Genes play a large part in determining the sexual orientation of men, scientists have shown.
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 largest investigation conducted into the subject.
The US researchers stress that environmental forces, such as hormones in the womb, play a more important role.
But they said this did not imply that upbringing or other social factors, or individual choice, had a bearing on sexual orientation.
"Sexual orientation has nothing to do with choice," one of the lead researchers, Dr Michael Bailey, from Northwestern University in Chicago, said.
"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 research involved testing the DNA in blood samples taken from more than 409 gay brothers and their heterosexual relatives.
It confirmed that a region previously linked to male sexuality on the X chromosome, known as Xq28, is more likely to be shared by gay pairs of brothers than siblings who do not have homosexuality in common.
A second genetic region, on chromosome 8, also appeared to increase the chances of a man being gay.
Dr Bailey did not rule out the possibility of a genetic "gayness" test before birth, but thought it would only provide a vague indicator of a baby's future sexual orientation.
"It would not be very accurate, as there are other factors that can influence the outcome," he said.
The issue of gay genes is almost as controversial as talk of links between inherited DNA and intelligence.
Previously some experts have dismissed the idea of a genetic component to homosexuality because of the so-called "Darwinian paradox".
This supposes that if gayness was genetic, the genes responsible for homosexuality would be extinguished by natural selection over time.
Gay men would be less likely than heterosexual men to reproduce and pass on their sexual orientation genes. The same would apply to their offspring, so that over a number of generations, the gay genes would be expected to die out altogether.
However, one study by Italian scientists has suggested that genes linked to male homosexuality may increase fertility in women, thereby helping to preserve them.
The new findings were presented at a Science of Sex and Attraction event attended by experts in Chicago.
It has no connection with the American Association for the Advancement of Science's annual conference also taking place in the city.