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to which extreme prosociality reflects an entirely innate human psychological trait. Recent cross-cultural studies have suggested that cultural norms that promote fairness and the punishment of violators may be more common in large-scale industrialized societies—in which people often interact with strangers—than in smaller, less market-based, communities, in which individuals interact primarily with familiar partners, and where mechanisms associated with kin selection and long-term reciprocity may be more relevant to social relations (Henrich et al., 2010a).

These observations emphasize again the importance of distinguishing between proximate and ultimate explanations when considering cooperation in animals. Whether animals have the cognitive capacity to engage in contingent cooperation is one question; whether it is always adaptive for them to do is another. It may well be that the relative rarity of contingent cooperation in animals stems less from the inability to keep track of recent interactions (and even, perhaps, to anticipate future ones) than from the willingness to tolerate short-term inequities with regular partners.

Finally, most studies of mental state attribution in animals to date have been conducted on captive animals, using paradigms and rewards determined by human experimenters. It is to be hoped that future investigations will attempt to address these questions under more natural conditions, on the animals’ own terms. Until such experiments are conducted, we can only speculate about the selective forces that might favor the evolution of a theory of mind, and its function in social interactions.


I thank Robert Seyfarth and Joan Silk for comments on this manuscript. I am also grateful to the Office of the President of the Republic of Botswana and the Botswana Department of Wildlife and National Parks for permission to conduct research in the Moremi Reserve. Research was supported by National Institutes of Health Grant MH62249 and was approved by the Animal Care and Use Committee of the University of Pennsylvania (Protocol 19001).

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