Evolution of responses to (un)fairness

Abstract

The human sense of fairness is an evolutionary puzzle. To study this, we can look to other species, in which this can be translated empirically into responses to reward distribution. Passive and active protest against receiving less than a partner for the same task is widespread in species that cooperate outside kinship and mating bonds. There is less evidence that nonhuman species seek to equalize outcomes to their own detriment, yet the latter has been documented in our closest relatives, the apes. This reaction probably reflects an attempt to forestall partner dissatisfaction with obtained outcomes and its negative impact on future cooperation. We hypothesize that it is the evolution of this response that allowed the development of a complete sense of fairness in humans, which aims not at equality for its own sake but for the sake of continued cooperation.

Fig. 1. 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 so as to maintain beneficial outcomes of cooperation, 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 there by maintain a successful cooperative relationship. Second-order IA requires advanced cognition and emotional control and thus far has only been seen in chimpanzees and humans. This is the foundation of the full-blown human sense of fairness.

Fig. 2. Subjects’ responses in the standard inequity task and the UG

(A) Capuchin monkeys during the original “monkeys reject unequal pay” experiment (4). The monkey on the left is rejecting the lesser reward, a cucumber slice, after viewing the partner receive a more preferred grape for the same amount of work. See video (5). (B) Chimpanzees during the UG (81). The chimpanzee who has just made the token choice (right) hands the token to her partner, who needs to accept and hand it over to the experimenter in order for both of them to receive the rewards corresponding with the token choice.