Chimpanzees play the ultimatum game

Abstract

Is the sense of fairness uniquely human? Human reactions to reward division are often studied by means of the ultimatum game, in which both partners need to agree on a distribution for both to receive rewards. Humans typically offer generous portions of the reward to their partner, a tendency our close primate relatives have thus far failed to show in experiments. Here we tested chimpanzees (Pan troglodytes) and human children on a modified ultimatum game. One individual chose between two tokens that, with their partner's cooperation, could be exchanged for rewards. One token offered equal rewards to both players, whereas the other token favored the chooser. Both apes and children responded like humans typically do. If their partner's cooperation was required, they split the rewards equally. However, with passive partners--a situation akin to the so-called dictator game--they preferred the selfish option. Thus, humans and chimpanzees show similar preferences regarding reward division, suggesting a long evolutionary history to the human sense of fairness.

Fig. 1.

Experimental setup for pairs of chimpanzees following a four-step sequence. Step 1: The proposer (P) is presented with a choice of two tokens, one representing an equal split of the rewards and the other representing an unequal split favoring the proposer. The proposer is free to select either token. Step 2: The proposer passes the selected token to the respondent (R) through a mesh panel. Step 3: The respondent either returns the token to the experimenter to accept the offer, drops the token, or does not return it for 30 s. Step 4: Six banana rewards are visibly divided on a tray in front of the chimpanzees according to the token selected. Here the dots represent an unequal 5:1 distribution of rewards in favor of the proposer. The tray is then pushed within reach of the chimpanzees so that each can collect its reward(s). Note that the experimental setup for children was similar, except that a commercially available baby gate was used to separate the participants and the experimenter.

Fig. 2.

Total percentage of offers selected by the chimpanzees. Chimpanzees were presented with two different tokens representing either an equitable or selfish (favoring the proposer) offer. We compared their choices in a preference test, where the partner was naïve and passive, with the UG, where the partner could affect reward outcomes for both individuals. Although chimpanzees preferred the selfish offer during the preference test, they significantly changed their preferences toward the equitable offer in the UG condition. See Table 1 for offer selections by each pair of chimpanzees. *Significant difference between equitable and selfish offers; binomial test, P < 0.05. ^†Significant change from preference test to UG; McNemar’s test, P < 0.05.