Inequity aversion is observed in common marmosets but not in marmoset models of autism induced by prenatal exposure to valproic acid.

Humans and various nonhuman primates respond negatively to inequity not in their favor (i.e., inequity aversion), when inequity between two individuals is introduced. Common marmosets, a highly prosocial species, further discriminated between human actors who reciprocated in social exchanges, and those who did not. Conversely, marmoset models of autism, induced via prenatal exposure to valproic acid (VPA marmosets), did not discriminate. Interestingly, previous studies of inequity aversion in marmosets have produced negative results, or were limited to males. Recent studies suggest that inequity aversion is highly influenced by the tasks employed. Here we show inequity aversion in both male and female marmosets using a novel task which required a relatively long duration of response. Marmosets were required to hold a spoon for 2 s to receive a reward. Marmosets successfully performed the task when they observed an unfamiliar conspecific partner obtaining the same reward (equity test). However, when they witnessed the partner receiving a more attractive reward for equal effort (inequity test), unexposed marmosets, which were not exposed to either valproic acid or saline during the fetal period refused to respond. This inequity aversion was not observed in unexposed marmosets when the partner was absent.　In contrast, marmosets with fetal exposure to valproic acid (VPA marmosets) successfully executed the task irrespective of their partners' reward conditions. As prenatal exposure to valproic acid is a well-known procedure to induce autism spectrum disorder (ASD)-like behaviors in rodents, we propose that VPA marmosets failed to show inequity aversion due to weak social motivation or interest towards others.