United States' researchers indicate that homosexuality might be a matter of biology rather than choice, as some genes were linked to men being attracted to other men.
Chicago University researchers analysed the DNA of 400 pairs of gay brothers and revealed two pieces of DNA that seem to be linked to homosexuality.
Dr Michael Bailey from Northwestern University in Illinois says that sexual orientation "has nothing to do with choice," as findings suggest there may be genes at play.
Qazi Rahman, a psychologist at King's College London, said the results were valuable for further understanding the biology of sexual orientation.
Other studies suggest that conditions in the mother's womb also influence sexual orientation. For instance, the more older male siblings a man has, the greater chance he will be gay. The phenomenon cannot be explained by the youngest boy being babied or other differences in the way they are brought up.
It is thought that carrying a male baby in the womb triggers an immune response in the mother, creating antibodies that attack part of the unborn child's brain linked to sexual orientation.
However, the latest findings also raise the prospect of a genetic test that could be used by insurance companies to discriminate against clients or by pregnant women to abort gay babies.