Integrating cooperative breeding with reciprocity and culture extends Hamilton's rule to humans.

We study how kinship and reciprocity shape giving and child care support in societies with limited access to markets. Extending Hamilton's rule to incorporate reciprocity, we test it using behavioral data from dictator games and surveys in remote matrilocal and patrilocal villages of northeast India (N = 406 parents of young children). We replicate with data from urban Manila neighborhoods (N = 301) and the Solomon Islands (N = 624). Consistent with predictions, transfers to recipients increase with genetic relatedness to the donor and even more with relatedness to the donor's child-a hallmark of reciprocity anchored in shared genetic investment. Nonanonymous conditions amplify giving, especially to nonkin. Cultural norms shape cooperation, but do not trump genetic relatedness. These results underscore the role of affinal ties and reciprocal exchange in sustaining cooperation, and suggest a mechanism through which cooperative breeding could contribute to the emergence of generalized reciprocity and human sociality.