An American study has found new evidence that male homosexuality is influenced by genes – but environmental and social factors also play a role.

In the study, which was presented at a Science of Sex and Attraction event in Chicago, scientists tested the DNA of more 400 sets of gay brothers and their heterosexual relatives, to find out if some men are predisposed to being gay.

Genetic factors account for between 30 per cent and 40 per cent of what decides whether a man is gay or straight, according to the research.

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Scientists found that a region of the X chromosome Xq28 had an impact on male sexuality, as did a stretch of DNA on chromosome 8.

Dr Michael Bailey, of Northwestern University, Illinois, who col-led the study said: “Sexual orientation has nothing to do with choice. Our findings suggest there may be genes at play – we found evidence for two sets that affect whether a man is gay or straight.

“But it is not completely determinative; there are certainly other environmental factors involved.”

The work has not yet been published, but it builds on a previous study carried out in 1993 by Dr Dean Hamer from the US National Cancer Institute, who also found that gay men shared genetic signatures on the X chromosome.

However, the findings of the study raise the prospect of a genetic test that could be used to determine a bay’s sexual orientation in the womb.

“Although this could one day lead to a pre-natal test for male sexual orientation, it would not be very accurate, as there are other factors that can influence the outcome,” Dr Bailey said.

“Clearly parents should not be allowed to torture or kill babies. But they can currently choose to terminate a pregnancy early on, so they should be allowed to have as much information on the future child as possible.”

But Dr Alan Sanders, clinical associate Professor of Psychiatry at Northwestern University, who led the study, said that identifying a specifically “gay gene” was “oversimplification” .

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“We also don’t think genetics is the whole story. It’s not. And whatever genes contribute to sexual orientation, you can think of it contributing to heterosexuality as much as you can think of it contributing to homosexuality. It contributes to variation of a trait,” he said.

Last year, Canadian scientists found that the more older male siblings a man has, the more likely he is to be gay.

They said this is because carrying a boy in the womb triggers an immune response in the mother, which has an impact on the unborn baby’s sexual orientation.

The more boys a woman carries, the stronger the response.

Dr Bailey said environmental factors were likely to have the biggest impact on homosexuality.

“Don’t confuse “environmental” with “socially acquired.” Environment means anything that is not in our DNA at birth, and that includes a lot of stuff that is not social,” he said.

Richard Lane, a spokesman for Stonewall, said: "These kinds of studies come along every so often; they purport to prove the origins of sexual orientation without really backing the research up.

"If someone is gay, then they deserve respect and equality regardless."