There are many people who seem to be physically normal representatives of one sex or the other but who prefer sexual contact with members of their own sex. Increased tolerance of this behaviour in recent decades has led some people to argue that no theoretical explanation of homosexuality is called for, any more than one needs to ‘explain’ heterosexuality. To the evolutionary theorist this is nonsense; heterosexuality has obvious survival benefits while homosexuality does not, so the latter is bound to arouse more scientific curiosity and demand special explanation.

A popular biological theory says that, during the process of masculinization of the developing male, some critical nuclei in the brain that are concerned with sexual preference somehow escape exposure to the circulating male hormone and the sex target ‘switch’ is therefore not reset in the male direction (Feldman and MacCulloch, 1971). This leaves us with an individual who is male in body but female in sexual preference. Less commonly, the theory supposes, this sex-target switch get accidentally masculinized in a constitutional female who otherwise remains normal, thus yielding a lesbian.

The strength of this hypothesis is shown in a recent review of research by Ellis and Ames (1987), although the effect of pre-natal sex hormones is probably more complex than that stated above. Apart from a masculinizing process that is largely under the control of foetal testosterone, we may have to recognize some separate feminizing effects upon the brain under the control of female hormones, especially luteinizing hormone (LH). This means that people can be more or less feminine, as well as more or less masculine, these two dimensions yielding a wider array of sexual types.

In addition, it is necessary to separate brain settings for sex orientation from brain settings for sex-typical behaviour. It is possible, for example, to have an individual who is macho both in body and personality but who prefers male sex partners, or vice versa. This is because the masculinization/feminization effects occur in different parts of the brain and, more importantly, at different times during pre-natal development. Indications are that sex orientation in humans depends critically upon the hormone balance prevailing during the third and fourth months of pregnancy, while secondary sex characteristics and sex-typical behaviour patterns are influenced more by hormones circulating during the fifth and sixth months of pregnancy. If the hormone balance changes from one phase of foetal development to the next, inconsistencies between sexual orientation and sex-role behaviour may be observed. Sex orientation is fixed relatively early in the old ‘limbic’ part of the brain, whereas sex-role behaviours are laid down later on in pregnancy in more diverse, ‘newer’ parts of the brain.

Ellis and Ames go on to review an impressive body of animal and human research which establishes pretty much beyond doubt that what we call homosexuality (an inversion of the normal relationship between body type and sexual orientation) may be created by five different procedures:

1. Direct alteration of hormones (by injection or castration) during foetal development.
2. Using drugs to block or augment the effects of androgens. The list of drugs which may at least partially divert masculinization of the brain include Depo-Provera, cyproterone acetate, barbiturates, diazepam and marijuana.
3. Exposure of the mother to severe emotional stress. Stress hormones carried in the mother’s blood may cross the placenta and interfere with testosterone production in the baby.
4. In some circumstances the immune system may react against the biochemicals necessary for sexual differentiation, identifying them as foreign substances and attacking with antibodies that destroy them. Ultimately this can also reverse sex orientation.
5. Raising young animals in isolation from the opposite sex also impedes heterosexual adaptation, though the problem may be one of social skill and confidence rather than sex orientation as such.