The production of a public good by the contribution of individual volunteers is a social dilemma because, if contributing is costly, volunteers have a lower fitness than individuals that do not contribute. Therefore it is generally believed that cooperation is unstable in public goods games and that public goods can be produced only in the presence of repeated interactions (which allow reciprocation, reputation effects and punishment) or relatedness (kin selection). Cooperation, however, often occurs in the absence of iterations and relatedness. We show that when the production of the public good is a Volunteer´s Dilemma, cooperators and defectors persist in a mixed equilibrium, without iterations and without relatedness. This mixed equilibrium disappears if the production of the public good is modeled as a Prisoner´s Dilemma. We also show that the Prisoner´s Dilemma and the Volunteer´s Dilemma are the two opposite extremes of a general public goods game, and that all intermediate cases can have a mixed equilibrium like the Volunteer´s Dilemma. This changes our perspective on public goods: the coexistence of cooperators and defectors is a typical outcome of most social dilemmas, and the production of public goods in general requires neither relatedness nor iterations.
István Scheuring is a senior research fellow at Loránd Eötvös University, Budapest and at the Hungarian Academy of Sciences. He studied physics and biology at Loránd Eötvös University (PhD in theoretical biology, 1994). His interests cover many different areas of theoretical biology, including ecology, the origins of life, and the evolution of cooperation and mutualism. Dr. Scheuring has been a visiting fellow at the Collegium Budapest in 1995, at the Max Planck Institute of Complex Systems in Dresden, Germany 2006 and at the KLI in 2009-2010.