Chimpanzees’ responses to inequity in a group context are influenced by food quality and their relationships with the group members present

Abstract Chimpanzees, like humans, react negatively to receiving lower-value rewards than their peers for the same task, though results vary across individuals and studies. This variation may, in part, stem from research focusing on dyads, which, while providing better experimental control, reduce the complexity of natural social environments. Here, we studied chimpanzees’ (n = 27) reactions to inequity using a token-exchange task that allowed unrestricted participation by all group members. As in previous studies, subjects reacted negatively to disadvantageous inequity when receiving low-value food. However, medium- and high-value rewards were rarely rejected, possibly explaining some of the variation across earlier studies. Notably, responses to inequity were stronger when closely bonded individuals were present, which may relate to inequity’s proposed function to maintain equitable relationships. Our findings highlight the importance of naturalistic conditions for understanding primate social decision-making, which is essential to understand the evolution of responses to unfairness.