The researchers used MRI scans to find out the overall volume and shapes of brains in a group of 90 volunteers consisting of 25 heterosexuals and 20 homosexuals of each gender.

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 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.

The team next used PET scans to measure blood flow to the amygdala, part of the brain that governs fear and aggression. The images revealed how the amygdala connected to other parts of the brain, giving clues to how this might influence behaviour.

They found that the patterns of connectivity in gay men matched those of straight women, and vice versa (see image). 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 Ivanka Savic, who conducted the study at the Karolinska Institute in Stockholm, Sweden.

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".

In straight men and lesbians, the amygdala fed its signals mainly into the sensorimotor cortex and the striatum, regions of the brain that trigger the "fight or flight" response. "It's a more action-related response than in women," says Savic.

The differences are likely to have been forged in the womb or in early infancy, says Savic.

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

"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.

Last update: 01-31-2009 14:08