trait, though the process is more complicated than pure inheritance because other biological factors influence whether the genes that cause left-handedness are switched on in the growing foetus or are left unactivated. This means that many people carry the genes that appear to cause left-handedness, but are not actually left-handed themselves. If you are an identical twin and left-handed there is only a 12% chance that your identical sibling will also be left-handed, even though he or she will carry the left-handed genes.32 Genes, by themselves, are not enough.
Is there a gene that causes homosexuality? In 1993 Dan Hamer and his colleagues announced the discovery of just such a gene,33 but its existence is still controversial and Hamer’s research has been under assault ever since the announcement. Some scientists complained that the sample from which he had drawn his genetic material was skewed because it comprised only self-proclaimed gays, while others believed he had over-simplified a horrendously complex process.34 The gay lobby disliked Hamer’s research because it suggested a frightening scenario: if gayness was indeed genetically induced then pregnant women might choose to abort a foetus if they discovered that it carried the homosexual gene.35
That concern is genuine. Parents regularly abort foetuses that carry the Down’s Syndrome gene, and that process could easily be extended to provide ‘designer’ babies, tested in the womb and guaranteed to be free of any unwanted genetic trait – whether of hair colour, sexual orientation or left-handedness. The danger is real, but some way off because the process of how genes are expressed is still not wholly understood. Dan Hamer’s ‘gay’ gene might not cause gayness at all, but merely predispose its possessor to the real causes of homosexuality. If that is true, then for the gay gene to be expressed requires further biological action, and that seems most likely because, confusingly, some homosexuals do not possess the so-called ‘gay’ gene at all. Something else, either an unidentified gene or a biological process, made them gay.
The most likely explanation is a biological process that occurs in the womb. Few scientists dispute the influence that is wielded on the developing foetus by hormones, and hormones are central to the process of sexual development. Hormones (among other things) are the ‘switches’ that activate genes, and in turn those genes instruct the growing foetus whether to be male or female. It is to that process, and to its effects on sexual orientation, that we must now turn.
It seems obvious that hormones will determine our gender, but, until very recently, the further assertion that the same hormones determined our brain structure into either a male or a female pattern was very controversial. The idea of a differently patterned brain was anathema to most hardline feminists, who wanted to assert their equality (by which they too often meant sameness) to men; if it could be proved that the brains of men and women were distinctly different in structure and function, then it was an alarmingly short step to believing they might be different in abilities as well. Their problem was that male and female brains did turn out to be distinctly different, and what was once a politically controversial theory quietly became the standard stuff of undergraduate textbooks.
Women might take some consolation from the fact that the basic human template is female. Every foetus begins as a female, but, at six weeks, boys begin to be made by a flood of hormones that drench the developing baby and so convert sugar and spice into slugs and snails. The male foetus is capable of making high levels of androgens – or male hormones such as testosterone. The male starts making the hormone at six weeks. It is not a one-off action: it goes on for months in the womb, each successive dose of hormones doing its bit to turn what was a female into a male. For our purposes, the crucial moment appears to come in the third month of pregnancy when a heavy dose of testosterone affects the developing boy’s brain. Among other effects this dose of testosterone sets his sexual orientation. Up until now ‘his’ brain has been effectively female and like any female his sexual longings, if he had any, would be focused on males; the testosterone drench reverses his polarity and from now on he will be attracted to girls. But if the testosterone dose falls below a critical high level the brain remains female. All foetuses receive some testosterone, even those destined to be born girls, but samples taken from the amniotic fluid suggest that the ‘brain-sexing’ drench of testosterone is eight to nine times higher for boys than it is for girls.36
Now it does not take much imagination to hypothesize that a shortfall in testosterone at the crucial moment of pregnancy might leave an otherwise conventional male with a female sexual orientation. The result would be an adult man who is, quite naturally, attracted to other males. It is possible that a ‘gay gene’ influences the crucial testosterone levels, but whether that is the case or not, the evidence for this hormonal cause of homosexuality is overwhelming.
Overwhelming but not absolutely proven, for we cannot experiment on developing human foetuses to test the hypothesis. So the evidence, however compelling, is indirect. A study by Lee Ellis has shown that mothers who suffer from severe stress (stress reduces the levels of testosterone) during the third month of pregnancy produce a higher than average incidence of homosexual offspring.37 We cannot prove this absolutely because, rightly, ethics forbids us to experiment on human foetuses, but animal studies support the biological explanation. Humans and rats share specific sex hormones and have similar areas at the base of their brains that control sexual behaviour (the hypothalamus). Roger Gorski and his team have demonstrated that a rat’s sexual orientation can be changed at will by manipulation of foetal hormones.38 A male rat deprived of testosterone in its early foetal stage becomes female in its sexual behaviour. No amount of male hormones given in later pregnancy can reverse this behaviour – the animal’s brain has been permanently organized into the female pattern.
A female rat dosed in the same critical period with male hormones becomes masculine in its sexual behaviour and, again, no amount of later female hormonal influence will reverse the orientation. Gorski’s work suggests that there is a critical stage during the development of the mammalian brain when male or female sexuality is established.39 Once that critical moment is passed no amount of ‘corrective’ hormone will make any difference. The sexual orientation of rats, and most probably that of humans too, is determined in the womb.
The researchers went on to investigate whether there were any structural differences between the brains of male and female rats and discovered an area of the hypothalamus that was seven times larger in the male brain than the female brain. ‘The difference is so large,’ one researcher wrote, ‘that you can see it with the naked eye.’40 Other researchers agreed with the finding, and confirmed, moreover, that it was just this area of the brain that controlled sexual behaviour. ‘Experimental damage to this area produces a marked and significant reduction in masculine sexual behaviour.’41
Roger Gorski and his team then experimented by manipulating the hormones delivered to a developing rat foetus to see if they made any difference to the hypothalamus, and discovered they could determine the hypothalamus’s structure by restricting the hormone dosage.42 This was a breakthrough discovery for, though it had been inferred that hormones changed brain structure and behaviour, it was the first time anyone had demonstrated that process in a laboratory. Gorski and his team had shown that sexual orientation was determined by hormones, and that the brain’s physical structure could be manipulated by the same hormones, and all this in an area of the brain that was well established as central to controlling sexual behaviour.
These experiments have been replicated by many different laboratories and in other animal species,43 and inevitably lead to the question of whether homosexuality occurs outside the laboratory in species other than man. For a long time this has been denied (thus providing ammunition to those who ascribe homosexuality to social or cultural causes), but more recent research has demonstrated frequent male–male sex in primates and in mountain sheep. Such sex is often ‘rape’, in which the dominant male uses sexual assault to demonstrate his higher status, but genuine, consensual homosexuality has been observed in domestic sheep. It was first observed in Iowa where farmers were disturbed by the number of ‘dud studs’: rams that were not interested in ewes, but in other
