Another important factor in determining the direction of evolution, and perhaps ultimately the only factor that needs to be considered, is reproductive success. In order to pass on its genes to the next generation an animal has to do more than just survive the environment; he or she has to mate successfully with a member of the opposite sex and raise offspring who themselves will be reproductively successful. Individuals who do not breed are doomed to genetic extinction.

Now in this domain it is clear that the optimal strategies for males and females are quite different. At least this is so for mammals, in which the burden of child production falls on the female. In mammals, the offspring grow inside the body of the female and are dependent on her for succour for some time thereafter. Later on in life the protection of two parents may be better than one, but that is usually much less essential. Generally speaking, mothers invest more in their offspring than do fathers.

For the male, multiple partnerships may be profitable. Theoretically he has the capacity to impregnate several females at the same time, or at least in quick succession, so that their periods of pregnancy overlap. He invests little time and energy in the act of fatherhood and it is therefore to his genetic advantage to distribute his sexual attentions widely. If there is any chance at all that some of the offspring he has sired will survive to sexual maturity themselves, then he maximizes his breeding potential by sequestering several mates or even behaving promiscuously, moving quickly from one female to the next. Impregnating the ‘wrong’ female at one time (one that is not ideally ‘fit,’ genetically speaking) does not preclude getting the ‘right’ one pregnant on another occasion. Millions of male sperm are generated for every one egg produced by the female, so wastage is relatively unimportant. Whereas a woman produces only a few hundred eggs in her entire lifetime, a man could theoretically fertilize every woman in Britain with a single ejaculation. This capacity is inevitably associated with the evolution of polygamous urges in men.