A new Scientific Reports study looking into the links between homosexuality in men and their genes has made a fascinating discovery: Two regions on two chromosomes do indeed seem to be different in gay men compared to heterosexual men.

It’s known that, for men, sexual orientation is partly based on the genes you inherit. Yes, it’s more complicated than that, but this is an unequivocal fact. The authors of the new study – led by geneticists and psychiatrists from the NorthShore University HealthSystem Research Institute – point this out early on.

“Male sexual orientation is moderately heritable (30 to 40 percent), but is multifactorial, with evidence of multiple genetic and environmental contributions,” they explain.

In the last three decades, it’s emerged that genetic variations do seem to have an influence over whether a man is homosexual or heterosexual. This new study sought to build on this evidence, and appears to have succeeded.

Using 1,077 self-identified homosexual and 1,231 self-identified heterosexual men of primarily European descent, they conducted a genome-wide association study, or a GWAS. This means they looked for differences in the genetic constituents of their subjects across their entire genome.

Ultimately, they managed to find two genetic regions that differed between the two populations. These regions were located on two different chromosomes, which are the thread-like structures that carry our genes.

One region is linked to a gene named SLITRK6, and can be found on chromosome 13; the other region is associated with a gene called TSHR, and was identified on chromosome 14. Variants of these genes appear to influence whether or not a man is homosexual.

Just having these variants doesn't mean the person will always be gay, by the way. There are certainly other genetic and external factors that influence this too.

The researchers note that “SLITRK6 is expressed especially in the diencephalon,” a segment of the brain that occurs very early on during embryonic development. Interestingly, the team point out that the diencephalon “contains a region previously reported as differing in size in men by sexual orientation.”

TSHR has a variety of associations, including with the hippocampus, the part of the brain that consolidates short-term memories into long-term ones. More importantly, this gene is linked to the behavior of the thyroid gland.

For example, this gene is connected to Graves’ disease, an autoimmune condition that triggers an overreactive thyroid gland. The team highlight that previous studies have found that homosexual men have a higher incidence of Graves’ disease.

Differing thyroid functions have been linked to the prevalence of homosexual men before. An overactive thyroid gland has been linked to lower body masses for homosexual men compared to heterosexual men, even when controlling for diet and other health factors.

Additionally, some tentative associations between mothers with a history of thyroid dysfunction during pregnancy and offspring that are attracted to the same sex have emerged in recent years.

So, in sum, both the SLITRK6 and the TSHR gene families appear to play a role, although exactly how they do so, and why this association emerged, remains unknown. Remember, they play other roles too, so it’s not really correct to call them “gay genes”.

Incidentally, although this is a thorough study, the sample size is still fairly small, so more research is needed to confirm these findings.

Regardless, the takeaway message from this new study is that sexual orientation is definitely not something people just “decide”; although our upbringing and daily life changes us all the time, whether we’re homosexual, heterosexual, or anything in-between is, to some extent, hardwired.

As has been pointed out elsewhere, research on women in this regard is both poorer and less common. Although a genetic component has been hinted at by several studies, it remains far more (embarrassingly) enigmatic than it should be at this stage.