Chapter 100 Evolution of Fairness and Conditional Cooperation in Public Goods Dilemmas Sven Van Segbroeck, Jorge M. Pacheco, Tom Lenaerts, and Francisco C. Santos Abstract Cooperation prevails in many collective endeavours. To ensure that co- operators are not exploited by free riders, mechanisms need to be put into place to protect them. Direct reciprocity, one of these mechanisms, relies on the facts that individuals often interact more than once, and that they are capable of retaliating when exploited. Yet in groups, strategies targeting retaliation against specific group members may be unfeasible, because individuals may not be able to identify clearly who contributed and who did not. Still, they may assess what constitutes a fair in- come from a collective endeavour. We discuss here how conditional cooperation in group interactions emerges naturally and how natural selection leads populations to evolve towards a specific level of fairness (Van Segbroeck et al., Phys. Rev. Lett., 108:158104, 2012), contingent on the nature and size of the collective dilemma faced by individuals. Keywords Reciprocity · N -Player games · Repeated games · Fairness · Evolutionary game theory Darwinian evolution dictates that cooperation, which is the act of helping someone at a personal cost, is not evolutionary viable as a behavioral strategy since others may profit directly from this act and are not required to behave in the same way. S. Van Segbroeck (B ) · T. Lenaerts MLG, Université Libre de Bruxelles, Brussels, Belgium J.M. Pacheco Departamento de Matemática e Aplicações, Universidade do Minho, Braga, Portugal J.M. Pacheco · F.C. Santos ATP-group, CMAF, Complexo Interdisciplinar, Lisbon, Portugal T. Lenaerts AI-lab, Vrije Universiteit Brussel, Brussels, Belgium F.C. Santos DEI & INESC-ID, Instituto Superior Técnico, TU Lisbon, Lisbon, Portugal T. Gilbert et al. (eds.), Proceedings of the European Conference on Complex Systems 2012, Springer Proceedings in Complexity, DOI 10.1007/978-3-319-00395-5_100, © Springer International Publishing Switzerland 2013 827