  From these expressions for the fitnesses of the two types of organism, we can immediately deduce that the altruistic type will only be favoured by selection if there is a statistical correlation between partners, i.e., if altruists have greater than random chance of being paired with other altruists, and similarly for selfish types. For suppose there is no such correlation—as would be the case if the pairs were formed by random sampling from the population. Then, the probability of having a selfish partner would be the same for both S and A types, i.e., P(S partner/S) = P(S partner/A). Similarly, P(A partner/S) = P(A partner/A). From these probabilistic equalities, it follows immediately that W(S) is greater than W(A), as can be seen from the expressions for W(S) and W(A) above; so the selfish type will be favoured by natural selection, and will increase in frequency every generation until all the altruists are eliminated from the population. Therefore, in the absence of correlation between partners, selfishness must win out (cf. Skyrms 1996). This confirms the point noted in section 2—that altruism can only evolve if there is a statistical tendency for the beneficiaries of altruistic actions to be altruists themselves.