CESifo working paper: Empirical and Theoretical Methods 3378
Nash proposed an interpretation of mixed strategies as the average pure-strategy play of a population of players randomly matched to play a normal-form game. If populations are finite, some equilibria of the underlying game have no such corresponding 'mass-action' equilibrium. We show that for mixed strategy equilibria of 2 × 2 games, the requirement of such a correspondence is equivalent to neutral evolutionary stability.
mass action finite population games evolutionary stability