Mathematical Biology Seminar Manfred Milinski, Max-Planck-Institute for Limnology, Plon Wednesday, April 4, 2007 2:55pm LCB 219 Reputation and punishment in human public goods games Abstract: The problem of sustaining a public resource that everybody is free to overuse emerges in many social dilemmas. Public goods experiments usually confirm that the collective benefit will not be produced. Because individuals and countries often participate in several social games simultaneously, the interaction of these games may provide a sophisticated way by which to maintain the public resource. Indirect reciprocity, 'give and you shall receive', is built on reputation and we found that it can sustain a high level of cooperation. We show, through alternating rounds of public goods and indirect reciprocity games that the need to maintain reputation for indirect reciprocity maintains contributions to the public good at an unexpectedly high level and leads to high profits for all players. Directly punishing defectors can also induce cooperation in public goods games although it incurs salient costs both for the punisher and the punished while reputation mechanisms discipline by withholding action immediately saving costs for the "punisher". Consequently costly punishment should become extinct in environments in which effective reputation building is possible. We study experimentally the interaction between punishment and reputation building and the consequences for cooperative efficiency in public goods games.