Attending to the outcome of others: disadvantageous inequity aversion in male capuchin monkeys (Cebus apella).

Brosnan and de Waal [Nature 425:297-299, 2003] reported that capuchin monkeys responded negatively to unequal reward distributions between themselves and another individual when comparing their own rewards with that of their partner. It was suggested that social emotions provided the underlying motivation for such behavior and that this inequity aversion is specific to the social domain. However, alternative hypotheses such as the "frustration effect" or the "food expectation hypothesis" may provide more parsimonious explanations for Brosnan and de Waal's [Nature 425:297-299] results, while others have argued that these findings are not congruent with the Fehr-Schmidt inequity aversion model cited by the authors. The claim that inequity aversion behavior is specific to the social domain has also been questioned, as primates also develop expectations about rewards in the absence of partners, and react negatively when those expectations are violated. In this study, a modified Dictator game was used to investigate whether capuchins would exhibit either disadvantageous inequity aversion behavior or reference-dependent expectancy violation in social and nonsocial conditions, respectively. When given the choice between an equitable and an inequitable outcome, the subjects showed disadvantageous inequity aversion behavior, choosing the equitable outcome significantly more in the social condition. In the nonsocial condition, however, subjects did not show negative expectancy violation resulting from the formation of reference-dependent expectations, choosing the equitable outcome at chance levels. These results suggest that capuchins attend to differential payoffs and that they are averse to inequity, which is disadvantageous to themselves.