Title: A causal understanding of warfare, based on the origins of human cooperation: Case study of cattle-raiding among Turkana pastoralists in Kenya
Name: Sarah Mathew
Year: 2010
Type: Dissertation Fellowship

Unlike other mammals, humans evolved a unique capacity to cooperate in large groups, even with unrelated and unfamiliar individuals. In contrast to other mammalian societies, where individual interest overrides an organisms behavior, the ability to cooperate has allowed human societies to mobilize its members for solving various collective action problems. Warfare is a key domain in which a collective action problem is solved: individuals bear substantial personal risks when they take up arms and join combat, while the intended gains may potentially be experienced by their social group at large. Using evolutionary models of the mechanisms underpinning human cooperation, my project aimed to understand how, and at what scale, politically uncentralized societies solve the collective action problem in warfare.

I studied warfare among the Turkanaa politically uncentralized, nomadic pastoral society in East Africa with a social organization comparable to many historically known hunter-gatherers. I obtained detailed accounts of a representative sample of 88 recent Turkana raids through in-depth interviews with 118 warriors who participated in these raids. The results of this study reveal that the Turkana sustain high levels of cooperation during combatwarriors experience a 1% chance of being killed per raid, and they produce significant collective benefits. Cowardice, desertion, and other forms of free riding are common and are punished through voluntarily imposed informal third-party sanctions, including collective corporal punishment and fines. Cooperation occurs at a remarkably large scale. Raiding parties comprise hundreds of unrelated warriors. Warriors are mobilized from a wide range of Turkana society, so many members of the raiding party are strangers. A vignette study showed that Turkana norms regarding warfare benefit the ethnolinguistic groupa population of half million people, and not smaller social groups within Turkana society. Models of cooperation based on kinship, reciprocity and repeated interactions among well-known people are insufficient to explain the very large scale cooperation occurs among the Turkana.

This study provides a uniquely detailed, quantitative picture of high-stakes cooperation and sanctioning in a society without formal political structure or coercive institutions, and so makes a singular empirical contribution to understanding pre-historic warfare, cooperation and sanctioning. These data imply that cultural evolutionary processes most likely played a role in shaping the evolution of human cooperation in warfarecultural group selection models posit that competition between culturally distinct groups has selected for cooperation within culturally similar groups such as ethnolinguistic units. The data also imply that cooperation warfare among large groups of strangers, sustained by informal third party sanctions, may have occurred for a long period of human evolutionary historywell before the advent of state societies.


Mathew, S. and R. Boyd. 2011. Punishment sustains large-scale cooperation in prestate warfare. Proceedings of the National Academy of Sciences. 108 (28): 11375  11380

Mathew, S. and R. Boyd. 2014. The cost of cowardice: third-party punitive sentiments towards free riders in Turkana raids. Evolution and Human Behavior 35: 58-64