A BBC programme earlier in the summer, The Making of Me, had actor John Barrowman on a mission to prove that his sexuality was something he was born with, a quest that had him measuring gay men's fingers at a Pride parade and watching porn with electrodes wired to his penis.
The approach was far from what I'd call scientific, but did demonstrate some of the routes by which biological factors can create "gayness" in men.
And there is increasing evidence that the basis for our sexual orientation comes down to biology. A study published in June showed that certain brain structures of gay people resemble those of opposite-sex heterosexuals. This also seems to be reflected in the way people with different gender identities use their brains. The documentary pointed out that, generally, women tend to have greater verbal fluency than men, but that gay men's brains work in a similar way to women's. Barrowman's verbals, predictably, were right off the scale.
And the assertion that male homosexuality comes down in part to genetics is supported by studies showing that if a gay man has an identical twin (who is a genetic clone of himself) his twin has a 40 per cent chance of also being gay, while if a gay man has a male non-identical twin (who is genetically simply a sibling), the twin has only a 20 per cent chance of being homosexual.
Basically, there's no reason to think that homosexuality is anything other than completely natural: after all, plenty of other species, from bears to flamingos, indulge in boy-on-boy and girl-on-girl sex. In fact, a lot of animals get up to much more risqu stuff than that. An exhibition called The Sex Lives of Animals has opened at the Museum of Sex in New York and features animals engaged in all manner of pastimes, including penis-fencing grey whales and self-fellating kangaroos. According to New Scientist magazine, the aim of the exhibition is to demolish preconceived ideas about sexuality and gender stereotypes. But I digress.
Back to humans and the relatively straightforward issue of homosexuality, the puzzle that scientists have been battling to solve is this: given that homosexuals, on average, have far fewer children than straight people, how have genes for homosexuality been maintained in the population?
An often-mooted idea is that gay people help their relatives via childcare or money, leading to more siblings or nephews and nieces thus ensuring the survival of shared genes. But actually there's no good evidence for this.
However, the female relatives of gay men do turn out to have more children. Research by Andrea Camperio Ciani and colleagues at the University of Padua, Italy, suggests that genes which make men more likely to be homosexual also make women likely to have more babies than average, and the team published work a couple of weeks ago showing the same is true for genes for bisexuality in men. It seems that the genes that make men tend to like men also cause women to like men – a kind of hyper-heterosexuality in women. The genes stick around in the population because the extra children produced by the women carrying them make up for the lower output of their gay male relatives.
The situation is far from clear – and even less so for the basis of homosexuality in women. Most researchers seem to agree that a variety of genetic, environmental and possibly social and psychological factors too, converge to determine our sexual orientation. Some also suggest that prenatal exposure to hormones, particularly testosterone, has a major influence.
For a man, one factor which influences the likelihood of being gay is the number of big brothers he has. Psychologist Ray Blanchard carried out a study showing that the odds of being gay increased 33 per cent for each older brother, and it is thought that this effect is due to the interaction between male foetuses and the mother's immune system.
Well, this news had Barrowman whooping it up because he has a big brother and his mother had miscarried another male baby earlier on, so he took this as proof that he was born gay.
Not exactly a bulletproof theory, but it made him happy.