Social and Non-social Mechanisms of Inequity Aversion in Non-human Animals

Abstract

Research over the last decades has shown that humans and other animals reveal behavioral and emotional responses to unequal reward distributions between themselves and other conspecifics. However, cross-species findings about the mechanisms underlying such inequity aversion are heterogeneous, and there is an ongoing discussion if inequity aversion represents a truly social phenomenon or if it is driven by non-social aspects of the task. There is not even general consensus whether inequity aversion exists in non-human animals at all. In this review article, we discuss variables that were found to affect inequity averse behavior in animals and examine mechanistic and evolutionary theories of inequity aversion. We review a range of moderator variables and focus especially on the comparison of social vs. non-social explanations of inequity aversion. Particular emphasis is placed on the importance of considering the experimental design when interpreting behavior in inequity aversion tasks: the tasks used to probe inequity aversion are often based on impunity-game-like designs in which animals are faced with unfair reward distributions, and they can choose to accept the unfair offer, or reject it, leaving them with no reward. We compare inequity-averse behavior in such impunity-game-like designs with behavior in less common choice-based designs in which animals actively choose between fair and unfair rewards distributions. This review concludes with a discussion of the different mechanistic explanations of inequity aversion, especially in light of the particular features of the different task designs, and we give suggestions on experimental requirements to understand the “true nature” of inequity aversion.

Anterior Cingulate Cortex Signals Attention in a Social Paradigm that Manipulates Reward and Shock

“…For example, we have shown that rats emit appetitive vocalizations and DA is released during rewards delivered to conspecific but only early during learning. After rats experienced several trials where the conspecific received reward and they did not, vocalizations became aversive and DA was inhibited during conspecific reward delivery [51,56,57]. Further, we have shown that DA is released when the recording rat observes the conspecific receive shocks, suggesting that observation of the conspecific receiving shock, instead receiving shock itself, is an event that is better in value than expected [36].…”

Exploring the Cognitive Capacities of Japanese Macaques in a Cooperation Game

“…Nevertheless, as the specifics of the reward division were left to the macaques, they could at most be disappointed in the experimenter for loading the apparatus with unequal food rewards. Such inter-conspecific negotiation tasks with minimal researcher interference are suggested by Oberliessen and Kalenscher to control for the “social disappointment hypothesis” [ 72 ].…”

Leveraging Social Learning to Enhance Captive Animal Care and Welfare

“…Once socially housed, animals can be influenced by their social environment in a number of ways. For example, an individual's social group might influence their movement patterns [65], how they weigh the relative importance or value of things [66], or even their affect [67]. Moreover, an animal's social environment can impact their health and mortality [68], which not only has welfare ramifications but may also affect animals' responses to research protocols and the validity of research outcomes [69].…”