Fig. 4.

Fitness plot of the average within-group fitness of selfish punishers and altruistic nonpunishers. Arrows indicate the population trajectories at any given frequency. Both altruistic nonpunishers and selfish punishers exhibit negative frequency dependence. Each strategy maintains a greater fitness when present at low frequencies and loses the fitness advantage as that strategy becomes more common, causing the population to settle at the stable equilibrium point (p̂).