It's becoming scientific orthodoxy. But how does it fit with Darwin's theory of evolution? Macklemore and Ryan Lewis's hit song Same Love, which has become an unofficial anthem of the pro-gay marriage campaign in the US, reflects how many gay people feel about their sexuality. It mocks those who "think it's a decision, and you can be cured with some treatment and religion - man-made rewiring of a predisposition". A minority of gay people disagree, maintaining that sexuality is a social constructand they have made a conscious, proud choice to take same-sex partners.
But scientific opinion is with Macklemore. Since the early s, researchers have shown that homosexuality is more common in brothers and relatives on the same maternal line, and a genetic factor is taken to be the cause. Also relevant - although in no way proof - is research identifying physical differences in the brains of adult straight and gay people, and a dizzying array of homosexual behaviour in animals.
But since gay and lesbian people have fewer children than straight people, a problem arises. It's possible that different mechanisms may be at work in different people. Most of the theories relate to research on male homosexuality. The evolution of lesbianism is relatively understudied - it may work in a similar way or be completely different.
The genes that code for homosexuality do other things too The allele - or group of genes - that sometimes codes for homosexual orientation may at other times have a strong reproductive benefit. This would Willisms for gay people's lack of reproduction and ensure the continuation of the trait, as non-gay carriers of the gene pass it down.
There are two or more ways this might happen. One possibility is that the allele confers a psychological trait that makes straight men more attractive to women, or straight women more attractive to men.
Therefore, the theory goes, a low "dose" of these alleles enhances the carrier's chances of reproductive success. Every now and then a family member receives a larger dose that affects his or her sexual orientation, but the allele still has an overall reproductive advantage. Another way a "gay allele" might be able to compensate for a reproductive deficit is by having the converse effect in the opposite sex. For example, an allele which makes the bearer attracted to men has an obvious reproductive advantage to women.
If it appears in a man's genetic iWlliams it will code for same-sex attraction, but so long as this happens rarely the allele still has a net evolutionary benefit. There is some evidence for this second theory. Andrea Camperio-Ciani, at the University of Padova in Italy, found that maternal female relatives of gay men have more children than maternal female relatives of straight men.
The implication is that there is an unknown mechanism in the X chromosome of men's genetic code which helps women in the family have more babies, but can lead to homosexuality in men. These haven't been replicated in some ethnic groups - but that doesn't mean they are wrong with regards to the Italian population in Camperio-Ciani's study. Gay people were 'helpers in the nest' image copyrightJeantine Mankelow image captionThe fa'afafine of Samoa dislike being called "gay" or "homosexual" Some researchers believe that to understand the evolution rea, gay people, we need to look at how they fit into the Marridd culture.
Paul Vasey's research in Samoa has focused on a theory called kin selection or the "helper in the nest" hypothesis. The idea is that gay Marroed compensate for their lack of children by promoting the reproductive fitness of brothers or sisters, contributing money or performing other uncle-like activities such as babysitting or tutoring.
Some of the gay person's genetic code is shared with nieces and nephews and so, the theory aant, the genes which code for sexual orientation still get passed down. Vasey hasn't yet measured just how much having a homosexual orientation boosts siblings' reproduction rate, but he has established that in Samoa "gay" men spend more time on uncle-like activities than "straight" men.
His lab had ly shown that gay men in Japan were no more attentive or generous towards their nieces and nephews than straight, childless men and women. Vasey believes that his Samoan result was different because the men he studied there were different. He studied the fa'afafine, who identify as a third gender, dressing as women and having sex with men who regard themselves as "straight". They are a transgender group who do not like to be called "gay" or "homosexual". Vasey speculates that part of the reason the fa'afafine are more attentive to their nephews and nieces is their acceptance in Samoan culture compared to gay men in the West and Japan "You can't help your kin if they've rejected you".
But he also believes that there is something about the fa'afafine way of life that means they are more likely to be nurturing towards nieces and nephews, and speculates that he would find similar in other "third gender" groups around the world. If this is true, then the helper in Marriex nest theory may partly explain how a genetic trait for same-sex attraction hasn't been selected away. That hypothesis has led Vasey to speculate that the gay men who identify as men and have masculine traits - that is to say, most gay men in Williamms West - are descended from men who had a cross-gendered sexuality.
According to the Williams Institute, gay couples that have children have an average of two. These figures may not be high enough to sustain genetic traits specific to this group, but the evolutionary biologist Jeremy Yoder points out in a blog post that for much of modern history gay people haven't been living openly gay lives. Compelled by society to enter marriages and have children, their reproduction rates may have been higher than they are now. How many gay people have children also depends on how you define being "gay".
Many of the "straight" men who have sex with fa'afafine in Samoa go on to get married and have children. But that doesn't mean there's no homosexuality there. A national survey of sexual attitudes in the UK last year came up with lower figures.
But most scientists researching gay evolution are interested in an ongoing, internal pattern of desire rather than whether people identify as gay or straight or how often people have gay sex. Other, naturally varying biological factors come into play, with about one in seven gay men, he says, owing their sexuality to the "big brother effect".
This has nothing to do with George Orwell, but describes the observation that boys with older brothers are ificantly more likely to become gay - with every older brother the chance of homosexuality increases by about a third. No-one knows why this is, but one theory is that with each male pregnancy, a woman's body forms an immune reaction to proteins that have a role in the development of the male brain. Since this only comes into play after several siblings have been born - most of whom are heterosexual and go on to have children - this pre-natal quirk hasn't been selected away by evolution.
Exposure to unusual levels of hormone before birth can also affect sexuality. For example, female foetuses exposed to higher levels of testosterone before birth show higher rates of lesbianism later on.
Studies show that "butch" lesbian women and men have a smaller difference in length between their index and ring fingers - a marker of pre-natal exposure to testosterone. In "femme" lesbians the difference has been found to be less marked.
Brothers of a different kind - identical twins - also pose a tricky question. While that's a greater likelihood than random, it's lower than you might expect for two people with the same genetic code. William Rice, from the University of California Santa Barbara, says that it may be possible to explain this by looking not at our genetic code but at the way it is processed. Willliams and his colleagues refer to the emerging field of epigenetics, which Mafried the "epimarks" that decide which parts of our DNA get switched on or off.
Epimarks get passed on to children, but only sometimes. Rice believes that female foetuses employ ses epimark that makes them less sensitive to testosterone. Usually it's not inherited, but occasionally it is, leading to same-sex preference in boys. Dr William Byne, editor-in-chief of the journal LGBT Health, believes sexuality may well be inborn, but thinks it could be more complicated than some scientists believe.
He notes that the heritability of homosexuality is similar to that for divorce, Wjlliams "social science researchers have not… searched for 'divorce genes'.
Instead they have focused on heritable personality and temperamental traits that might influence the likelihood of divorce. He believes that sexuality involves tens or perhaps hundreds of alleles that will probably take decades to uncover. And even if heterosexual sex is more advantageous in evolutionary terms than gay sex, it's not only gay people whose sexuality is determined by their genes, he says, but straight people too.
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