Figure 3.

Evolution of η for cooperators and defectors. The solid (dashed) lines show the cumulative distribution C(η) of the parameter η of cooperators (defectors) for different values of T (W = 5, β = 0.005, N = 10³, z = 30). The inset provides the level of cooperation for values of T between 1 and 2.5. The values of η of all individuals are uniformly distributed in [0,1] at the start of each evolution, as indicated in black. Cooperators that react swiftly to adverse ties are strongly favoured by natural selection only when the tension of the game is high (1.8 ≤ T ≤ 2.1). Swift defectors, on the other hand, are always selected for any tension of the game, but the strength of this selective pressure drops as T increases. C(η₀) is defined as the fraction of individuals who have η ≥ η₀.