Abstract

Humans often display extensive forms of prosocial behavior, frequently beyond what is predicted by inclusive fitness. This observation has led to various models attempting to explain the unique extent of human altruism. While many of these models have been empirically tested, one model, the ancestor–descendant conflict model, which is based on a maximum descendant-leaving strategy, has yet to be explicitly examined with observational human data. The ancestor–descendant conflict model posits that ancestors influence the behavior of their descendants, through traditions passed down from generation to generation, and predicts that humans in traditional societies will favor, both altruistically and cooperatively, their closer kin even at greater genealogical distances than predicted by inclusive fitness. In this paper, we specifically examine the strength of ancestor influence on prosociality using observational (n = 863) and interview (n = 28) data collected with the Iban of West Kalimantan, Indonesia (Borneo). We evaluate kinship cooperation norms, kinship’s role in cooperative interactions, and the strength of ancestor influence on those interactions with dyadic data (unique dyads = 668). We find strong norms and behaviors for individuals cooperating more often with closer kin, while also finding limited support for the influence of ancestors on actual cooperative behavior.