  The question we are interested in is: which type will be favoured by selection? To make the analysis tractable, we make two simplifying assumptions: that reproduction is asexual, and that type is perfectly inherited, i.e., selfish (altruistic) organisms give rise to selfish (altruistic) offspring. Modulo these assumptions, the evolutionary dynamics can be determined very easily, simply by seeing whether the S or the A type has higher fitness, in the overall population. The fitness of the S type, W(S), is the weighted average of the payoff to an S when partnered with an S and the payoff to an S when partnered with an A, where the weights are determined by the probability of having the partner in question. Therefore,