggression (3)

Instinct Theory: FreudInstinct Theory: FreudTwo 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

In 1966, the ethologist Konrad Lorenz published On Aggression

Argued that aggression is adaptiveSuccessful aggression leads to securing

of Food Territory Mates

Instinct Theory: LorenzInstinct Theory: Lorenz

Tautological, untestable hypothesesDue 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 itThe problems with instinct theory,

however, do not refute the influence of evolution

Instinct Theories: ProblemsInstinct Theories: Problems

Sex Differences in Aggression Sex Differences in Aggression (1)(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

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 Sex Differences in Aggression (2)(2)

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Sex Differences in Aggression Sex Differences in Aggression (3)(3)

Campbell (1995)

Young Male SyndromeYoung Male SyndromeYoung men have the greatest degree of

intrasexual competition for matesThus, they employ riskier strategies

Hunting Combat Defense

Through reputation, these behaviours serve to impress females and deter rivals

Young Female Syndrome?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 malesProportion of resource-rich males

Context Effects of Aggression Context Effects of Aggression (1)(1)

Male-Male Marital and employment status Status and reputation Sexual jealousy and intrasexual

rivalryFemale-Female

Intrasexual rivalry

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 Context Effects of Aggression (2)(2)

Circannual Rhythms of TCircannual Rhythms of T

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Frequency of ViolentIncidents

2 (1) = 10.007, p = .002Krupp et al. (2002)

Heat EffectsHeat Effects

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Frequency of ViolentIncidents

2 (3) = 34.44, p < .001Krupp et al. (2002)

Warfare (1)Warfare (1)Extremely sexually-dimorphic

behaviourBenefits

Increased sexual access Increased resources Improved reputations

Costs Death or injury Lowered reputation

Warfare (2)Warfare (2)Unique aspect of warfare is its cooperative

natureConditions 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

Evolutionary Predictions (1)Evolutionary Predictions (1)Males will have adaptations for warfare

Historically, only males seek warSexual access will be primary benefit

For gangs and Yanomamö, this is trueAdaptations for defecting might also

have evolved when likelihood of death was high

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)Evolutionary Predictions (2)

Group ProcessesGroup 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

The Wrap-UpThe Wrap-UpBenefits to aggressive behaviour Instinct TheoriesSex DifferencesYoung male & young female syndromesContext effectsEvolutionary perspectives on warfare

Things to ComeThings to ComeSexual conflict

Occurrence and timing of sex Jealousy Mate retention tactics Access to resources Rape