The evidence for nature over nurture underpins the argument for human rights for gay men and women but is still far from conclusive, writes Dominic James.

• The "dominant mother" theory is discredited
• Some evidence of a gay gene
• Something seems to happen in the womb
• Is a virus involved?
• It's the size of your index finger!

My friend Simon admitted recently to stealing his sister’s Barbie Dolls as a 5 year-old and taking the heads off his Action men.

Rufus Wainwright shared during his Judy Garland tribute concert that he used to dress up in his mother’s shoes and Laura Ashley dresses (to his dad’s horror).

And, I personally remember playing hop-scotch with the girls in the playground rather than football with the lads!

There's been considerable research into the phenomenon of boys who consistently exhibit a host of strongly feminine traits and interests while avoiding boy-typical behavior like rough-and-tumble play. One study showed that boys who consistently demonstrated such behaviour from an early age have a very good chance of growing up to be gay.

Of course, not all gay men show this extremely feminine behavior as young boys. But the research indicates that, of the boys who do exhibit such behaviours, about 75 percent of them turn out to be gay or bisexual.

Since many studies identify such behaviour as present from a very early age, the implication is that being gay is more nature than nurture; we are born gay. However, the studies of identical twins throw this into question.

Identical twins begin as genetic clones. From the moment they leave their mother's womb, their environment is generally about as close to identical as possible; having similar relationships with the same nurturing father and mother. Yet, studies show that before either boy talks, one might develop highly feminine traits while the other will present as all butch!

So, what happens between their identical genetic starting point and their births? They spent nine months in their mother’s womb. In the hunt for what causes people to be gay or straight, that's now the most interesting and potentially enlightening frontier.

Does this really matter?

I would argue it does. For much of the 20th century, most psychiatrists drew upon the work of Freud and asserted being gay resulted from an overprotective mother (nurture) and therefore could be changed (see my piece on gay cures).

If research can argue that being gay is genetic, the implication for gay rights would be immense. It would add weight to the argument for protection against discrimination in the same way as other innate characteristics are protected; race and gender.

The “nature” argument really gained momentum in 1991 when an American scientist showed that a tiny clump of neurons (believed to control sexual behavior) was, on average, more than twice the size in heterosexual men than in homosexual men.

Then twin studies that showed that if one identical twin was gay, there was a 50% chance the other would be. In the case of fraternal twins the odds are just 20%. The difference is explained by the fact that identical twins share the same genetics, whereas fraternal twins only share about half.

The debate finally hit the headlines in 1993 with the discovery of the “Gay Gene”: gay brothers shared a specific region of the X chromosome, called Xq28, at a higher rate than gay men shared with their straight brothers.

In recent years, researchers who suspect that homosexuality is inborn - whether because of genetics or events happening in the womb - have looked everywhere for clues: Prenatal hormones. Birth order. Finger length. Fingerprints. Stress. Sweat. Eye blinks. Spatial relations. Hearing. Handedness. Even "gay" sheep!

Earlier this year brain scans showed that when straight men smelled a female urine compound, their hypothalamus lit up. That didn't happen with gay men who responded more to male sweat.

At the same time, there has been a whole body of research indicating that exposure levels to male hormones in the uterus, combined with the mother’s immune system determines sexuality.

For example, take finger length. In general, men have shorter index fingers in relation to their ring fingers; in women, the lengths are generally about the same (try this at home!). Researchers have found that lesbians generally have ratios closer to males. Other studies have shown marked differences in spatial awareness between gay and straight men; we’re hopeless at map reading!

Taken together, the research suggests that early on in the womb something fundamental to sexual orientation is happening. Nobody's sure what's causing it. But here's where genes may be involved, perhaps by regulating hormone exposure. This returns us to the question of whether there might be a gay gene.

However, if a prime motivation of all species is to pass genes on to future generations, and gay men are estimated to produce 80 percent fewer offspring than straight men, why would a gay gene not have been wiped out by the forces of natural selection? This evolutionary disadvantage is what led some scientists to argue that homosexuality might be caused by a virus. Others argue that it's not a question of what genes you have, but rather which ones you use (or get “switched on”).

This is all still hypothetical until scientists are able to prove the existence of one or more relevant genes. In the meantime, the message seems clear: while nurture may well play a role, the roots of homosexuality, at least in men, appear to be in place by the time a child is born.

British researchers Glenn Wilson and Qazi Rahman come to the conclusion in their book Born Gay: The Psychobiology of Sex Orientation: "Sexual orientation is something we are born with and not 'acquired' from our social environment."

So, let’s get on with it and embrace what Mother Nature blessed us with!

Dominic James Dominic is the founder of Tom Dick and Sally.