Sarah Mathew: Arizona State University

Explaining why humans cooperate in sizable groups requires detailed knowledge of how people cooperate in politically uncentralized societies. I will present findings from the Turkana, a politically uncentralized population of pastoralists in Kenya, which indicate that: a) the Turkana maintain costly large-scale cooperation in warfare through peer sanctioning of free riders; and b) Turkana norms regulating punishment mitigate the second-order free rider problem and promote group-beneficial punitive behavior. Additionally, with data from 750 individuals drawn from nine clans of four neighboring ethnolinguistic groups, the Turkana, Samburu, Rendille and Borana, I will show that: a) between-group cultural variation is sufficiently high for cultural group selection to operate; and b) the scale at which cultural variation is maintained can explain the scale at which people cooperate. This suite of patterns indicates that peer sanctioning and cultural group selection in combination played a key role in enabling large-scale cooperation in humans.