The human sense of fairness presents an evolutionary puzzle because it appears to run counter to the short-term interests of at least some parties. We not only react negatively to getting less than a partner but sometimes also to getting more, which seems illogical. Following an ideal of impartiality, we seek appropriate outcomes for everyone within the community, not just a few individuals, and, in particular, not just ourselves. Why do we react this way? Are we the only species to do so? Here, we consider the evidence with regard to nonhuman primates and other animals to illuminate the evolution of the sense of fairness. Because social ideals escape measurement, we focus on behavioral responses to equal versus unequal reward division. Moreover, a true sense of fairness includes taking account of receiving both less than a partner and more than a partner. Therefore, we consider the evidence for both of these in other species and how this informs our understanding of the evolution of fairness in humans.

A diagram of the relationship between first-order IA and second-order IA. Individual A received high-level rewards, and individual B received low-level rewards. Individuals who recognize when they receive less than another may react against this situation—for instance, by finding a new cooperative partner. As reliance on cooperation increases, individuals also benefit from recognizing when they receive more, as this allows them to forestall first-order IA reactions in their partners and thereby maintain a successful cooperative relationship. This is the foundation of the full-blown human sense of fairness. A diagram of the relationship between first-order IA and second-order IA . Individual A received high-level rewards, and individual B received low-level rewards. Individuals who recognize when they receive less than another may react against this situation—for instance, by finding a new cooperative partner. As reliance on cooperation increases, individuals also benefit from recognizing when they receive more, as this allows them to forestall first-order IA reactions in their partners and thereby maintain a successful cooperative relationship. This is the foundation of the full-blown human sense of fairness.

Advances

There is widespread evidence for sensitivity to receiving less than a partner, or “first-order inequity aversion” (IA), in species that cooperate outside mating bonds and kinship. In these studies, animals are paired with a social partner who receives a preferred reward for completing a task. Subjects may respond by refusing to participate or refusing to accept the food reward, and such reactions are compared with those following control tests in which both subjects receive the same reward for the same effort. Increased responses when the partner receives a preferred reward are indicative of a sensitivity to inequity.

Thus far, passive and active protest against unfavorable outcomes has been documented in monkeys, apes, dogs, and birds. It is thought that these species compare their outcomes with those of others so as to judge the merit of their partnerships. They may turn away from partners that appropriate more than their fair share of the yields of cooperation.

A complete sense of fairness also includes second-order IA, however, which seeks to equalize outcomes even at a short-term cost to the self. This requires individuals to give up an immediate benefit to stabilize a long-term valuable cooperative relationship. Second-order IA reactions have thus far been found only in humans and apes. We hypothesize that second-order IA requires anticipation of first-order IA in the partner and its negative impact on the relationship. To forestall these consequences, and ensure continued cooperation, outcomes are equalized between partners.