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species and differences in performance across tasks are not yet fully understood and raise new questions for further study.

It is not from the benevolence of the butcher, the brewer, or the baker that we expect our dinner, but from their regard to their own interest.

Adam Smith, The Wealth of Nations

How selfish soever man may be supposed, there are evidently some principles in his nature, which interest him in the fortune of others, and render their happiness necessary to him, though he derives nothing from it except the pleasure of seeing it.

Adam Smith, The Theory of Moral Sentiments

As Adam Smith pointed out more than 250 years ago, humans often act out of self-interest but also feel concern for the welfare of others. These sentiments come into conflict when selfish behavior produces negative impacts on others and when concern for others leads to altruistic behavior that reduces one’s own welfare. For evolutionary biologists, selfishness is a straightforward consequence of selective forces that favor behaviors that enhance individual fitness. Natural selection is not expected to favor indiscriminate altruism, because altruists bear the costs of the altruistic behaviors that they perform; this reduces their relative fitness. Altruism can only evolve if altruists confer benefits selectively on others who carry the same altruistic alleles. Kin selection (Hamilton, 1964a) and reciprocal altruism (Trivers, 1971; Axelrod and Hamilton, 1981) both rely on this principle. Selection can favor altruism to close relatives, because recent common descent provides a reliable cue of genetic similarity. In the case of reciprocity, past behavior of other group members provides a cue about whether they carry alleles that lead to altruistic behavior. These processes can generate biases in favor of kin and reciprocating partners but not a general predisposition to behave altruistically to others.

Altruism is also paradoxical for many economists. Selfishness is the expected outcome when, as is often assumed, utility functions only include personal consumption. However, as Adam Smith realized, human behavior deviates from the expected behavior for self-interested actors. Experimental studies in behavioral economics that are designed to bring conflicts between self-interest and altruism into sharp relief show that people value their own welfare but also value the welfare of others (Henrich et al., 2004; Fehr and Schmidt, 2006). This body of work provides insight about some of the dimensions of our altruistic social preferences.

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