Understanding cooperation and punishment in small-scale societies is crucial for explaining the origins of human cooperation. We studied warfare among the Turkana, a politically uncentralized, egalitarian, nomadic pastoral society in East Africa. Based on a representative sample of 88 recent raids, we show that the Turkana sustain costly cooperation in combat at a remarkably large scale, at least in part, through punishment of free-riders. Raiding parties comprised several hundred warriors and participants are not kin or day-to-day interactants. Warriors incur substantial (...) risk of death and produce collective benefits. Cowardice and desertions occur. (shrink)

Here, we revise Pietraszewski's model of groups by assigning participant pairs with two triplets, denoting: the type of game that models the interaction, its critical switching point between alternatives, and the perception of strategic similarity with the opponent. These triplets provide a set of primitives that accounts for individuals' strategic motivations and observed behaviors.