The models above are based on computer simulations, but we can derive analytical insights from a simplified model. Suppose that there are only two image levels, 0 (for bad) and 1 (for good). The image of a player depends on his or her last action as a donor: players who defected have score 0, and players who cooperated have score 1. Let us only consider two types of player: first, defectors, who never provide assistance; and second, discriminators who help players having image 1, but not players having image 0. A given player knows the score of only a fraction, q, of the population.

DEFINE: Introduces the two-level (0/1) image score model and its player types: defectors and discriminators.

Nowak, M. A., & Sigmund, K, Evolution of Indirect Recip…, loc. 74