Figure 1.

The evolution of cooperation is strongly affected by the presence of punishment. (a) The long run average frequency of cooperation (i.e., the sum of the frequencies of contributors and punishers) as a function of group size when there is no punishment (p = k = 0) for three different conflict rates, 0.075, 0.015, and 0.003. Group selection is ineffective unless groups are quite small. (b) When there is punishment (p = 0.8, k = 0.2), group selection can maintain cooperation in substantially larger groups.