Common marmosets (Callithrix jacchus) evaluate third-party social interactions of human actors but Japanese monkeys (Macaca fuscata) do not.

Reciprocity and cooperation are fundamental to human society and are observed in nonhuman primates. Primates are not only sensitive to direct reciprocity and its violation but also indirect reciprocity. Recent studies demonstrated that some primate species adjusted their behavior by observing others' interactions. Capuchin, marmoset, and squirrel monkeys avoided taking food from human actors who behaved nonreciprocally; however, no such empirical evidence among Old World monkeys is available. Here, we show that common marmosets, which are a highly prosocial species, discriminated between human actors who reciprocated in social exchanges and those who did not; however, Japanese monkeys, who are renowned for despotic social relationships, did not. In the reciprocal condition, 2 human actors exchanged food equally, whereas in the nonreciprocal condition, 1 actor (nonreciprocator) ended up with all the food and the other actor with none. The common marmosets avoided receiving food from the nonreciprocator in the nonreciprocal condition. Nevertheless, the Japanese monkeys did not show differential preferences in either condition. These results suggest a crucial role for prosocial tendencies in monkeys' responses to asymmetric exchanges and indicate that third-party social evaluations are not homologous among primates. Further comparative studies with direct comparisons will be required to explore the underlying mechanism of third-party social evaluations. (PsycINFO Database Record (c) 2019 APA, all rights reserved).