Equilibria for a population of DISC players under heterogeneous assignment rules. (a), (b) Cooperativeness (ψSTand ψJG) and ingroup bias (ρST and ρJG) for groups employing ST and JG. (c), (d) Payoff difference between a player in a ST group and that in a JG group (ΠJG−ΠST). We set θ = 0.6 and c = 1. We also set M = 8 in (a) and (c), M = 20 in (b) and (d), and vary the number of JG groups (i.e., m) and b.