Talk of a "gay gene" has raised fears that genetic engineering could eliminate homosexuality. But Simon Le Vay argues that its discovery may have a positive effect on public attitudes

A recent spate of scientific reports suggesting a biological basis for sexual orientation has fuelled public debate about homosexuality. But what does the science say, and how might it impact on public attitudes and gay rights?

In 1991, while on the faculty of the Salk Institute in San Diego, California, I published the results of my research on brain structure and sexuality in men. A group of cells in the hypothalamus, I reported, was more than twice as large in heterosexual men as in gay men. This cell group lies in a brain region involved in the generation of "male-typical" sexual drive, and is usually larger in men than in women. So my findings suggested that there might be a close connection between the biological processes of brain development and men's ultimate experience of sexual attraction to the same or the other sex.

Around the same time, several research groups reported on studies of twins. Although the exact results differed from study to study, there was general agreement that identical twins (who share all the same genes) are much more likely to share the same sexual orientation than are fraternal twins (who share only about half their genes). These findings suggested that sexual orientation in both men and women has a substantial inherited component.

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This conclusion was bolstered, for men, by the molecular genetic research of Dean Hamer and his colleagues at the National Cancer Institute in Washington DC. By studying the DNA of pairs of brothers, both of whom were gay, they found a region of the X chromosome, named Xq28, where such brothers have an above-chance likelihood of inheriting the same genetic sequences. The heterosexual brothers of these same men, in contrast, generally do not inherit the same sequences.

Hamer's findings suggest there is a gene on the X chromosome that influences (but does not fully determine) men's sexual orientation. At this point, the gene itself has not been identified. Also, Hamer's group has not so far been able to pin down the location of any gene influencing sexual orientation in women.

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These scientific reports are part of a larger body of biological research on human sexuality that suggests both sexual orientation and other gender-related traits are strongly influenced by the sexual differentiation of the brain before birth, a process that itself is under the influence of sex hormones. The science does not yet provide a fully coherent account of psychosexual development. Nevertheless, it suggests that theories that see sexual orientation and gender as developing out of a web of family and other interpersonal relationships - are at the least inadequate and perhaps wholly incorrect.

Public reactions to these reports have been mixed. To some people, especially to many gay men like myself, a biological explanation resonates with their own sense of having been "born that way". It offers gay people an objective identity, one that has too long been denied them. In effect it counters the view, put forward by St Paul, that there is nothing more to homosexuality than straight people acting wrongly.

In the United States, where the gay rights debate takes place in a highly confrontational setting, the biology indeed offers ammunition to the pro-gay side: it strengthens the notion that homosexual feelings and behaviour are the natural attributes of a "sexual minority," as entitled to explicit legal protection as a minority defined by skin colour. For this reason, high-profile gay-rights cases, such as the Amendment Two case in Colorado, have featured prolonged wrangling between expert witnesses concerning the accuracy and significance of the recent studies. Although there is as yet no global resolution to the legal status of gay people in the US, the "biological argument" has clearly had an effect: three federal statutes and numerous state laws now recognise sexual orientation as a nondiscrimination category like skin colour.

The situation in Europe is somewhat different. Early this century the gay rights movement was led by the German sexologist Magnus Hirschfeld, who promoted a biological theory of homosexuality based on the "third sex"- the notion that gays and lesbians were psychic (and perhaps even bodily) intermediates between "full men" and "full women." In the post-second world war era, however, biological theories became suspect. They were tainted by the spectre of Nazi eugenics, even though the Nazis mostly subscribed to the seduction theory of homosexuality, not to genetic theories. Fanciful ideas were put forward to explain almost every aspect of mental diversity in terms of learning or life experiences, or even as the consequence of arbitrarily applied societal labels. And at the same time explicit oppression of gay people gave way to repressive tolerance: gays and lesbians could live their own lives so long as they accepted second-class citizenship and kept a low profile. By agreeing to this dubious bargain, gays and lesbians in Europe have made the question of causation largely irrelevant to their destiny.

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To some people, biological theories of sexual orientation offer the prospect of a technical fix. "If we could by some form of genetic engineering eliminate these trends, we should - so long as it is done for a therapeutic purpose," said British Chief Rabbi, Lord Jakobovitz, shortly after my research was published. (The ethics adviser to the rabbinical cabinet later explained that the programme was to be a voluntary one.) The history of research is not especially comforting in this regard: hormone theories have led to testicular transplants and hormone "therapy", brain theories to operations on gay men's hypothalamus and so on. In the 1970s German endocrinologist Gunter Dorner suggested the hormone levels in the amniotic fluid of pregnant women be routinely monitored and "corrected" if they were such as to predispose the fetus to homosexuality. (Dorner has now become more positive in his view of homosexuality.) If genes predisposing to homosexuality are identified and sequenced, the prospects for the elimination of gay people through genetic engineering will come a lot closer to reality. Even now, if Hamer's findings are correct, DNA evidence could in rare circumstances be used to say something about the future sexual orientation of a male fetus. This would be in the situation where a fetus has two brothers old enough to know that they are gay. If the fetus shared the same DNA markers at Xq28 as his two brothers, he would be more likely to become gay; if he had different markers, he would be more likely to become straight. The test would be far from 100 per cent accurate but would be right far more often than could be achieved by guesswork. In the future, as the Xq28 gene and other genes influencing sexual orientation are identified and sequenced, the predictive possibilities will become greater, and it will no longer be necessary to have gay relatives for the tests to be applied. The ethical problems associated with such tests are, of course, immense: they are mere examples, however, of the many dilemmas that are going to face us as the Human Genome Project runs its course.

But persecution of gay people is not tied to particular theories of causation. It uses whatever theories are offered, biological or not. Learning theories have led to programmed "unlearning" through aversion therapy and the like. Theories that invoke defective parenting have led to psychoanalytic treatments. What matters more than anything is the prevailing social attitude towards gay people. If malign, this attitude will permit and encourage any form of oppression and "treatment"; if accepting, treatment will not be seen as desirable or necessary.

In large part, it is the actions of gays and lesbians themselves that change social attitudes. As more gay people come out of the closet, reject "repressive tolerance" and demonstrate their own worth to society, it seems likely that the prospects for mistreatment of gay people by biological or any other means will fade, whatever the science may come up with.

Yet the biology does have a role to play. Homophobia results, paradoxically, from a failure to distance gay people. Faced with the fact that some people engage in homosexual behaviour, straight people tend to apply a "test-by-empathy". They put themselves mentally in the shoes of a person engaging in such behaviour and experience the aversion that they, as heterosexuals, would naturally feel in those circumstances. They then apply this aversion to gay people. By contrast, people see animals engaging in all kinds of potentially disgusting behaviour, but they do not hate animals for it because they see animals clearly as "other", and hence do not apply the test-by-empathy. The recognition of otherness, often demonised as a wellspring of human conflict, can also be the basis for mutual acceptance. And the science makes this recognition and this acceptance easier.

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Simon LeVay left the Salk Institute to co-found the Institute of Gay and Lesbian Education.