No Response to Inconsistently Experienced Inequity in Four Primate Species

Understanding when and how nonhuman primates respond to inequity is essential for uncovering the evolutionary roots of human fairness. Previous studies have shown that some species will refuse to participate or refuse rewards in some contexts when outcomes are unequal, but in these studies, subjects experienced inequity on every trial. This is likely not a realistic reflection of daily life, and we do not know whether these responses persist when inequity is experienced inconsistently. We tested four primates'-chimpanzees, capuchins, squirrel monkeys, and owl monkeys-responses in conditions in which inequity occurred in either 25% or 50% of trials. None exhibited increased refusal rates in response to inequity or contrast relative to controls, despite the fact that in prior studies with inequity on every trial, these capuchin monkeys and chimpanzees responded to inequity and these squirrel monkeys showed contrast effects. Their lack of response suggests that while primates will refuse to participate due to inequity in some cases, such inequity must be experienced relatively frequently or consistently to trigger refusals to participate. Future work should explore the thresholds at which inequity prompts these refusals and develop other paradigms to test if they notice and/or respond in other ways to inequity, particularly at lower thresholds.