(2010) found that negative responses to inequity were consistently found when subjects were required to perform a task to obtain food, whereas no responses to inequity were found when subjects were able to obtain food without performing a task. However, this generalization does not fit at least one study of great apes that used a token exchange task modeled on the task that S. F. Brosnan et al. (2010) used. Bräuer et al. (2009) found no evidence for inequity aversion among orangutans, bonobos, or chimpanzees.
Altruism and mutualistic cooperation play important roles in the lives of nonhuman primates, but there are important differences in the scope of altruistic behavior between humans and other primates. In other primates, altruism is strongly biased in favor of kin and reciprocating partners, and it is never extended to strangers. Primates use aggression to deter competitors and rivals, but there is no compelling evidence of third-party punishment. Unlike humans, nonhuman primates show no aversion to inequitable distributions of resources that favor themselves.
It is important to continue efforts to chart the size and dimensions of the gap between humans and other primates if we want to understand the evolutionary forces that have shaped human social preferences. Evidence that closely related primates, particularly great apes, have altruistic social preferences would suggest that our social preferences were built on a set of ancestral motivations that facilitated altruism to kin and reciprocating partners, mutualistic activities with group members, punitive behavior to competitors, antagonistic attitudes to strangers, and concern for reputational status. At the same time, evidence that closely related primates lack the kinds of altruistic social preferences that characterize modern humans would suggest that emergent forces, possibly including cultural group selection (Richerson and Boyd, 2005), demands of raising slow-growing offspring (Hrdy, 2005b, 2007; Jaeggi et al., 2010), knowledge-intensive human foraging strategy (Kaplan et al., 2000, 2003), or risks associated with uncertain hunting returns (Winterhalder, 1986), have transformed us in consequential ways and given rise to important differences in the scope and scale of cooperation, our capacity for empathy and compassion, the development of moral sentiments, and the willingness to enforce culturally specified social norms.
We believe that well-designed experimental studies of social preferences in other primates provide important insights about the nature of social preferences. Such studies should be designed to test hypotheses that are grounded in evolutionary theory and our knowledge of the natural history, social organization, and cognitive capacities of our study subjects.