The Evolution of Intergroup Violence

By: Raymond C. KellyPresented By: Boris & Len

Introduction• Documented reports of Chimpanzees attacking

and/or killing their intergroup neighbors from 1979 to 1986

• Kelly’s Hypothesis looks to evaluate the factors that lead to chimpanzee violence and HOPEFULLY determine if those parallels converged with humans.

• Kelly ALSO looks to evaluate Wrangham’s conclusion that “selection has favored a hunt and kill propensity in chimpanzees and humans, and that coalitional killing has a long history in the evolution of both species.”

Wrangham’s Exaplanation

• Wrangham applies his explanation using collected data on coalitionary killings amongst Chimps, Bonobos, and Wolves.

• The main point of Wrangham’s explanation claims that fitness is highly correlated with food resources…

• THUS increasing ones territory size ALSO increases ones resource for food… THEREBY increasing fitness.

• Therefore attacks of any kind on other communities allow for a selective advantage toward fitness; so long as the cost is LOW for the attacker/s.

Andaman Islanders

• Hunting Conflicts– The cost of hunting is especially HIGH when hunting on border

property or intruding on enemy territory.– Possible Scenarios:

• Lone hunter comes across a group of intruding hunters.• Two opposing parties become aware of each other.

– Possible Outcomes:• Lone Hunter uses knowledge of area to stalk and kill party. (Ambush Technique)• The smaller party concedes defeat to the larger party (w/o regard to territory

ownership)

• The Andaman Islanders are a hunter-gatherer society organizationally comparable to Upper Paleolithic (35,000 –10,000 B.P.) societies.

• Analysis of Andamanese interactions shows that conflict over resources is reported more frequently as compared to similar hunter–gatherer tribes.– 13 documented cases of conflict– Mainly due to hunting parties “running into” each other.– 5 seriously injured, 6 killed in action

• Overall 85% casualty rate

• Ethnographic data of the Adaman Islanders allowed Kelly to conclude:– Costs are high when intruding upon others territory.– THUS Wrangham’s imbalance-of-power hypothesis, where one group can attack another without serious cost, is

NOT met.– There is no real reason to obtain intercommunity dominance because one group cannot encroach on the territory

of another at will without cost.– THUS the costs incurred by intergroup hostility are irrelevant/insignificant.

Andaman Islanders

NORTH vs. SOUTH• Border Avoidance: The assumption that conflict is more likely to occur near the edges of a

territory THUS members of a group congregate towards the middle of the territory.

• Analysis of the Southern region of the Andaman Islands reveals the population density to be only 73% of that found in the Northern region.

• The SOUTH– Hostile intergroup relations still remains – Population density most likely correlated to frequency of conflicts

• The NORTH– Promotion of peace gatherings

• Neighboring groups come together and jointly partake in resources• Gatherings allow for reconciliation amongst tribes• Promote intergroup marriage, trade, copulation, child-rearing

• Effects of Joint Peace Gatherings– The joint efforts of two or more tribes to engage in activities and festivities as such significantly

increased the utilization of resources in North Andaman.– The better use of food resources ALSO yielded a higher population density in North Adaman over

time.

NORTH vs. SOUTH cont’d

• Conclusions:– Data shows when relations are good between neighbors the average

territory size is 16.4 sq miles. (capable of sustaining 45 individuals) • HOWEVER…poor relations between neighbors only yields a

territory size of 12.0 sq miles. (capable of sustaining 33 individuals)

– If fitness is positively correlated with territory size…THEN initiation of peaceful joint activities can be considered adaptive.

– THUS engaging in violent or conflictual behavior is maladaptive.

Results/Findings• Kelly’s analysis of the data shows:

– Factors that influenced coalitionary killing among primates do not influence humans in the same way.

– Fitness is correlated with territory size. (i.e. – the larger a tribes territory, the greater the tribes fitness)

– Higher population densities tend to see more conflict, HOWEVER…the ability of one group to dominate the other is negated by a number of factors that yield a HIGH cost-to-benfit ratio. (i.e. – you could die if you start a fight) • Accurate assessment of oppositional strength (could be an ambush).• Intrinsic territorial advantages (home court advantage, visitor disadvantage)• Weapon Technology (spear vs. knife)

– The potential for intergroup violence exists in both Primates and Humans allowing us to conclude that intergroup violence was/is a significant feature of human adaptability and evolution at the start of the Paleolithic Era.

• Primates differ from Humans:– Findings reveal that initiation of peaceful alternatives set humans apart from chimps and Bonobos.

• The ability of humans to resolve conflict peacefully has lead to better use of resources, increase in average territory size, and decrease in overall frequency of conflict among tribes.

Other Stuff• Development of Weaponry

– The earliest use of projectile weapons takes place over 400,000 years ago in the form of wooden spears.

– The development of weapons increased the risk factors of intergroup violence• Risk of fatal injury greatly increased• Weapons like spears or bows made ambushing prey or enemies easier.

– Increase in risk promoted peaceful alternative• Aggression and violence downplayed• More attention conflict avoidance, sharing, mutual gain, etc.

– Makes intergroup dominance IMPOSSIBLE

• 3 Major Periods of Intergroup Violence– Era of Coalitionary Killing

• Strength in numbers to increase fitness potential– Era of Intrinsic Defensive Advantage

• Knowledge of environment • Stealth/ ambush

– Era of War• Advancements in weapons allows for easier kills, stealthier tactics, range of weapon, and

applications