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.