Social Norms and Reciprocity* Andreas Diekmann Institut für Soziologie Universität Bern Thomas Voss Institut für Soziologie Universität Leipzig [ March 2003 ] Paper presented on the session „Solidarity and social norms – models and mechanisms“, Sektion Modellbildung und Simulation, Leipzig, October 2002 * Manuela Vieth detected an error in an earlier version of this paper. We would like to thank her very much for the highly valuable comments. Thomas Voss’ work on this paper has been supported by a fellowship of the Netherlands Institute for Advanced Study in the Humanities and Social Sciences (NIAS) in the academic year 2001/2002. Research grant Vo 684/5-1 of Deutsche Forschungsgemeinschaft is also gratefully acknowledged.