War and Peace
Aggression in an
Evolutionary Context
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
Aggression (2)

Benefits
 Co-opting others’ resources
 Defense against attack
 Intrasexual competition
 Status hierarchies
 Deter rivals
 Deter sexual infidelity in mates
Aggression (3)

Context-specificity
 Spousal violence due to jealousy
 Reputation and escalating retaliation
 Ability to retaliate
 Size, strength, skill
 Proximity of kin
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
Instinct Theory: Lorenz



In 1966, the ethologist Konrad Lorenz
published On Aggression
Argued that aggression is adaptive
Successful aggression leads to securing of
 Food
 Territory
 Mates
Instinct Theories: Problems
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

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
Sex Differences in Aggression (2)


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 (3)
Arrest Rate per 100,000
1500
1000
Males
Females
500
0
0
1014
1519
2024
Age
2529
3039
40+
Campbell (1995)
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
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
Context Effects of Aggression (1)


Male-Male
 Marital and employment status
 Status and reputation
 Sexual jealousy and intrasexual rivalry
Female-Female
 Intrasexual rivalry
Context Effects of Aggression (2)



Male-Female
 Sexual jealousy
Female-Male
 Defense against attack
Other effects
 Variation in testosterone (T)
 Heat effects
 Hypoglycemia (e.g., in Qolla)
Circannual Rhythms of T
5000
4000
3000
Frequency of Violent
Incidents
2000
1000
0
Fall
Spring
2 (1) = 10.007, p = .002
Krupp et al. (2002)
Heat Effects
5000
4000
3000
Frequency of Violent
Incidents
2000
1000
0
Summer
Fall
Winter
Spring
2 (3) = 34.44, p < .001
Krupp et al. (2002)
Warfare (1)
Extremely sexually-dimorphic behaviour
 Benefits
 Increased sexual access
 Increased resources
 Improved reputations
 Costs
 Death or injury
 Lowered reputation

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
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
Evolutionary Predictions (2)



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
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
The Wrap-Up
Benefits to aggressive behaviour
 Instinct Theories
 Sex Differences
 Young male & young female syndromes
 Context effects
 Evolutionary perspectives on warfare

Things to Come

Sexual conflict
 Occurrence and timing of sex
 Jealousy
 Mate retention tactics
 Access to resources
 Rape