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Relatedness is a cornerstone of the evolution of social behavior. In the human lineage, the existence of cooperative kin networks was likely a critical stepping stone in the evolution of modern social complexity. Here we report the results of the first experimental manipulation of a putative cue of human kinship (facial self-resemblance) among ostensible players in a variant of the “tragedy of the commons,” the one-shot public goods game, in which group-level cooperation—via contributions made to the public good and the punishment of free riders—is supported at a personal cost. In accordance with theoretical predictions, contributions increased as a function of the “kin density” of the group. Moreover, the distribution of punishment was not contingent on kin density level. Our findings indicate that the presence of a subtle cue of genealogical relatedness facilitates group cooperation, supporting the hypothesis that the …