What makes homosexuality natural

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. Similarly in the West, there is evidence that many people go through a phase of homosexual activity. 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.

Qazi Rahman says that alleles coding for same sex attraction only explain some of the variety in human sexuality. 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 significantly 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.

Rice and his colleagues refer to the emerging field of epigenetics, which studies 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 an epimark that makes them less sensitive to testosterone. But these studies all had very small sample sizes and most focused on men, says Mills.

In the recent study, Ganna and his colleagues used a method known as a genome-wide association study GWAS to look at the genomes of hundreds of thousands of people for single-letter DNA changes called SNPs. If lots of people with a trait in common also share certain SNPs, chances are that the SNPs are related in some way to that characteristic. Then the researchers performed two separate analyses. In one, they evaluated more than one million SNPs and looked at whether people who had more SNPs in common with each other also reported similar sexual behaviours.

For their second analysis, Ganna and his colleagues wanted to see which particular SNPs were associated with same-sex sexual behaviours, and found five that were more common among those individuals. An even larger sample size could help to identify those missing variants, he says. One is near a gene related to smell, which Ganna says has a role in sexual attraction.

Another SNP is associated with male-pattern baldness — a trait influenced by levels of sex hormones, which suggests that these hormones are also linked to same-sex sexual behaviour. The results demonstrate the complexity of human sexuality, says Ganna. They also presented a challenge to the study researchers, who knew that explaining nuanced findings on such a sensitive topic to the general public would be tricky.

To ensure that their results are not misinterpreted, the study researchers worked with LGBTQ advocacy groups and science-communication specialists on the best way to convey their findings in the research paper and to the public. Their efforts included the design of a website that lays out the results — and their limitations — to the public, using sensitive, jargon-free language.

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In a issue of Science magazine , geneticist Andrea Ganna at the Broad Institute of MIT and Harvard, and colleagues, described the largest survey to date for genes associated with same-sex behavior.

By analyzing the DNA of nearly half a million people from the U. Numerous studies have established that sex is not just male or female. Nonetheless, misconceptions persist that same-sex attraction is a choice that warrants condemnation or conversion , and leads to discrimination and persecution. I am a molecular biologist and am interested in this new study as it further illuminates the genetic contribution to human behavior. The new finding is consistent with multiple earlier studies of twins that indicated same-sex attraction is a heritable trait.

As the ease and affordability of genome sequencing increased, additional gene candidates have emerged with potential links to homosexual behavior. So-called genome-wide association studies identified a gene called SLITRK6 , which is active in a brain region called the diencephalon that differs in size between people who are homosexual or heterosexual.