Scientists are reporting what could amount to be the firmest evidence yet of genetic links to male sexual orientation, in the first published genome-wide association study (GWAS) examining the trait.
Researchers recruited more than 2,000 men of both homosexual and heterosexual orientation and analysed their DNA, identifying two genetic regions that appear to be linked to whether individuals are gay or straight.
"Because sexuality is an essential part of human life – for individuals and society – it is important to understand the development and expression of human sexual orientation," says psychiatrist Alan Sanders from NorthShore University HealthSystem in Evanston, Illinois.
"The goal of this study was to search for genetic underpinnings of male sexual orientation, and thus ultimately increase our knowledge of biological mechanisms underlying sexual orientation."
To do so, Sanders' team studied 1,077 homosexual men and 1,231 heterosexual men of primarily European ancestry, who were respectively recruited from community festivals and a nationwide survey.
For the purposes of the study, the men's sexual orientation was based on their self-reported sexual identity and sexual feelings. Each individual taking part provided a sample of their DNA in the form of blood or saliva samples, which were genotyped and analysed.
When the researchers sifted through the data, they isolated several genetic regions where variations called single nucleotide polymorphisms (SNP) signalled single-letter changes in the DNA, with two of the most prominent congregations located near chromosomes 13 and 14.
"The genes nearest to these peaks have functions plausibly relevant to the development of sexual orientation," the researchers explain in their paper.
On chromosome 13, the variants were located next to a gene called SLITRK6, which is expressed in the diencephalon – a part of the brain that's previously been shown to differ in size depending on men's sexual orientation.
While the mechanisms here aren't fully understood, the researchers explain the SLITRK gene family is important for neurodevelopment and could be of relevance for a range of behavioural phenotypes, not just sexual orientation.
On chromosome 14, the strongest associations were centred around the thyroid stimulating hormone receptor (TSHR) gene, and it's thought the cluster of SNP variants here could conceivably affect sexual orientation due to altered expression in the hippocampus – in addition to producing atypical thyroid function.
It's not the first time scientists have examined our genetic code looking for hints as to predictors of sexual persuasion.
While there are numerous environmental factors to consider, previous research - that has not yet been replicated - linked a genetic marker in the X chromosome called Xq28 to male sexual orientation back in the 1990s.
This gave rise to the idea of the so-called 'gay gene', even though that's technically a misnomer, since the Xq28 band actually contains several genes, and the science on the region remains unclear.
More recently, a controversial study presented in 2015 by UCLA researchers suggested an algorithm analysing epigenetic markers that affect gene expression could predict male sexual orientation with up to 70 percent accuracy, but the findings were never published.
Similarly controversial – but in a completely different field of science – researchers from Stanford University made headlines in September when they claimed an AI they had developed could correctly distinguish between gay and heterosexual men and women (81 percent of the time and 74 percent of the time respectively).
While those findings produced an uproar, the claims – if true – serve as another illustration that our biology may contain innumerable clues about things like our sexual orientation that science is only beginning to reveal.
In terms of the new results, there's bound to be a lot of interest in the study, but the researchers are eager to emphasise their findings are largely speculative for now, since there's still a lot we don't know about what these genetic variations really mean.
There's also the relatively small size and skewed European basis of the sample – not to mention the fact that it's all men – which limit what it can tell us about genetic underpinnings to sexual orientation more broadly across race and sex lines.
Despite those shortcomings, there's a lot for other researchers to consider here, and the team hopes this could lay the groundwork for future investigations that could more deeply penetrate the genetic factors that help influence our sexual identities.
"What we have accomplished is a first step for GWAS on the trait, and we hope that subsequent larger studies will further illuminate its genetic contributions," says Sanders.
"Understanding the origins of sexual orientation enables us to learn a great deal about sexual motivation, sexual identity, gender identity, and sex differences, and this and subsequent work may take us further down that path of discovery."
The findings are reported in Scientific Reports.