Capuchins (Sapajus apella) and their aversion to inequity

Humans have a strong sense of fairness and are usually averse to unequal treatment for the same action. Ever since Brosnan and de Waal showed a similar effect in capuchin monkeys (Sapajus apella), numerous studies using different experimental methods have been conducted to investigate whether animals show inequity aversion like humans do. Capuchin monkeys have become one of the best-studied animals in this area. Our first aim in this chapter was to synthesise the findings in this literature. We found that there is mixed evidence for inequity aversion in capuchin monkeys. Our second aim was to understand this variation by focusing on the following factors: the type of task used, the feeding regime outside the experiment and the monkeys’ social environment. To obtain data on some of these factors, as they are not always reported in published studies, we contacted researchers in the main laboratories conducting this work. We found that responses to inequity systematically varied as a function of the task demands and the feeding regime, but not the social environment. Tasks, in particular pulling tasks, that required participants to expend effort to get the food were more likely to detect evidence of inequity aversion. Moreover, monkeys with access to food before or after testing, were more likely to show inequity aversion than those whose access to food was temporarily restricted. We note that our survey is an explorative approach to investigate the variation in reports on inequity aversion in capuchin monkeys. We hope this chapter raises awareness of the complexity of the concept and generates new testable hypotheses, which might advance our understanding of the theoretical foundations of inequity aversion.