Within-group competition reduces cooperation and payoffs in human groups

Social organisms in many taxa cooperate to produce resources that are shared among group members. Some cooperatively produced resources may be monopolized by individuals who invest in within-group competition, but these have largely been overlooked in empirical and theoretical research on human cooperation, which has focused on noncontestable public goods. In this study, we allow for the potential of within-group competition over cooperatively produced resources and use a game theoretic ‘‘tug-of-war’’ model and empirical test to show that such competition decreases the degree of cooperation within human groups and hence decreases group members’ payoffs. Our study thus sheds light on how cooperative production and equal division of shared resources may have evolved, expands on current models of human cooperation to reflect the many natural conditions with opportunities for within-group competition, and demonstrates unifying principles in cooperation and competition across the animal kingdom.