biostudies-literatureTitchener RLeibnizScience CampusDFG, German Research Foundation221225https://www.ebi.ac.uk/biostudies/studies/S-EPMC9974291biostudies-literatureUnknown10(3)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, <i>Macaca fascicularis</i>. We tested 12 monkeys in a novel 'inequity aversion' paradigm. Subjects had to pull a lever and were rewarded with low-value food; in Royal Society open scienceSocial disappointment and partner presence affect long-tailed macaque refusal behaviour in an 'inequity aversion' experiment.PMC9974291SF2018/02/LSC254142454/GRKTitchener RScherberger HThiriau CFischer JHuser TKeupp SfalseSocial disappointment and partner presence affect long-tailed macaque refusal behaviour in an 'inequity aversion' experiment.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, <i>Macaca fascicularis</i>. We tested 12 monkeys in a novel 'inequity aversion' paradigm. Subjects had to pull a lever and were rewarded with low-value food; in 2023-01-01T00:00:00Z2023 Mar2025-04-19T02:45:24.632Z2025-02-19T04:54:30.974ZS-EPMC99742913686607910.1098/rsos.221225