In this paper we explore the relationship between the individual’s preference for cooperation and the establishment of cooperative norms. Our aim is to provide an experimental test of the evolutionary hypothesis (see Carpenter, 2004, Fehr and Gachter 2002; Gintis 2000; Boyd, Bowles, Gintis and Richerson 2003; Bowles and Gintis 2004), according to which individuals are prepared to punish defectors in experimental social dilemma games because they want to enforce a social (“altruistic”) norm which may conduce to increasing their future payoffs, as in the case of sanctions against free riding behaviour. According to this line of research , the high levels of cooperation we observe in our societies can, therefore, be strictly related to the establishment of social norms which are able to enforce and maintain cooperation in the long run. We study the results of two experiments in which the individuals decided both whether to participate in a common project and the institutional rule according to which the profits of the project had to be shared among each of the participants in the group.