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.
Highlights
We suggest that a different IA paradigm—choicebased IA task designs—might be a promising complement to the existing IA literature as they offer the potential to avoid some of the interpretational caveats mentioned in the preceding section
Preferences for equality are compared between two conditions: a social condition with a conspecific present, and a non-social control condition in which the outcome distributions are identical to the social condition, but the conspecific is absent; e.g., rewards are dropped in an empty, adjacent chamber or compartment. Using such choice-based tasks, it has been shown that both rats (Oberliessen et al, 2016) and capuchin monkeys (Fletcher, 2008) preferred equal over unequal outcome distributions when paired with a conspecific, and that this preference for equal distributions was weaker, or entirely absent, in a non-social control condition with no conspecific present
The clear advantage of such choice-based IA designs is that the animals do not need to forego own rewards to express their aversion to inequity; they differ from the impunity-like flavor of previous IA tasks that involved costly refusals of ownrewards
Summary
Other-regarding preferences, i.e., the consideration of the well-being of others when making decisions, are pertinent in human behavior and economic decision making (Fehr and Schmidt, 1999). The disadvantaged monkey refused to exchange the token, or rejected the cucumber reward entirely, tentatively reminiscent of the behavior of human responders in the impunity game (see below for critical discussion) Since this early study, IA was replicated in capuchin monkeys (van Wolkenten et al, 2007; Fletcher, 2008; Takimoto et al, 2010; Takimoto and Fujita, 2011), and reported in macaques (Massen et al, 2012; Hopper et al, 2013), chimpanzees (Brosnan et al, 2005, 2010), cotton top tamarins (Neiworth et al, 2009), dogs (Range et al, 2009, 2012; Brucks et al, 2016; see McGetrick and Range, 2018 for an overview), wolves (Essler et al, 2017), crows (Wascher and Bugnyar, 2013), rabbits (Heidary et al, 2008) and rats (Oberliessen et al, 2016). The social disappointment hypothesis (Engelmann et al, 2017) suggests that, rather than being sensitive to the relative advantage of the conspecific, animals respond to TABLE 1 | Evidence for and against inequity aversion in non-human animal species using different task designs
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