... In the field of psychology, and especially in social psychology, the concept of reciprocity has been used by many scientists to explain a variety of phenomena. Reciprocity meets in different aspects of everyday life, such as attitude change (Cialdini, Green and Rusch, 1992), intimacy in close relationships (Surra and Longstreth, 1990), interpersonal perception (Kenny et al., 1996), altruism (Krebs, 1975), aggression (Robarchek and Robarchek, 1998), cooperation (Komorita and Parks, 1999), intergroup interactions (Goren and Bornstein, 1999), consumer shopping and behavior (Miller and Kean, 1997), restaurant tipping (Rind and Strohmetz, 1999), and gift giving (Cialdini, 1988). It also has been used in several other sciences and their disciplines and sub-disciplines, like in sociology, sociobiology, economics, political science, anthropology, animal behavior, and evolutionary psychology and biology (Axelrod, 1984;Cosmides and Tooby, 1989;de Waal and Berger, 2000;Fehr, Gachter and Kirchsteiger, 1997;Hoffman, McCabe and Smith, 1998;Nowak and Sigmund, 1998;Trivers, 1971;Wedekind and Milinski, 2000). ...