The brains of gay men and women look like those found in heterosexual people of the opposite sex, research suggests.
The Swedish study, published in the Proceedings of the National Academy of Sciences journal, compared the size of the brain's halves in 90 adults.
Gay men and heterosexual women had halves of a similar size, while the right side was bigger in lesbian women and heterosexual men.
A UK scientist said this was evidence sexual orientation was set in the womb.
Scientists have noticed for some time that homosexual people of both sexes have differences in certain cognitive abilities, suggesting there may be subtle differences in their brain structure.
This is the first time, however, that scientists have used brain scanners to try to look for the source of those differences.
A group of 90 healthy gay and heterosexual adults, men and women, were scanned by the Karolinska Institute scientists to measure the volume of both sides, or hemispheres, of their brain.
When these results were collected, it was found that lesbians and heterosexual men shared a particular "asymmetry" in their hemisphere size, while heterosexual women and gay men had no difference between the size of the different halves of their brain.
In other words, structurally, at least, the brains of gay men were more like heterosexual women, and gay women more like heterosexual men.
A further experiment found that in one particular area of the brain, the amygdala, there were other significant differences.
In heterosexual men and gay women, there were more nerve "connections" in the right side of the amygdala, compared with the left.
The reverse, with more neural connections in the left amygdala, was the case in homosexual men and heterosexual women.
The Karolinska team said that these differences could not be mainly explained by "learned" effects, but needed another mechanism to set them, either before or after birth.
'Fight, flight or mate'
Dr Qazi Rahman, a lecturer in cognitive biology at Queen Mary, University of London, said that he believed that these brain differences were laid down early in foetal development.
"As far as I'm concerned there is no argument any more - if you are gay, you are born gay," he said.
The amygdala, he said, was important because of its role in "orientating", or directing, the rest of the brain in response to an emotional stimulus - be it during the "fight or flight" response, or the presence of a potential mate.
"In other words, the brain network which determines what sexual orientation actually 'orients' towards is similar between gay men and straight women, and between gay women and straight men.
"This makes sense given that gay men have a sexual preference which is like that of women in general, that is, preferring men, and vice versa for lesbian women."