Justice- and fairness-related behaviors in nonhuman primates

Abstract

A distinctive feature across human societies is our interest in justice and fairness. People will sometimes invest in extremely costly behavior to achieve fair outcomes for themselves and others. Why do people care so much about justice? One way to address this is comparatively, exploring behaviors related to justice and fairness in other species. In this paper, I review work exploring responses to inequity, prosocial behavior, and other relevant behaviors in nonhuman primates in an effort to understand both the potential evolutionary function of these behaviors and the social and ecological reasons for the individual differences in behavior. I also consider how these behaviors relate to human behavior, particularly in the case of experimental studies using games derived from experimental economics to compare nonhuman primates' responses to those of humans in similar experimental conditions. These results emphasize the importance of a comparative approach to better understand the function and diversity of human behavior.

Keywords: behavioral phylogeny; comparative economics; cooperation; decision-making; game theory.

Fig. 1.

Schematic diagram of the UG.

Fig. 2.

Schematic of the IG. The full IG has not yet been explored with other species; however, a typical inequity procedure is reminiscent of the responder’s choice in the IG (indicated by the dotted line).

Fig. 3.

Schematic diagram indicating the responses to inequity of species that have been tested using the typical inequity procedure in which subjects exchange tokens for rewards while sitting next to a partner. The photos representing species that show a negative response to inequity in this task are outlined in green, and those that do not are outlined in red. The names of species that show extensive social cooperation, such as coalitions and alliances, are in green, those that do not are in red, and interdependent species (e.g., with biparental care) are in violet. Humans show both extensive social cooperation and biparental care. Note that this diagram excludes gorillas, for whom too little information is known. The photographs of the macaque and bonobo are by F. B. M. de Waal. The photograph of the orangutan is by C. Talbot. The photograph of Darwin is from the public domain. All other photographs are by the author.