Scientists have found the strongest evidence for what many people take as granted, that being gay is caused by your genes.

The results of the research, which only applies to gay men and not lesbians or bisexual women, has found the strongest evidence yet for the existence of genes that increase the likelihood of a man being gay.

After studying the genetic profiles of 409 gay brothers and hetrosexual members of their families, recruited at gay pride events, scientists identified two small areas on the male genome that are likely to be linked to sexuality.

Professor Michael Bailey, from the Northwestern University in Chicago said: “Sexual orientation has nothing to do with choice. Our findings suggest there may be genes at play and we found evidence for two sets that affect whether a man is gay or straight.”

It is not, of course, the first scientific study to suggest genetic causes for homosexuality. Two studies in 1993 and 1995, by American geneticist Dean Hamer claimed to identify a genetic marker, known as Xq28, that appeared to influence male sexual attraction on the X chromosome, which men inherit from their mothers.

The most recent research indicates that male homosexuality is between 30 and 40 per cent caused by genetic factors. Other factors are believed to be related to the hormones that a baby is exposed to in the womb.

Professor Bailey is keen to point out that his research should not lead to tests for homosexuality. “The possibility of testing is not the reason why we are conducting this research. No genes had been discovered that influenced female sexuality.”