Inequity aversion plays a central role in human cooperation. Some animals similarly show frustration and become demotivated when rewarded more poorly than a conspecific, which has been taken as evidence of inequity aversion. An alternative explanation - social disappointment - shifts the cause of frustration from the unequal reward to the human experimenter who could – but elects not to – treat subject and partner equally. This study investigates whether social disappointment could explain frustration patterns in long-tailed macaques, Macaca fascicularis. We tested twelve monkeys in a novel inequity aversion paradigm. Subjects had to pull a lever and were rewarded with low-value food; in half of the trials a partner worked alongside the subjects receiving high-value food. Rewards were distributed either by a human or a machine. In line with the social disappointment hypothesis monkeys rewarded by the human refused food more often than monkeys rewarded by the machine. Our study extends previous findings in chimpanzees and suggests that both social disappointment and food competition drive refusal patterns.

Protest in response to unequal reward distribution is thought to have played a central role in the evolution of human cooperation. Some animals refuse food and become demotivated when rewarded more poorly than a conspecific, and this has been taken as evidence that non-human animals, like humans, protest in the face of inequity. An alternative explanation—social disappointment—shifts the cause of this discontent away from the unequal reward, to the human experimenter who could—but elects not to—treat the subject well. This study investigates whether social disappointment could explain frustration behaviour in long-tailed macaques, Macaca fascicularis . We tested 12 monkeys in a novel ‘inequity aversion’ paradigm. Subjects had to pull a lever and were rewarded with low-value food; in half of the trials, a partner worked alongside the subjects receiving high-value food. Rewards were distributed either by a human or a machine. In line with the social disappointment hypothesis, monkeys rewarded by the human refused food more often than monkeys rewarded by the machine. Our study extends previous findings in chimpanzees and suggests that social disappointment plus social facilitation or food competition effects drive food refusal patterns.

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