The game

"No instinct has been produced for the exclusive good of other animals, but each animal takes advantage of the instincts of others" (Darwin 1859).
Ever since Darwin the evolutionary success of a seemingly obvious contradiction to his statement has raised the interests of naturalists all over the world: some cooperating animals are clearly mutualistic or even altruistic. Before the 1960s only a few scientists attempted to understand the evolutionary processes underlying cooperation. since group selection seemed to explain cooperative societies. Yet. research in later years could not support a pervasive group-benefit view of selection; how then can cooperative genotypes spread in an environment of selfish genes? Currently, the evolution of cooperation can be divided into several general categories: e.g. 1) by-product mutualism where cooperation is an incidental outcome from genuinely selfish behaviour (Dugatkin et al. 1992) kin-selected altruism (Hamilton 1964) with its climax in the social insects and 3) reciprocal altruism (Trivers 1971) among unrelated individuals. The 'Prisoner's Dilemma' is used as the standard metaphor to conceptualise the conflict between mutual support and selfish exploitation among interacting non-relatives in biological communities.

Fig. 1. The payoff matrix of the Prisoner's Dilemma: The two participants have two options: to cooperate or to defect. The payoff to player one is shown. Obviously, he gains more by defection not only if player two cooperates (5 instead of 3 points), but also if player two defects (1 instead of 0 points). Since this is also valid for player two, both end up defecting and score instead of 3 points each if they had cooperated. Hence the dilemma.

It has yielded a plethora of investigations concerned with a theory of cooperation based on reciprocal altruism. The Prisoner's Dilemma is a simple two-person game, where each player (for instance two prisoners accused of the same crime) can choose either to cooperate (C) or to defect (D = not cooperate). Fig. 1 shows the payoff matrix: if one player defects, the other has the option to cooperate yielding S (the sucker's payoff or to defect and obtain P (the punishment for mutual defection). On the other hand. if the opponent cooperates, one receives R (the reward for cooperation) for a C or gains T (the temptation to defect) for a D. Provided the payoffs satisfy

T>R>P>S, with R>(S+T)/2, (1)

and the players meet only once, each player should defect no matter what the adversary does, in order not to become the 'sucker'. In a population of interacting pairs of individuals as described above, no single mutant adopting a different strategy can invade and secure a foothold. Defection is the primeval state and the only evolutionary stable strategy (ESS; Maynard Smith 1982). However, when played repeatedly, there is no trivial answer to the question of how cooperation can arise from an non-cooperative state.