Rationality and Society, 13: 5-32 (2001) The Role of Social Structure in the Maintenance of Cooperative Regimes Michael D. Cohen ∗ , Rick L. Riolo † , Robert Axelrod ‡ {mdc,rlriolo,axe}@umich.edu September 2000 ∗ School of Information, University of Michigan, Ann Arbor, MI 48109 USA. † Center for Study of Complex Systems, University of Michigan, Ann Arbor, MI 48109 USA. ‡ School of Public Policy, University of Michigan, Ann Arbor, MI 48109 USA. 1