 (As before, Prob(W partner/W) denotes the conditional probability of having a weakly altruistic partner given that one is weakly altruistic oneself, and so-on.)  From these expressions, it is easy to see that W(W) &gt; W(N) even if the there is no correlation among partners, i.e., even if Prob(W partner/W) = P(W partner/N) and P(N partner/W) = P(N partner/N). Therefore, weak altruism can evolve in the absence of donor-recipient correlation; as we saw, this is not true of strong altruism. So weak and strong altruism evolve by different evolutionary mechanisms, hence should not be co-classified, according to this argument.