Homosexuality is the desire to have sexual relations, either in fantasy or in fact, with a member of one’s own sex rather than a member of the opposite sex. It is an habitual pattern of sexual behaviour which, by its very nature, precludes reproduction, and it is, therefore, appropriate to discuss it in this section. This in no sense implies a negative moral judgement. Indeed, it is possible to argue, on the contrary, that any mode of adjustment which brings sexual fulfilment without procreation is to be welcomed in an overpopulated world. A common condition of great cultural significance, homosexuality constitutes a major puzzle for evolutionary psychology: how is it possible to account for the inclusive fitness of genes which predispose people to homosexual behaviour? Clearly such genes must possess some adaptive advantage otherwise they would have disappeared from the genome. What can this be?