A vast new study has quashed the idea that a single “gay gene” exists, scientists say, instead finding homosexual behaviour is influenced by a multitude of genetic variants which each have a tiny effect.
The researchers compare the situation to factors determining a person’s height, in which multiple genetic and environmental factors play roles.
“[This study] highlights both the importance of the genetics as well as the complexity of the genetics, but genetics is not [the] whole story,” said Dr Benjamin Neale, co-author of the study from the Broad Institute in the US.
Writing in the journal Science, an international team of researchers report how they drew on existing genetic databases to conduct the largest study yet into genetics and same-sex sexual behaviour.
In the first part of the study, they looked at data from about 500,000 individuals collected as part of the UK Biobank project: about 4% of men and nearly 3% of women said they had ever had a same-sex sexual experience. The team stress that they did not focus on identity or orientation, and did not include transgender individuals.
By looking at sexual behaviour and relatedness of individuals, they estimated that about a third of the variation in same-sex behaviour is explained by genetics. That, they say, chimes with previous twin studies that put the figure at about 30% to 50%.
Dr Brendan Zietsch, co-author of the research from the University of Queensland in Australia, said that does not mean the rest is due to upbringing or culture. “For example, it is thought that non-genetic factors before birth, such as the hormonal environment in the womb, also play an important role,” he told the Guardian.
The team then looked at which genetic variants might be behind the link, using data from more than 400,000 participants in the UK Biobank project and more than 68,000 individuals whose data was collected by the company 23andMe.
Researchers found five genetic variants – tiny differences in DNA – that showed a clear link to same-sex sexual behaviour, two in both men and women, two found only in men and one found only in women. The team believe one, found only in men, might be involved in sex hormone regulation, not least because it is linked to male-pattern balding.
Even taken together, though, these five genetic variants explain less than 1% of the variation in same-sex behaviour among participants – suggesting many other variants are involved, each playing a very small role.
Neale stressed that the scale of the influence of non-genetic factors, complexities of sexual behaviour, and difficulties in precisely measuring the size of any variant’s effects, means it is not possible to use genetic information to predict whether an individual will have same-sex partners.
The study provides a number of insights, including that there is overlap between genetic predisposition to same-sex sexual behaviour and traits such as openness to experience, as well as predisposition to mental health problems.
“One possibility is that stigma associated with same-sex sexual behaviour causes or exacerbates mental health issues. This could create a genetic correlation,” said Zeitsch.
The authors also say their findings call into question the idea that sexuality exists on a single scale.
“[There] seem to be genes associated with opposite-sex attraction and other genes associated with same-sex attraction, and these are not related,” added Zeitsch. “These results suggest we shouldn’t be measuring sexual preference on a single continuum from straight to gay, but rather two separate dimensions: attraction to the same sex and attraction to the opposite sex.”
However, the study has limitations including that it is based mainly on people of European ancestry, while the age range of participants does not fully reflect that of the wider population. It also relied on self-reported behaviour.
The idea that genetics might play a role in same-sex attraction was propelled into the spotlight in 1993 when Dean Hamer, a scientist at the US National Cancer Institute, and his team found links between DNA markers on the X chromosome and male sexual orientation.
The findings caused considerable controversy, with the media dubbing the discovery the “gay gene”.
Subsequent research has thrown up mixed results, although recent studies have supported the theory that genetics plays a role in sexual orientation.
Hamer said he was delighted by the latest research. “This provides yet another strand of evidence, another approach, showing that there is a strong and significant genetic contribution to people’s sexual behaviour,” he said.
However, unlike Hamer’s work, the latest study does not show any special importance of the X chromosome. The researchers say that is not surprising, adding the previous findings would not meet today’s bar for importance, and were based on a small sample size.
Hamer said he was not surprised either – although he put it down to the latest study looking at same-sex behaviour rather than sexual orientation.
Qazi Rahman, a leading authority on sexual orientation research from King’s College London, welcomed the study but said that the databases involved only captured information from a small percentage of people who were invited to participate. That, he said, means that the genetic variants found in the latest research might reflect another trait particular to those who chose to respond.
Rahman added he was not surprised that the genetic variants identified only have small individual effects, pointing out that since same-sex behaviour is linked to having to fewer offspring, evolution is likely to hide such variants in effect.
A website set up by the researchers to explain the study gives a clear message: “This study provides further evidence that diverse sexual behaviour is a natural part of overall human variation.”
Rahman agreed, stressing that the findings do not change the fact that everyone should be treated with respect. “The causes of a trait shouldn’t influence how we see it,” he said.