Fig. 1.

Phase portraits of classical games with $r_1=r_2=1$. In the Prisoners' dilemma, (a), strategy 2 (defection) dominates strategy 1 (cooperation). In the Hawk–Dove game, (b), the two strategies coexist. The Stag hunt game, (c), is bistable. The dashed lines in (b) and (c), given by $x_2/x_1=(a-c)/(d-b)$, are invariant under the dynamics.