It is clear, however, that the cause of homosexuality lies in some unusual balance of hormonal influence in the womb but not later on, a fact that further supports the idea that the mentality of sexual preference is affected by prenatal sex hormones. This is not incompatible with the growing evidence that homosexuality is genetically determined. The "gay gene" that I will discuss in the ext chapter is widely expected to turn out to be a series of genes that affect the sensitivity of certain tissues to testosterone. It is both nature and nurture.

It is no different from genes for height. Fed on identical diets, two genetically different men will not grow to the same height. Fed on different diets, two identical twins will grow to different heights. Nature is the length of the rectangle, nurture the width. There can be no rectangle without both. The genes for height are really only genes for responding to diet by growing.

The taboo against homosexuality is especially interesting. There is not generally accepted biological explanation of homosexuality and superficially it does not appear to be adaptive. Nevertheless, evidence is accumulating that there is an inherited predisposition for homosexuality. Assuming this is the case, the taboos of the past would, paradoxically, have favoured the survival of these genes by forcing the people who carried them, against their wishes, to marry and have children.

This suggests an interesting prediction for the future. As horizontal transmission increases the taboo should lose its power and so can be expected to disappear, as indeed it is doing in many societies. Homosexuals are then free to have sex with other homosexuals, to have long-term relationships with their own sex, and not to have children at all. The short-term effect is much more overt homosexual behavior and acceptance of that behavior by everyone, but the long-term effect may be fewer genes for homosexuality.