The Mysterious Gene For Homosexuality

Does it matter if there is a genetic tendency for being gay? It depends whom you ask.

Scientists would say that this is part of the current activity, which is huge, that is trying to map the function of the human genome, and assess how far a person’s genetic makeup contributes or determines his/her characteristics (for example, personality) or risks (for example, for depression, or schizophrenia, or becoming a drug addict). Those who consider being gay a disadvantage in life (which it still is, in certain societies), might regard gay people differently if they knew that being gay was an inherited trait, rather than a consequence of life events, such as a particular type of upbringing, or mixing with certain sorts of friends or even a deliberate decision. The law also might take a different view in those countries (again, there are still many) in which being gay is a criminal offense.

An early study claimed that if one pair of (male) identical twins (who have identical genetic patterns) was gay, there was a 100 percent chance of the other one also being gay. If true, this would suggest a huge genetic contribution. These twins are compared with non-identical ones, who have different genetic makeups: if the result were to be the same, it indicates a common environmental factor (for example, the family). It was quickly shown that, in fact, the real chance of two identical twins being gay was around 40-50 percent, whereas it was less in non-identical ones, though still somewhat greater than in the general population. So there was a genetic effect, but also an environmental one.

The search was on for the responsible gene. A dramatic claim was that there was a single genetic variation, the "gay gene," that was found in gay men. Furthermore, it was on the X chromosome, which is inherited from the mother. Alas, like so many other early reports of a "gay gene," this one could not be repeated. It’s become apparent that reliable genetic studies of this kind need very large numbers of subjects, far more than this (and many other studies) used.

And so the matter rested—until now, when a huge new study has just been published. It studied nearly half a million people, whose data had been stored either in a large UK biobank, collected to study genetic variations underlying a range of physical and mental disorders, or by the commercial company 23andMe. This immediately raises questions about selection. Both sets are largely of European descent. Assessment of sexuality was by self-report. Most subjects were middle-aged. Nevertheless, the results of this unique study are fascinating.

As you might expect, most subjects classed themselves as entirely heterosexual. There was another, much smaller, group who said they were entirely gay (about 6 percent), and a scattering of even smaller numbers who claimed to be bisexual. This fits with other population surveys.

The genetic analysis was surprising. Firstly, they found very few genes whose variations were correlated with being gay, or even having a single same-sex partner. There were five common to both genders, but only 2 for men and one for women (they were different). One of the combined genes replicated results of two previous studies, which is reassuring. But nothing on the X chromosome. Overall, genetic variations accounted for only around 1 percent of the likelihood of being gay. Either genes contribute very little, or they had missed the ones that do. They studied only one sort of genetic variation, and there are several others. So it’s possible that there are still genetic influences of a different sort to be found. But as it stands, there’s no sign of a "gay gene."

This huge study allowed them to look at other types of associations between being same-sex behaviour and a range of personal and social traits. Same-sex behaviour was associated with increased loneliness and ‘neuroticism’ (trait anxiety). As you might expect, the more same-sex activity, the less the chance of having children. And being gay increased the chance of drug use, socially risky behaviour and smoking. It also increased the risk of a range of mental disorders, such as bipolar, ADHD and depression.

Where are we in this debate? What determines whether someone will be straight or gay, or somewhere in between? It’s naive to separate genes from environmental events. Genes play a major role in biasing individuals towards a particular environment, and the environment can influence how genes function. The complex and variable psychological state that we label as "gay" will have equally complex determinants. Of course, genes will contribute. But how they do is still as mysterious as it was before this landmark study.