Can you be gay 10 years old

In the last two decades, dozens of scientific papers have been published on the biological origins of homosexuality - another announcement was made last week. 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 constructexternaland can you be gay 10 years old have made a conscious, proud choiceexternal 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. Scientists don't know the answer to this Darwinian puzzle, but there are several theories. 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 allele - or group of genes - that sometimes codes for homosexual orientation may at other times have a strong reproductive benefit. This would compensate 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 code 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 results 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. The fa'afafine of Samoa dislike being called "gay" or "homosexual". Some researchers believe that to understand the evolution of gay people, we need to look at how they fit into the wider culture.

Adult LGBT Population in the United States

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 people 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 goes, 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.