Chimpanzee and hunter-gatherer intergroup aggression differ in important ways, including humans having the ability to form peaceful relationships and alliances among groups. This paper nevertheless evaluates the hypothesis that intergroup aggression evolved according to the same functional principles in the two species—selection favoring a tendency to kill members of neighboring groups when killing could be carried out safely. According to this idea chimpanzees and humans are equally risk-averse when fighting. When self-sacrificial war practices are found in humans, therefore, they result (...) from cultural systems of reward, punishment, and coercion rather than evolved adaptations to greater risk-taking. To test this “chimpanzee model,” we review intergroup fighting in chimpanzees and nomadic hunter-gatherers living with other nomadic hunter-gatherers as neighbors. Whether humans have evolved specific psychological adaptations for war is unknown, but current evidence suggests that the chimpanzee model is an appropriate starting point for analyzing the biological and cultural evolution of warfare. (shrink)

Building on and partially refining previous theoretical work, this paper presents an extended simulation model of ancestral warfare. This model (1) disentangles attack and defense, (2) tries to differentiate more strictly between selfish and altruistic efforts during war, (3) incorporates risk aversion and deterrence, and (4) pays special attention to the role of brutality. Modeling refinements and simulation results yield a differentiated picture of possible evolutionary dynamics. The main observations are: (i) Altruism in this model is more likely to evolve (...) for defenses than for attacks. (ii) Risk aversion, deterrence and the interplay of migration levels and brutality can change evolutionary dynamics substantially. (iii) Unexpectedly, one occasional simulation outcome is a dynamically stable state of ‘tolerated intergroup theft’, raising the question if corresponding patterns also exist in real intergroup conflicts. Finally, possible implications for theories of the co-evolution of bellicosity and altruism in humans are discussed. (shrink)

In the absence of explicit punitive sanctions, why do individuals voluntarily participate in intergroup warfare when doing so incurs a mortality risk? Here we consider the motivation of individuals for participating in warfare. We hypothesize that in addition to other considerations, individuals are incentivized by the possibility of rewards. We test a prediction of this “cultural rewards war-risk hypothesis” with ethnographic literature on warfare in small-scale societies. We find that a greater number of benefits from warfare is associated with a (...) higher rate of death from conflict. This provides preliminary support for the relationship between rewards and participation in warfare. (shrink)

The classic opening scene of 2001, A Space Odyssey shows an ape-man wreaking havoc with humanity's first invention--a bone used as a weapon to kill a rival. It's an image that fits well with popular notions of our species as inherently violent, with the idea that humans are--and always have been--warlike by nature. But as Douglas P. Fry convincingly argues in Beyond War , the facts show that our ancient ancestors were not innately warlike--and neither are we.