The joint emergence of group competition and within-group cooperation

Between-group conflict and within-group cooperation can be seen as two sides of the same coin, coevolving in a group-structured population. There is strong support for between-group competition facilitating the evolution of human cooperative tendencies, yet our understanding of how competition arises is less clear. We show that groups of randomly assembled individuals spontaneously engage in costly group competition, and that decisions promoting between-group conflict are associated with high levels of within-group cooperation. Remarkably, when groups were given the possibility to compete against other groups, net earnings for individuals were higher than when groups were not allowed to interact. The joint emergence of conflict and cooperation along even weakly defined group boundaries, and the apparent benefits of this strategy, suggest the existence of behavioral biases influencing human social behavior and organization.