A sense of fairness has long been considered purely human – but animals also react with frustration when they are treated unequally by a person. In a study with long-tailed macaques (Macaca fascicularis), researchers at the German Primate Center – Leibniz Institute for Primate Research (DPZ) have now confirmed an alternative explanatory approach. A combination of social disappointment with the human experimenter and some degree of food competition best explains their behavior in an ‚inequity aversion‘ experiment.