  The fact that correlation between donor and recipient is the key to the evolution of altruism can be illustrated via a simple ‘one shot’ Prisoner's dilemma game. Consider a large population of organisms who engage in a social interaction in pairs; the interaction affects their biological fitness. Organisms are of two types: selfish (S) and altruistic (A). The latter engage in pro-social behaviour, thus benefiting their partner but at a cost to themselves; the former do not. So in a mixed (S,A) pair, the selfish organism does better—he benefits from his partner's altruism without incurring any cost. However, (A,A) pairs do better than (S,S) pairs—for the former work as a co-operative unit, while the latter do not. The interaction thus has the form of a one-shot Prisoner's dilemma, familiar from game theory. Illustrative payoff values to each ‘player’, i.e., each partner in the interaction, measured in units of biological fitness, are shown in the matrix below.