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Monkeys reject unequal pay


During the evolution of cooperation it may have become critical for individuals to compare their own efforts and pay-offs with those of others. Negative reactions may occur when expectations are violated. One theory proposes that aversion to inequity can explain human cooperation within the bounds of the rational choice model1, and may in fact be more inclusive than previous explanations2,3,4,5,6,7,8. Although there exists substantial cultural variation in its particulars, this ‘sense of fairness’ is probably a human universal9,10 that has been shown to prevail in a wide variety of circumstances11,12,13. However, we are not the only cooperative animals14, hence inequity aversion may not be uniquely human. Many highly cooperative nonhuman species seem guided by a set of expectations about the outcome of cooperation and the division of resources15,16. Here we demonstrate that a nonhuman primate, the brown capuchin monkey (Cebus apella), responds negatively to unequal reward distribution in exchanges with a human experimenter. Monkeys refused to participate if they witnessed a conspecific obtain a more attractive reward for equal effort, an effect amplified if the partner received such a reward without any effort at all. These reactions support an early evolutionary origin of inequity aversion.

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Figure 1: Mean percentage ± s.e.m. of failures to exchange for females across the four test types.
Figure 2: Mean percentage ± s.e.m. of failures to exchange in the first 15 trials (black bars) versus the last 10 trials per test (white bars).
Figure 3: A juvenile capuchin monkey shows cheek-to-cheek begging to an eating adult male, cupping his hand next to the adult's food in solicitation.

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We thank L. Antonucci, K. Bouxsein, J. Davis, C. Freeman and A. Katz for help with data collection and coding, L. Ruttan for comments on the manuscript and R. Earley for statistical advice. Photo by F.d.W.

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Brosnan, S., de Waal, F. Monkeys reject unequal pay. Nature 425, 297–299 (2003).

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