Figure 1. “In-group/out-group” evolutionary dynamics.

Individuals are distributed over groups. The interaction structure of the population is given by a two-colored graph: from the point of view of a focal individual, its encounters are either in-group or out-group. Individuals may use different strategies for in-group and out-group interactions, and even different games can be played within and between groups. Individuals interact with everyone else according to their prescribed behavioral strategies. Both individual strategies and group memberships are updated proportional to payoff.