Figure 1

Mean-field approximation. Cooperation frequencies in the TS-plane achieved by a population arranged on a fully connected network. This result is in full accordance with the expected Nash equilibrium for the PD. Parameter S indicates the payoff obtained by cooperators that face defectors, that in turn gain a payoff equal to T (when facing cooperators)—see matrix 1. Colors indicate the averaged degree of cooperation achieved by the population. We recall that red indicates strong cooperation, while blue defection (i.e., no cooperation).