Human social preferences cluster and spread in the field

Significance How does society maintain the high levels of prosociality in humans, which are so puzzling to explain? Previous laboratory evidence suggests that human social networks play a critical role; however, if and how this role is played out in field settings is not well understood. We demonstrate that intrinsic social preferences, such as fairness and altruism, cluster and spread in human real-world friendship networks. More importantly, however, our findings show that individuals do not choose friends based on cooperativeness—friendship networks influence whether individuals become cooperative or selfish. We conclude that social learning contributes substantially to the ontogeny of prosociality, signifying the role of culture for the development and maintenance of large-scale human cooperation. While it is undeniable that the ability of humans to cooperate in large-scale societies is unique in animal life, it remains open how such a degree of prosociality is possible despite the risks of exploitation. Recent evidence suggests that social networks play a crucial role in the development of prosociality and large-scale cooperation by allowing cooperators to cluster; however, it is not well understood if and how this also applies to real-world social networks in the field. We study intrinsic social preferences alongside emerging friendship patterns in 57 freshly formed school classes (n = 1,217), using incentivized measures. We demonstrate the existence of cooperative clusters in society, examine their emergence, and expand the evidence from controlled experiments to real-world social networks. Our results suggest that being embedded in cooperative environments substantially enhances the social preferences of individuals, thus contributing to the formation of cooperative clusters. Partner choice, in contrast, only marginally contributes to their emergence. We conclude that cooperative preferences are contagious; social and cultural learning plays an important role in the development and evolution of cooperation.