Reciprocity is an important concept in many disciplines because it helps to explain how selfish individualists may cooperate without a coercive authority, the Hobbesian problem of order (Ellis, 1971). Sociologists, anthropologists, economists, and biologists have studied how reciprocity uses selfish motives to maintain social order and integration. Sociologist Peter Blau (1964, p. 314) builds a theory of social exchange on the principle that people often do favors for their associates and by doing so they obligate them to return favors. Alvin Gouldner ( 1960, p. 173) points out that reciprocity contributes to social order, because a debtor is morally bound not to harm a creditor, while a creditor will not wish to forgo the opportunity of being repaid. Bronislaw Malinowski ( 1926, p. 26) observed that the social behavior of the Trobriand islanders was based on a well-assessed give-and-take, always mentally ticked off and in the long run balanced. A coastal village delivered fish to an inland village, because otherwise it would not receive any vegetables after the harvest. Max Gluckman (1955) found that hostile African tribes who herded cattle developed an exchange system so that each could use the other's territory to water its cattle during a dry season. If one tribe attacked another venturing into its territory, it might be deprived of its only source of water in the future. Biologist Robert Trivers ( 1971 and 1983) showed how reciprocal altruism evolved through natural selection. Biologists have observed reciprocal helping among baboons, dolphins, and cleaner-fish. International relations theorists have also turned to reciprocity to explain why states cooperate despite the lack of a coercive central authority (e. g. , see Keohane, 1984 and 1986; Oye, 1986; and Snidal, 1985). Robert Axelrod (1984) stimulated much interest in reciprocity through his suggestive analysis of a computer tournament of repeated prisoner's dilemma (PD) games. He found that tit for tat -a strategy which cooperates on the first move and thereafter