Chimpanzees are rational maximizers in an ultimatum game

Traditional models of economic decision-making assume that people are self-interested rational maximizers. Empirical research has demonstrated, however, that people will take into account the interests of others and are sensitive to norms of cooperation and fairness. In one of the most robust tests of this finding, the ultimatum game, individuals will reject a proposed division of a monetary windfall, at a cost to themselves, if they perceive it as unfair. Here we show that in an ultimatum game, humans ’ closest living relatives, chimpanzees (Pan troglodytes), are rational maximizers and are not sensitive to fairness. These results support the hypothesis that other-regarding preferences and aversion to inequitable outcomes, which play key roles in human social organization, distinguish us from our closest living relatives. Humans are able to live in very largegroups and to cooperate with unrelatedindividuals whom they expect never to encounter again, conditions that make the stan-dard mechanisms for cooperation unlikely (1), namely kin selection (2) and reciprocal altruism (3). Nevertheless, people help others, sometimes at great personal cost. But people are not ob