Inequity responses in nonhuman animals

Abstract

Aversion to inequity is a universal human trait, but recent evidence in other primate and nonprimate species suggests that humans are not the only species to respond negatively to unequal outcomes. Although only about a dozen species have been tested thus far, the emerging phylogenetic patterns help us to understand the evolution of responses to inequity. Negative responses to inequity are not universal, even among the primates, nor are they based upon social tolerance, social group size, or brain size. Instead, what we see is that all of the species that respond to inequity habitually cooperate with nonkin in various contexts, including food sharing, cooperative hunting, coalitions, and alliances. This indicates that inequity responses emerged in conjunction with cooperation. Their likely function is to compare one’s own effort and payoff with that of a partner, and therefore help individuals determine the value of a cooperative partner. In other words, negative reactions to inequity may stabilize cooperation by helping individuals recognize when a cooperative partner is getting more than their “fair share,” which is a cue that it may be time to find a new, more equitable, partner. These negative responses to inequity do not necessarily involve rational or even conscious decision-making, suggesting that they are widespread among animals that cooperate. We end by speculating what these data mean for our understanding of the evolution of fairness in humans, and what this says about the human sense of morality.