Imagine a population of individuals with the option to help one another or not. Random pairs of players are chosen, of which one is the potential donor of some altruistic act and the other is the recipient. The donor can cooperate and help the recipient at a cost c to himself, in which case the recipient receives a benefit of value b (with b . c). If the donor decides not to help, both individuals receive zero pay-off. Each player has an image score, s, which is known to every other player. If a player is chosen as a donor and decides to cooperate then his (or her) image score increases by one unit; if the donor does not cooperate then it decreases by one unit. The image score of a recipient does not change.

Nowak, M. A., & Sigmund, K, Evolution of Indirect Recip…, loc. 36