 By now it should be clear what is wrong with this type of argument—it does not follow from the fact that altruistic individuals are less fit than selfish ones in every population that populations which average over selfish and altruistic individuals cannot be fitter than populations with just selfish individuals. It could be that being an altruist is correlated with being in a population with more altruists, and that populations with more altruists are fitter. This dispenses with the naive argument. Note, however, that within every single group selfish individuals are fitter, so if the groups change membership only through reproduction (as opposed to migration and mutation) then over enough generations every group will end up consisting only of selfish individuals. So whether groups selection can occur depends on additional facts about population structure and dynamics. Hamilton’s (1964) Kin Selection theory explains how altruism can evolve in cases where altruists are more likely to associate with other altruists (possibly because it runs in the family).