The figure shows the actual (in blue) and expected (in black) proportion of

-strategists in a population of

individuals who, at each iteration (of time length

), are randomly matched in pairs to play a symmetric 2×2 game. The two possible actions (or pure strategies) in the game are labeled

and

. Thus, each individual in the population is either an

-strategist or a

-strategist. The payoffs of the game are

,

,

, and

(parameters), where, for instance,

denotes the payoff obtained by an

-strategist when he plays with a

-strategist.

At the end of each iteration, after all individuals have played the game, one randomly selected player revises her strategy—

or

—according to the following rule: "I look at another (randomly selected) individual; if and only if she got a payoff higher than mine, I adopt her strategy".