Cycles of cooperation and free-riding in social systems Y. P. Ma, S. Gonçalves, S. Mignot, J.-P. Nadal and M. B. Gordon Eur. Phys. J. B, 71 4 (2009) 597-610 Published online: 07 October 2009 DOI: 10.1140/epjb/e2009-00325-x