Social disappointment explains chimpanzees' behaviour in the inequity aversion task

Jan M. Engelmann, Jeremy B. Clift, Esther Herrmann, Michael Tomasello

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Chimpanzees’ refusal of less-preferred food when an experimenter has previously provided preferred food to a conspecific has been taken as evidence for a sense of fairness. Here, we present a novel hypothesis—the social disappointment hypothesis—according to which food refusals express chimpanzees' disappointment in the human experimenter for not rewarding them as well as they could have. We tested this hypothesis using a two-by-two design in which food was either distributed by an experimenter or a machine and with a partner present or absent. We found that chimpanzees were more likely to reject food when it was distributed by an experimenter rather than by a machine and that they were not more likely to do so when a partner was present. These results suggest that chimpanzees’ refusal of less-preferred food stems from social disappointment in the experimenter and not from a sense of fairness.
Original languageEnglish
Article number20171502
Number of pages8
JournalProceedings of the Royal Society B: Biological Sciences
Issue number1861
Early online date23 Aug 2017
Publication statusPublished - 30 Aug 2017


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