A Hypothesis of the Co-evolution of Cooperation and Responses to Inequity

Sarah F Brosnan

doi:10.3389/fnins.2011.00043

Abstract

Recent evidence demonstrates that humans are not the only species to respond negatively to inequitable outcomes which are to their disadvantage. Several species respond negatively if they subsequently receive a less good reward than a social partner for completing the same task. While these studies suggest that the negative response to inequity is not a uniquely human behavior, they do not provide a functional explanation for the emergence of these responses due to similar characteristics among these species. However, emerging data support the hypothesis that an aversion to inequity is a mechanism to promote successful long-term cooperative relationships amongst non-kin. In this paper, I discuss several converging lines of evidence which illustrate the need to further evaluate this relationship. First, cooperation can survive modest inequity; in explicitly cooperative interactions, individuals are willing to continue to cooperate despite inequitable outcomes as long as the partner's overall behavior is equitable. Second, the context of inequity affects reactions to it in ways which support the idea that joint efforts lead to an expectation of joint payoffs. Finally, comparative studies indicate a link between the degree and extent of cooperation between unrelated individuals in a species and that species’ response to inequitable outcomes. This latter line of evidence indicates that this behavior evolved in conjunction with cooperation and may represent an adaptation to increase the payoffs associated with cooperative interactions. Together these data inform a testable working hypothesis for understanding decision-making in the context of inequity and provide a new, comparative framework for evaluating decision-making behavior.

Highlights

When making decisions about resources, humans show an intense interest in how their outcomes compare to those of others
In our study, squirrel monkeys did not respond negatively to inequity, completing the interaction whether or not their partner received a greater reward (Talbot et al, in press). They were sensitive to the experimental paradigm; subjects were more likely to refuse to participate if the rewards violated their expectations than in the control condition. These results indicate that the inequity response is not homologous within the Family Cebidae, and the distribution across new world monkeys and great apes suggests that neither sociality nor cognition are sufficient feature to explain it
Responding to inequity would be a mechanism to do this; situations in which one’s payoffs deviate substantially from one’s social partners could be a reliable signal to evaluate other options. In this scenario inequity aversion would serve as the mechanism to evaluate when there is the possibility to increase one’s benefit from cooperative interactions, and as such function to maximize the outcomes from cooperation

Summary

Introduction

When making decisions about resources, humans show an intense interest in how their outcomes compare to those of others. It has been proposed that refusals of rewards may be due to a “frustration effect,” in which subjects compare their current outcomes to those which they received previously, and protest if the comparison comes up wanting (Roma et al, 2006) Such individual contrast effects are seen in a wide variety of species Hypotheses for the function of the response to inequity It has been proposed that the inequity response functions to increase the success of long-term cooperative relationships amongst unrelated individuals (hereafter cooperation; Fehr and Schmidt, 1999; Brosnan, 2006) This was originally proposed with respect to humans, new evidence provides support for this hypothesis in an evolutionary context. Note that while the original inequity aversion formulation assumed that individuals were averse to decisions which increased relative inequity (Fehr and Schmidt, 1999), even reactions that increase short-term inequity (e.g., in the Impunity game) may serve to increase long-term equity by moving actors in to relationships which are more beneficial

Both high value Both low value Both high value

Thoughts on the evolution of inequity

What is needed next?

Conclusion