1. Department of Anthropology, University of California, Los Angeles, California 90095, USA
2. Department of Anthropology, Emory University, Atlanta, Georgia 30322, USA
3. Michale E. Keeling Center for Comparative Medicine and Research, The University of Texas M. D. Anderson Cancer Center, Bastrop, Texas 78602, USA
4. Cognitive Evolution Group, University of Louisiana at Lafayette, New Iberia, Lousiana 70560, USA

Correspondence to: Joan B. Silk¹ Correspondence and requests for materials should be addressed to J.B.S. (Email: jsilk@anthro.ucla.edu).

Abstract

Humans are an unusually prosocial species—we vote, give blood, recycle, give tithes and punish violators of social norms. Experimental evidence indicates that people willingly incur costs to help strangers in anonymous one-shot interactions^{1, 2}, and that altruistic behaviour is motivated, at least in part, by empathy and concern for the welfare of others (hereafter referred to as other-regarding preferences)^{1, 2, 3}. In contrast, cooperative behaviour in non-human primates is mainly limited to kin and reciprocating partners, and is virtually never extended to unfamiliar individuals⁴. Here we present experimental tests of the existence of other-regarding preferences in non-human primates, and show that chimpanzees (Pan troglodytes) do not take advantage of opportunities to deliver benefits to familiar individuals at no material cost to themselves, suggesting that chimpanzee behaviour is not motivated by other-regarding preferences. Chimpanzees are among the primates most likely to demonstrate prosocial behaviours. They participate in a variety of collective activities, including territorial patrols, coalitionary aggression, cooperative hunting, food sharing and joint mate guarding^{5, 6, 7, 8, 9, 10, 11, 12}. Consolation of victims of aggression¹³ and anecdotal accounts of solicitous treatment of injured individuals suggest that chimpanzees may feel empathy^{14, 15}. Chimpanzees sometimes reject exchanges in which they receive less valuable rewards than others, which may be one element of a 'sense of fairness', but there is no evidence that they are averse to interactions in which they benefit more than others^{16, 17, 18}.

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