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Our results2, which have been replicated in an independent study of a different group of chimpanzees3, show that they behave very differently from the way we would expect humans to behave. In a study very similar to ours, 3–5-year-old children were asked whether they would prefer to have one sticker for themselves and one sticker for a young female experimenter, or just one sticker for themselves4. Most of the children chose the prosocial option, and some were even willing to give up stickers to the experimenter.

Moreover, we question Beninger and Quinsey's assumption that chimpanzees would behave differently towards kin and non-kin in our experimental protocol. Although female chimpanzees sometimes share plant foods with their infants5, analysis of these food transfers indicates that the infants take the initiative in these events, not the mothers. Mothers offer their infants only the unpalatable parts of their own food, and generally seem reluctant to pass them nutritious foods6. The role of kinship in cooperation among adults is also uncertain, as cooperation among males often involves reciprocal exchanges between unrelated partners7.