Scientists have again debunked the idea of a single "gay gene", in the largest study to date of the genetics of same-sex sexual behaviour.

Key points: Researchers scanned the human genome for genetic markers associated with same-sex sexual behaviour

Researchers scanned the human genome for genetic markers associated with same-sex sexual behaviour Many genes influence a person's likelihood of having had same-sex partners, but they have only a small effect on behaviour

Many genes influence a person's likelihood of having had same-sex partners, but they have only a small effect on behaviour Some people question whether the benefits of this type of research outweigh the potential dangers

Rather, their findings paint a diverse and complex picture of human sexuality, and the genetic factors that influence it.

Nearly half a million people took part in the study, mostly from the United Kingdom and the United States, which was published in the journal Science today.

While we've known from previous twin and family studies that our sexual preferences are influenced by our genes, it's been difficult for scientists to pinpoint whether any specific genetic markers could play a role.

While most previous studies have involved only a few hundred or a few thousand participants, this international collaboration is at least 100 times larger and more able to draw robust conclusions, the researchers said.

"We truly consider this a team scientific effort intended to help increase our collective understanding of the genetics of sexual behaviour, which is actually a very, very personal topic for me as a scientist, but also as a gay man," said study co-author Fah Sathirapongsasuti from genetic testing company 23andMe.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Watch Duration: 2 minutes 38 seconds 2 m 38 s There is no single gay gene, researchers find (Supplied: Francesca Cattaneo, Miriam Palopoli, Alessandro Nepote Vesin, Andrea Ganna)

What the researchers did

The data used in the study, which was predominantly drawn from the UK Biobank and 23andMe, looked at the male and female participants' DNA and questions they had answered about their sexual behaviour, including whether they'd ever had sex with someone of the same sex.

The researchers scanned the entire human genome to find markers that were significantly associated with same-sex sexual behaviour.

In the past, some studies have pointed to a genetic marker on the X chromosome, Xq28, that could influence male sexual orientation.

The researchers didn't find this marker was significantly associated with same-sex behaviour.

Instead, they found five markers on other chromosomes were particularly associated with same-sex behaviour, although they say thousands more also appear to be involved.

Interestingly one of these markers was from a region of DNA that has genes related to our sense of smell, and another was associated with male-pattern baldness.

"There is no one 'gay gene'," said study co-author Brendan Zietsch, a psychologist at the University of Queensland.

"Instead there are many, many genes that influence a person's likelihood of having had same-sex partners."

But the five markers identified by the researchers have only a very small effect.

Taken together, they contributed to less than 1 per cent of the variation in same-sex sexual behaviour.

Looking across the entire genome, all the markers only account for between 8 and 25 per cent of variation.

Geneticist and LGBT activist Fahad Ali of the University of Sydney said the study confirmed what we already knew: that genetic contribution to same-sex attraction was quite low.

"These are very, very small genetic changes we're talking about, like a single change in one of the letters to the genetic code," said Mr Ali, who was not involved in the study.

"You're really just changing something from like one spelling of a word to a different spelling of a word that are both equally valid, and really has a very, very, very small impact on how the biological processes in your body work."

Like sexual behaviour, our height is influenced by both our genes and our environment. ( Unsplash CC: Brendan Church )

In this way sexual behaviour is similar to most other human traits, influenced by a complex interplay between our genes and our environment, said geneticist Nina McCarthy of the University of Western Australia, who wasn't part of the study.

"Something like height is not determined by a single gene, rather it's the influence of thousands of different genes spread across our genome and individually each of these genes have a tiny effect on human height," Dr McCarthy explained.

"We're seeing almost exactly the same pattern here with human sexuality."

It is impossible to predict an individual's sexual behaviour from their genome, said study co-author Ben Neale of the Broad Institute.

"Genetics is less than half of this story for sexual behaviour, but it's still a very important contributing factor."

Not oversimplifying sexuality

As well as capturing whether the study participants had ever had sex with someone of the same sex, the researchers also gathered data on what proportion of someone's sexual partners were of the same sex.

This led them to challenge the idea of human sexuality being a single continuum, like we see represented with measures like the Kinsey scale.

"We showed that the genes that distinguish people who'd never had a same-sex partner from those who had are not the same as those that distinguish people with lower versus higher proportions of same-sex partners," Dr Zietsch said.

"These results suggest we shouldn't measure 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."

Mr Ali said this finding is interesting because you'd expect the gene variants related to these two things to be the same but they're not quite.

"There's this assumption that there's a sort of binary from super gay, if I may, to super straight and ... it may be a bit more complicated than that," he said.

But he thinks a lot of people in the LGBT community can readily attest to the fact that there are any number of permutations of attraction.

Why undertake research like this?

Understanding the biological basis of sexual preference is associated with greater acceptance of sexual minorities. ( Unsplash CC: Allie Smith )

The results of the study reinforce the extreme complexity of sexual behaviour and attraction, Dr Zietsch said.

But despite their findings posing as many questions as answers, and the limitations of the study, he's excited by what more there is to learn.

"Greater understanding of the biological basis of sexual preference is associated with greater acceptance of sexual minorities, so I believe that shining a light on these topics has benefits beyond academia," he said.

Anna Brown, CEO of Equality Australia, said the research demonstrated the complexity and multiple factors that contribute to sexuality and sexual orientation.

"I think it's really important that we have a scientific understanding of how that diversity comes to be, and our place in the world as natural and healthy," she said.

However she was concerned the researchers excluded gender-diverse people from the study.

"It's really important that scientific studies reflect the full diversity of society, when they actually analyse and take population samples and undertake these sorts of tests."

But bioethicist Bridget Haire of the University of New South Wales questioned whether the greater knowledge gained through researching this area was worth the potential consequences it posed.

"I think that there are many very, very good reasons to be highly concerned about studies that look into a genetic basis of same sex-sexual attraction," Dr Haire said.

"Currently there are more than 70 countries in the world that criminalise same-sex sexual practices. And in about 12 of them laws can be used to put people to death for their sexuality."

She fears a future where a genetic key that could predict someone's sexual behaviour could be used against them when travelling to countries where same-sex sexual behaviour is illegal.

"As a person who is in a same-sex relationship and as someone who travels to those countries, I understand already how problematic that is when I am in control of where that information goes," Dr Haire said.

"I actually think there are a million things in the world to research, and the genetics of same-sex sexual behaviour really isn't one that's screaming out for more work because of the dangers that it involves."