Fig. 3.

Triangle plot illustrating the population trajectories for altruistic nonpunishers (AN), altruistic punishers (AP), and selfish nonpunishers (SN) at all possible initial frequencies. Each corner represents the population as that pure strategy. Both altruistic punisher and selfish nonpunisher strategies exhibit positive density dependence, attracting populations to their respective corners. Under conditions of AP advantage, populations will approximate the AP–AN axis, and because of the absence of the SN strategy both strategies will exhibit equal fitness. The population will traverse the AN–AP axis via drift, allowing the population to drop below the region of AP attraction and become vulnerable to SN invasion and subsequent fixation.