Gay and straight men have different variations of at least two genes that somewhat correlate with their sexual orientation, a new study appears to show, lending credence to the theory that sexual preferences are inherited, not chosen.

The study appeared Thursday in the Scientific Reports journal. Scientists at the North Shore University in Illinois compared the genomes of 1,077 gay and 1,231 straight men of “primarily European ancestry,” and found differences in two genes.

The first, SLITRK6, is behind brain development and hormone production. It is particularly active in the hypothalamus region of the brain, which previous studies have shown are up to 34 percent larger in gay men. Differences were also found in another gene, TSHR, responsible for thyroid function, which previous studies also linked to sexual preferences as well as weight loss.

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“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,” lead author Dr Alan Sanders told The Telegraph.

“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.”

Not all men who have the “gay” variants of the gene end up that way, however.

“There are probably multiple genes involved, each with a fairly low effect,” Sanders told the New Scientist. “There will be men who have the form of gene that increases the chance of being gay, but they won’t be gay.”

In their report, the scientists warned that their findings are “best described as speculative,” though they may pave the way for more conclusive studies. Other experts too expressed caution, as studies such as this usually involve a much larger and more diverse sample size.

“As this study was carried out in European men, we do not know whether the findings will apply to homosexuality in women, or even to homosexuality in non-European men,” Dr Nina McCarthy of the University of Western Australia told Cosmos magazine.

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“All that is required to see a genetic association in this study is for slightly more homosexual men to carry the genetic variant than heterosexual men, and many times this will simply be due to chance.”

Research on genetic factors in sexual preferences has been ongoing since 1993, when genetic variations were found between gay and straight men on the X chromosome, a phenomenon that was dubbed by the media as the “gay gene.” Supporters of LGBT rights back the ‘gay gene’ theory, as it renders sexual attraction a factor outside of a person’s control rather than a hedonistic lifestyle choice.

But according to Dr Ilan Dar-Nimrod of the University of Sydney, this may inadvertently lead to more homophobia by framing gays as an ‘other.’

“In this instance, it results in a heterosexual person feeling a clear ‘us’ and ‘them’ that leads to distancing from ‘them’ (i.e. gay people). Such a feeling may fuel prejudice,” Dar-Nimrod told Cosmos. He said further that the recent movement for same-sex marriage in Australia was characterized by “seeing sexual choices among consensual adults as their own business, rather than a moral issue.”