How chimpanzees decide in the face of social and nonsocial uncertainty

Uncertainty can arise in interactions with both social partners and nonliving objects. Previous research has shown that humans display higher aversion to uncertainty arising from social interactions than to uncertainty caused by interactions with objects such as gambling machines, and that this difference may be mediated by betrayal aversion. We investigated whether chimpanzees, Pan troglodytes, differentiate between social and nonsocial forms of uncertainty. Subjects participated in two experiments, each involving a social and a nonsocial condition. In both experiments, choosing the safe option resulted in immediate access to low-value food. Choosing the uncertain option could result in access to high-value food, but only if the partner (social condition) or a machine (nonsocial condition) proved trustworthy. In experiment 1, where chimpanzees had no prior information on reciprocation rates (i.e. decided under uncertainty), chimpanzees were less likely to choose the uncertain option when they interacted with a partner than with a machine. When they did choose the uncertain option, chimpanzees also hesitated longer in the social condition. In experiment 2, where chimpanzees had learned the statistical probabilities on reciprocation rates (i.e. decided under risk), they did not distinguish between social and nonsocial situations and were generally risk averse. These results suggest that chimpanzees are more averse to engaging in uncertain choices when the source of uncertainty is a conspecific than when it is a machine; when confronted with risky choices, chimpanzees show no such tendency.