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According to the cooperative breeding hypothesis, human cooperation finds its root in the social arrangements and reciprocal networks solicited for infant care. We implement the first economic experiment to elucidate the relationship between allomaternal care – in which caregivers other than the mother participate in childrearing – and prosociality among a random sample of participants in remote areas of the Solomon Islands. We document three main findings. First, in a series of dictator games, donations to kin follow Hamilton’s rule, but from the perspective of the participant’s child. Second, allomaternal care elicits altruism and reciprocity among kin and friends, as well as prosociality toward strangers. This relationship generalises to other prosocial preferences and holds in a global sample, as shown based on a combination of ethnographic and folklore data. Third, allomaternal care is associated with the socioemotional well-being of children, as well as the prosociality of participants who have no children, suggesting an evolutionary role for allomaternal care in cultivating cooperation.