 If the correlation between partners is sufficiently strong, in this simple model, then it is possible for the condition W(A) &gt; W(S) to be satisfied, and thus for altruism to evolve. The easiest way to see this is to suppose that the correlation is perfect, i.e., selfish types are always paired with other selfish types, and ditto for altruists, so P(S partner/S) = P(A partner/A) = 1. This assumption implies that W(A)=11 and W(S)=5, so altruism evolves. With intermediate degrees of correlation, it is also possible for the condition W(S) &gt; W(A) to be satisfied, given the particular choice of payoff values in the model above.