On the surge of altruism

To date, many arguments to explain altruistic behavior were based on `kin selection'. This note shows that evolution can sustain altruism even if the players are not necessarily genetically linked. Depending on the payoff parameters in a prisoner's dilemma game, there can be four distinct types of equilibria, for each of them the proportion of altruists in the population is described. The author is grateful to an anonymous referee for valuable comments. Responsible editor: Alessandro Cigno.