Evolution and Human Behavior 24 (2003) 153 – 172

Explaining altruistic behavior in humans
Herbert Gintisa,b,*, Samuel Bowlesa,b, Robert Boydc, Ernst Fehrd
Santa Fe Institute, 1399 Hyde Park Road, Santa Fe, NM 87501, USA Department of Economics, University of Massachusetts, Amherst, MA 01003, USA c Department of Anthropology, University of California at Los Angeles, 405 Hilgard Avenue, Box 951361 Los Angeles, CA 90095-1361, USA d University of Zurich, Blumlisalpstrae 10 CH-8006 Zurich, Switzerland ¨ ¨ ¨ b a

Received 15 February 2002; received in revised form 29 November 2002

Abstract Recent experimental research has revealed forms of human behavior involving interaction among unrelated individuals that have proven difficult to explain in terms of kin or reciprocal altruism. One such trait, strong reciprocity is a predisposition to cooperate with others and to punish those who violate the norms of cooperation, at personal cost, even when it is implausible to expect that these costs will be repaid. We present evidence supporting strong reciprocity as a schema for predicting and understanding altruism in humans. We show that under conditions plausibly characteristic of the early stages of human evolution, a small number of strong reciprocators could invade a population of selfregarding types, and strong reciprocity is an evolutionary stable strategy. Although most of the evidence we report is based on behavioral experiments, the same behaviors are regularly described in everyday life, for example, in wage setting by firms, tax compliance, and cooperation in the protection of local environmental public goods. D 2003 Elsevier Science Inc. All rights reserved.
Keywords: Altruism; Reciprocity; Experimental games; Evolution of cooperation

1. Introduction The explanatory power of inclusive fitness theory and reciprocal altruism (Hamilton, 1964; Trivers, 1971; Williams, 1966) convinced a generation of researchers that what appears to be
* Corresponding