After studying scans from 90 volunteers, the researchers found that straight women and gay men had no difference between the size of the different halves of their brain while lesbian women and straight men shared a particular "asymmetry" in their hemisphere size. In heterosexual men, the scans showed the right side of the brain to be typically 2% larger than the left compared to 1% in lesbians.

This is the first time scientists have used brain scanners to try to look for the source of those differences although earlier research had identified differences in spatial and verbal abilities related to sex and sexual orientation. Gay men and straight women have been found to fare better at certain language tasks, while heterosexual men and lesbians tend to have better spatial awareness.

The Swedish researcher, Ivanka Savic from the Stockholm Brain Institute, says she and her colleagues chose to measure areas of the brain likely to have been fixed at birth.

"This is the most robust measure so far of cerebral differences between homosexual and heterosexual subjects… That was the whole point of the study, to show the parameters that differ, but which couldn't be altered by learning or cognitive processes," Savic was quoted by abc7news.com as saying.

The study noted similar characteristics in the area of the brain responsible for emotion response. In heterosexual men and lesbian 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 straight women. However neuroscientists are unsure whether the similarities are genetic or develop in the womb.

Savic's team has yet to confirm whether the differences in brain shape are responsible for sexual orientation, or are a consequence of it. To find out, they have begun another study to investigate brain symmetry in newborn babies, to see if it can be used to predict their future sexual orientation.

Above: Images showing the amygdala in heterosexual men and women (labeled HeM and HeW) and homosexual and women (labeled HoM and HoW). Image: National Academy of Sciences, PNAS.