At the end of each iteration, after all individuals have played the game, one randomly selected player from each population revises her strategy— or —according to the following rule: "I look at another (randomly selected) individual in my population; if and only if she got a payoff higher than mine, I adopt her strategy". Thus, the game is played between individuals of different populations, but imitation takes place within each population.

The figure shows a simulation of the proportion of -strategists in each population (in white), its expected dynamics (in dashed red), and the phase plane of the expected dynamics (mean field) in the background.