Scientists find homosexuality is caused by a combination of environmental and genetic factors 


Preference for same-sex relationships is determined by both environmental and assorted genetic factors, a large-scale study has confirmed.

This means that there is no such thing as a single ‘gay gene’ that determines your sexual preferences — just like many other human traits. 

Instead, thousands of genetic regions are involved, together accounting for between around 8–25 per cent of variation in sexual preferences between people.

Researchers confirmed this after studying genetic and survey data from over 470,000 volunteers taken from the UK Biobank and 23andMe.com.

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Preference for same-sex relationships is determined by both environmental and assorted genetic factors, a large-scale study has confirmed (stock image)

Preference for same-sex relationships is determined by both environmental and assorted genetic factors, a large-scale study has confirmed (stock image)

WHAT DO SCIENTISTS THINK LEADS TO SAME-SEX ATTRACTION?

Scientists have long sought to quantify the extent to which genetic and non-genetic (or environmental) factors impact a person’s preference for same-sex relationships.

Previous studies had hinted that genetic factors were complex, but their relatively small scales made it hard to draw reliable conclusions.

In the new study, researchers used data from over 470,000 people, over a 100 times more than previous works.

They confirmed that homosexuality stems from both environmental and genetic factors.

Rather than their being one single ‘gay gene’, however, the team found thousands of places – or loci – in the genome that seem to play a role in sexuality.

Only five of these had a ‘significant’ impact — and, combined, all the factors accounted for only 8–25% of the variation in sexual attraction between different people.

The factors at play are so complex that is impossible to predict from a person’s DNA whether they are attracted to members of the same-sex or not. 

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Geneticist Andrea Ganna of the Massachusetts General Hospital and colleagues surveyed over 470,000 individuals whose genetic data is recorded in either the UK Biobank or with the personal genomics company 23andMe Inc. 

Study participants were asked such questions as ‘Have you ever had sex with someone of the same sex?’ and ‘To whom are you sexually attracted?’

The authors performed so-called Genome Wide Association Studies to attempt to identify genetic patterns that were consistent with a variety of behavioural, personality and physical traits.

‘We established that the underlying genetic architecture is highly complex,’ the researchers wrote in their paper.

‘There is certainly no single genetic determinant – sometimes referred to as the “gay gene” in the media.’

This finding overturns the suggestion of a previous study, in 1993, which reported finding a genetic marker more commonly found in gay men that is passed down in the X chromosome from mother to child.

Instead, the researchers found that there were thousands of genetic loci  – areas of the genetic code – spread across the genome.

Each were found to have small individual effects that combine to contribute to differences in people’s predisposition to same-sex sexual behaviour.

Only five of the thousands of loci the team identified appeared to be ‘significantly’ associated with same-sex behaviour.

In total, all the tested genetic variants accounted for only 8–25 per cent of the variation in same-sex sexual behaviour between different people.

Some of the genetic variants are linked to the biological pathways for sex hormones and smelling, which the researchers said could provide clues towards some of the mechanisms that affect same-sex behaviour.

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‘All measured common variants together explain only part of the genetic heritability at the population level, and do not allow meaningful prediction of an individual’s sexual preference,’ said the researchers in their paper.

‘In my opinion, one of the most interesting findings […], is that from a genetic standpoint there is no single continuum from opposite-sex to same-sex sexual behaviours,’ said Dr Ganna.

This finding was reiterated by paper co-author and geneticist Benjamin Neale, also of the Massachusetts General Hospital.

‘We discovered that the Kinsey Scale, which places individuals on a continuum from basically exclusively opposite-sex partners to exclusively same-sex partners is really an oversimplification of the diversity of sexual behaviour in humans,’ he said.

This means that there is no such thing as a single 'gay gene' that determines your sexual preferences — just like many other human traits (stock image)

This means that there is no such thing as a single ‘gay gene’ that determines your sexual preferences — just like many other human traits (stock image)

The team also determined that the genetic influence on same-sex behaviour is slightly different between men and women, with the overlap between the sexes being lower for sexual preferences that for other behavioural traits.

‘This is not the first study exploring the genetics of same sex behaviour, but the previous study were small and underpowered,’ said Dr Ganna.

‘The results of those study were mostly not reputable.’ 

‘So, we decided to form a large international consortium — and collected data for more than 500,000 people.’

‘Just to give you a sense of the scale of the data, this is approximately 100 times bigger than previous study on this topic.’

The findings, Dr Ganna concluded, ‘underscore the importance of resisting simplistic conclusions because the behavioural phenotypes are complex, because our genetic insights are rudimentary, and because there is a long history of misusing genetic results for social purposes.’

Writing in an associated perspective article, sociologist Melinda Mills of the University of Oxford, who was not involved in the present research, highlighted the limitations of the study’s findings.

The researchers ‘did find particular genetic loci associated with same-sex behaviour,’ she noted, but their effects were still minuscule.

‘When they combine the effects of these loci together into one comprehensive score, the effects are so small (under 1%) that this genetic score could not be reliably used to predict same-sex sexual behaviour of an individual.’ 

‘Using these results for prediction, intervention or a supposed “cure” is wholly and unreservedly impossible,’ she added.

‘I think that the results of this paper indicate that the genetic underpinnings of human sexuality are extremely complex, diverse and nuanced,’ said Nina McCarthy, a University of Western Australia geneticist who was also not involved in the study.

‘I believe that well-conducted genetic studies into human sexuality such as this may help to inform, and perhaps have a positive impact on, our understanding of human sexual diversity,’ she added.

However, Dr McCarthy noted that the methodology had limitations ad to the .

‘The study sample did not include transgender persons, intersex persons, and other important persons and groups within the queer community, a limitation which the authors say they hope will be addressed in future work. 

The full findings of the study were published in the journal Science.



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