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. However, increased interaction with nonrelatives would have left individuals vulnerable to exploitation, imposing selection pressure on kin recognition systems. 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 to the public good increased as a function of the “kin density” of the group and 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 mechanisms fostering contemporary sociality took root in extended family networks. Consequently, humans may have evolved cooperative and competitive strategies to contend with attempts by nonrelatives to compromise the integrity and prosperity of their kin groups.
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