  Should weakly altruistic behaviours be classified as altruistic or selfish? This question is not merely semantic; for the real issue is whether the conditions under which weak altruism can evolve are relevantly similar to the conditions under which strong altruism can evolve, or not. Many authors argue that the answer is ‘no’, on the grounds that weakly altruistic behaviours are individually advantageous, so can evolve with no component of kin selection or donor-recipient correlation, unlike strongly altruistic behaviours (Grafen 1984, Nunney 1985, West et al. 2007). To appreciate this argument, consider a game-theoretic scenario similar to the one-shot Prisoner's dilemma of section 4, in which organisms engage in a pair-wise interaction that affects their fitness. Organisms are of two types, weakly altruistic (W) and non-altruistic (N). W-types perform an action that boosts their own fitness by 10 units and the fitness of their partner by 20 units; N-types do not perform the action. The payoff matrix is thus: