A collaborative study on human sexuality has found that the brains of homosexual individuals are structurally and functionally different from those of heterosexual people.
During the study, scientists found that lesbians appear to have a lower proportion of grey matter - the cells that process signals from the senses - in their brains than straight women, giving their brains a more 'male-like' structure.
And they found that the brains of gay men appear to have structural similarities to those of heterosexual women.
They also exhibit the same powerful response as straight women to the sex hormones released in male sweat.
For instance, it is known that the greater size of male brains and the higher proportion in females of grey matter are caused partly by sex hormones released during foetal development.
These hormones are also involved in determining sexual orientation.
Scientists have long thought this meant that there should be differences in the brains of homosexuals.
However, brain-scanning equipment has only recently become powerful enough to seek them out.
In one study, researchers at the Wellcome Trust Centre for Neuroimaging at University College London, examined the brains of 80 men and women - including 16 gay men and 15 lesbians - using magnetic resonance imaging, or MRI.
The scans showed that lesbians had a 'male-like' proportion and distribution of grey matter in their brain when compared with heterosexual women.
"In homosexual women the perirhinal cortex grey matter displayed a male-like structural pattern," Times Online quoted the authors, as saying.
The perirhinal area is linked to social and sexual behaviour.
In the other study, Ivanka Savic of the Karolinska Institute, Sweden, asked 12 gay men, 12 heterosexual women and 12 heterosexual men to smell hormones found in male sweat while she measured their brain responses.
She found that the anterior hypothalamus, an area linked to sexual behaviour, responded strongly in both heterosexual women and gay men. Straight men showed little response.
This means the brains of gay men have functional similarities to those of straight women.
Savic also measured the brain responses of 12 lesbians who were asked to smell male and female hormones.
She found that the lesbians responded much more strongly to female hormones.
According to Savic, the study suggests that sexual orientation is largely programmed into people early in life.
The first study is published in the Public Library of Science and the second study is published in the Proceedings of the National Academy of Sciences.