Title

A long standing problem in evolutionary biology is the evolution of cooperati on. Why do cooperators prevail that support others at a cost to themselves, whil e defectors would obtain a higher payoff? Recently, similar problems appear in s ocial communities on the internet. Some users just download from peer-to-peer ne tworks, others also contribute high-quality files. Many people use wikipedia, bu t few write or improve the articles. Different mechanisms for the evolution of c ooperation in these systems can be of interest. For example, visible tags can in dicate that someone is willing to cooperate. Another possibility is indirect re ciprocity, where individuals interact with others based on their reputation.