Fig. 2.
Fitness plot of the average within-group fitness of altruistic punishers, selfish nonpunishers, and the invading altruistic nonpunishers strategy at varying frequencies of altruistic punishers. At invasion at a frequency >0, the fitness of altruistic nonpunishers resides above that of altruistic punishers by the value of the cost of punishment c(psn). Arrows indicating the population trajectories illustrate the failure of altruistic punishers to maintain the highest fitness in their previous region of selective advantage, resulting in their decrease in frequency and the establishment of selfish nonpunishers as an ESS.