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Abstract

Disadvantageous inequity aversion (IA), a negative response to receivingless than others, is a key building block of the human sense of fairness.While some theorize that IA is shared by species across the animalkingdom, others argue that it is an exclusively human evolutionaryadaptation to the selective pressures of cooperation among non-kin.Essential to this debate is the empirical question of whether non-humananimals are averse towards unequal resource distributions. Over the pasttwo decades, researchers have reported that individuals from a wide rangeof taxa exhibit IA; tasks where participants can reject or accept a givendistribution of rewards delivered the bulk of this evidence. Yet these resultshave been questioned on both conceptual and empirical grounds. In thelargest empirical investigation of non-human IA to date, we synthesizethe primary data from 23 studies using accept/reject tasks, covering 60 430observations of 18 species. We find no evidence for IA in non-humananimals in these tasks. This finding held across all species in the datasetand pre-registered subsets (all species reported to exhibit IA, primatesreported to exhibit IA, chimpanzees and capuchin monkeys). Alternativeinterpretations of the data and implications for the evolution of fairness arediscussed.