Abstract
Displaying aversive reactions toward a disadvantageous inequity is a well known phenomena in humans. This kind of sensitivity to unfair outcomes, together with the sensitivity to unfair intentions or actions, is known to regulate reciprocity and to stabilize cooperation. Much work has demonstrated that this behavioral trait is not unique to humans, but also occurs in other primate species. However, it still needs to be examined whether this aversion to disadvantageous inequity can be further employed in an animal’s actual choice to punish the partner who gets better rewards then themselves, by being spiteful. The current study therefore was designed to test whether chimpanzees’ prosocial choices are influenced by differential reward distributions (equal/advantageous/disadvantageous) and also by unfair behaviors (prosocial/selfish) of human partners. Unlike other inequity tasks, we used a prosocial choice experiment to give the chimpanzees an active role to be prosocial or selfish toward their partner. In experiment 1, we tested 2 female chimpanzees at the Primate Research Institute, Kyoto University with 3 females from the same group to test aversion to outcome disparity. In experiment 2, the same chimpanzees who participated in experiment 1 as an actor, were tested with 4 familiar humans who were randomly assigned either to be prosocial or selfish by making 8 consecutive prosocial or selfish choices toward the chimpanzee. Our results show that the chimpanzees show aversive reactions when they themselves had a less preferred reward than their partners by rejecting to do the task. Especially, one chimpanzee, Pendesa, showed display behaviors toward the subordinate recipient, but not toward the dominant recipient who was having a more preferred reward than her. However, this behavioral tendency did not lead chimpanzees to make selfish choices. This was also true when they were paired with human partners. Further experiments should be carried out in order to test whether these tendencies resulted from relationship quality which influences the tolerance for inequity, suggested by other studies, or from other factors, such as avoidance of future revenge, etc.
