Scientists investigating human sexuality have found that the brains of homosexuals have structural and functional differences from those of “straight” people.

Lesbians appear to have a lower proportion of grey matter in their brains than straight women, giving their brains a more “male-like” structure.

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.

The research comes amid growing interest in how variations in brain structure are linked to human behaviour.

It is known, for example, that the greater size of male brains and the higher proportion in females of grey matter – the cells that process signals from the senses – 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 there should be differences in the brains of homosexuals. Brain scanning equipment has only recently become powerful enough to seek them out, however.

In one study, researchers at the Wellcome Trust Centre for Neuroimaging at University College London used magnetic resonance imaging, or MRI, to look at the brains of 80 men and women, including 16 gay men and 15 lesbians.

They found that lesbians had a “male-like” proportion and distribution of grey matter in their brain when compared with heterosexual women.

In a paper published in the Public Library of Science, the researchers said: “In homosexual women the perirhinal cortex grey matter displayed a male-like structural pattern.” The perirhinal area is associated with social and sexual behaviour.

In another study in the Proceedings of the National Academy of Sciences, 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 implies 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. The lesbians responded much more strongly to female hormones.

The research suggests sexual orientation is largely programmed into people early in life, according to Savic. However, other studies have shown a person’s lifestyle can lead to structural brain changes.

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