cooperators. The problem is that all these arguments, as well as the very definition of ‘cluster’, depend crucially on the structure of the network. The most important feature in this regard is the clustering coefficient $C$, which measures how connected is the neighbourhood of each node, on average. The existence of local transitive relationships, closely related to the clustering [@was], is what defines the possibility of survival of small clusters of cooperators. Paradoxically, it will be also responsible for the negative effect that an isolated cooperator may have on incipient cooperative clusters.

Here we use the definition of global clustering coefficient of Watts and Strogatz [@watts]. For each node $i$, its local clustering coefficient is defined as the quotient between the number of links joining nodes of the neighbourhood of $i$ divided by the total number of possible links ($k_i(k_i+1)$). $C$ is then defined as the average over $i$ of all local clustering coefficients. We study the influence of $C$ on the evolutionary dynamics of the iterated prisoner’s dilemma, but keeping the degree distribution constant, to disentangle both contributions. For this we analyze regular networks (i.e. with the same number of neighbours for every node) with different values of $C$, 