extremes: completely ordered (large $C$) and completely disordered (small $C$) networks. For this last class we concentrate on lattice networks with $k=6$ and $k=8$ which have clustering coefficients $C=2/5$ ($k=6$) and $C=3/7$ ($k=8$),

First, we analyze the fate of a cluster of 3 cooperators. In the case of the lattice networks with $k=6$ and $k=8$, there are two and three possible configurations, respectively (see Fig.\[k8t\]). However, all of them are unstable because, thanks to the large clustering, some neighbors of the cluster can be connected to 2 or 3 cooperators in the cluster, having thus a larger payoff than any of them. On the other hand, in a random network there is a finite probability ($1-3k^2/N+O(k/N)$) that all the neighbors of a 3-cluster are not neighbors of more than 1 cooperator, and thus the cluster is stable. Furthermore, if this cluster does not disappear it will grow to become a cluster of $k+1$ cooperators (a central cooperator surrounded by cooperators). This cluster, in turn, has a non vanishing probability of continuing its expansion. For example, if there is a link joining two of the new surrounding cooperators (which happens with probability $(k/N) k(k-1)/2)$, the cluster grows by turning into cooperators 