Fig. 2.

The dynamics of competing social norms in K = 2 groups, for different levels of insularity (out-group interaction parameter ω). The social norm used in group 1 is indicated at the top of each column; the norm used in group 2 is indicated at the right of each row. Each panel shows the rate of change of group 1’s size, ${\dot{\nu }}_1$, as a function of its current size, ν₁, with different colors corresponding to different values of the out-group interaction rate, ω. In all cases, insularity (ω < 1) tends to make Stern Judging even stronger in competition with every other norm. In all plots, b = 2, c = 1, u_a = u_x = 0.02. Values of ω are as inset in the Scoring–Scoring figure. The dotted line for ω = 1.0, when Stern Judging competes against Shunning, indicates the special situation that, as Shunning grows, the population passes through a regime where it is vulnerable to invasion by unconditional defectors.