Before revision:  The effect depends nontrivially on the combination of the social network structure (degree distribution and assortativity), the type of social dilemma, and the presence of Trojan horses , i.e. defectors placed within clusters of cooperators.
After revision:  The effect depends nontrivially on the combination of the social network structure , with positive effect in standard scale-free networks and no effect in standard Poisson networks. Furthermore, when degree assortativity is increased such that individuals cluster with others of similar social connectedness, we find that bridge areas between social clusters can act as barriers to the spread of defection, leading to strong enhancement of cooperation in particular in Poisson networks. But this effect is sensitive to the type of social dilemma, and the presence of Trojan horses , i.e. defectors placed within clusters of cooperators.