Inequity aversion in human adults: testing behavioural criteria from comparative cognition

Inequity aversion refers to an attempt to reduce a perceived discrepancy between one’s own input and output ratio (i.e. ratio between work invested and rewards obtained) and that of others. It has been proposed that inequity aversion might also play a role in the decision-making process of other animals. One issue, however, is that while studies in comparative cognition define clear behavioural criteria for an inequity aversion effect in animals, studies conducted on humans rely on an implicit definition of the concept involving assumptions for which there is yet no experimental evidence. In particular, the basis of inequity aversion is assumed to be a social comparison process that is further motivated by fairness concerns. Based on the studies on non-human animals, we tested whether inequity aversion in adult humans will satisfy the behavioural criteria for these assumptions. Although humans showed a decrease in working for a lower-value reward when working alone (non-social effect), this effect was enhanced when a partner was present (social effect) suggesting that their inequity aversion might be based on a social comparison process. Additionally, our tests ruled out the possibility that selfish motives underlie the subjects’ decisions, which raises the possibility that fairness concerns might have been the primary motive. Our results thus show that human behaviour satisfies two criteria imposed by the definition of inequity aversion. A similar pattern is shown in other species, suggesting that the same or similar processes might influence economic decision-making in both humans and non-human animals.