The influence of kin relationship and reciprocal context on chimpanzeesʹ other-regarding preferences

We investigated the evolutionary origin of other-regarding preferences, one of the strong underlying motivations for altruism, in the chimpanzee, Pan troglodytes. Although altruism is expected theoretically to be kin biased and frequent in a reciprocal context, few experimental studies to date have specifically tested these hypotheses from the viewpoint of proximate mechanisms. We examined the other-regarding preferences of individuals in mother–offspring pairs and in nonkin adult pairs in both reciprocal and nonreciprocal contexts. Based on the previously established choice paradigm with mutually or selfishly beneficial options, we developed a novel task using buttons. In experiment 1, chimpanzee participants involving three mother–offspring pairs were offered two options: delivering food rewards to their partner and themselves or only to themselves. We compared their choices between partner-present and partner-absent conditions. In experiment 2, we developed a reciprocal context in which the two participants alternately chose the two options. In contrast to the theoretical predictions, the chimpanzees did not show any prosocial tendencies even between mother and offspring or in a reciprocal context. We propose that the experimental set-up which prevented direct interactions between the participants might have influenced these results. In conclusion, the present study suggests that voluntary and/or strategic other-rewarding behaviour arose in humans after divergence from the common ancestor of humans and chimpanzees.