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Brain scans have provided the most compelling evidence yet that being gay or straight is a biologically fixed trait.

The scans reveal that in gay people, key structures of the brain governing emotion, mood, anxiety and aggressiveness resemble those in straight people of the opposite sex.

The differences are likely to have been forged in the womb or in early infancy, says Ivanka Savic, who conducted the study at the Karolinska Institute in Stockholm, Sweden.

"This is the most robust measure so far of cerebral differences between homosexual and heterosexual subjects," she says.

Previous studies have also shown differences in brain architecture and activity between gay and straight people, but most relied on people's responses to sexuality driven cues that could have been learned, such as rating the attractiveness of male or female faces.
The results showed that straight men had asymmetric brains, with the right hemisphere slightly larger – and the gay women also had this asymmetry. Gay men, meanwhile, had symmetrical brains like those of straight women.
They found that the patterns of connectivity in gay men matched those of straight women, and vice versa (see image, above right). In straight women and gay men, the connections were mainly into regions of the brain that manifest fear as intense anxiety.

"The regions involved in phobia, anxiety and depression overlap with the pattern we see from the amygdala," says Savic.

This is significant, she says, and fits with data showing that women are three times as likely as men to suffer from mood disorders or depression. Gay men have higher rates of depression too, she says, but it's difficult to know whether this is down to biology, homophobia or simply feelings of being "different".
"This study demonstrates that homosexuals of both sexes show strong cross-sex shifts in brain symmetry," says Qazi Rahman, a leading researcher on sexual orientation at Queen Mary college, University of London, UK.