crucial effects of spatial distribution were finally noticed [@now0; @now3]. Since then, several mechanisms for the evolution of cooperation have been proposed. Some of them are summarized in [@now1]: kin selection, direct reciprocity, indirect reciprocity, group selection and network reciprocity. Here we have chosen to focus on this last mechanism, which is associated to the fact that a cooperative individual can take advantage of the topology of the network to form clusters of cooperators that are often resilient to the invasion of cooperators.

Studies about the effect of network reciprocity have dominated the literature on spatial distributed games in the last years [@now3; @kup1; @kup2; @sza2; @lib; @oht; @roc]. They were the response to the need of studying the evolution of the strategies of players of a game beyond the simplifying assumption of a well-mixed population, where everybody interacts equally likely with everybody else. The observation that real populations are not well mixed and the fact that spatial structures could affect the evolution of a game and the strategies of the players demanded a new approach. A natural step was to consider complex networks as models for the underlying topology characterizing the spatial or social structures. In the case of 