Some scholars have raised the potential functional role of fictive kinship for religion, generally. This paper seeks to develop that idea. It is argued in this paper that fictive kinship language in religion (and some other non-religious contexts) recruits traits connected to Hamilton’s rule as it is expressed inHomo sapienspsychology. The effect is that cooperation is augmented within a population that generally shares the same religious worldview. The general position is that if religions are in the business of cooperation and (...) this partially accounts for their evolution and preservation, then it follows that we should take particular note of any significant feature of religions that might lend itself to the cooperation account of religion’s apparent evolutionary success. Fictive kinship is one such feature. (shrink)