Scientists at the Stockholm Brain Institute in Sweden report in the Proceedings of the National Academy of Sciences USA that gay men and straight women share similar traits—most notably in the size of their brains and the activity of the amygdala—an area of the brain tied to emotion, anxiety and aggression. The same is true for heterosexual men and lesbians.

Study author, neurologist Ivanka Savic–Berglund, says such characteristics would develop in the womb or in early infancy, meaning that psychological or environmental factors played little or no role.

"This is yet another in a long series of observations showing there's a biological reason for sexual orientation," says Dean Hamer, a molecular biologist at the National Institutes of Health (NIH), who was not involved in the study. "It's not just a reflection of people's behavior, nor is it a choice, nor is it something in their rearing environment. [The study] shows that it's something that people are born with."

Previous studies have examined brain differences between gay and straight people on the basis of their responses to various tasks, such as rating the attractiveness of other people. The problem was that there was no way to determine whether their responses were colored by learned social cues.

To get around this, Savic-Berglund focused on the structure and function of brain regions that develop during fetal development or early infancy—without using any cognitive tasks or rating systems.

The researchers used MRIs to determine the volume and shapes of the brains of 90 volunteers—25 straight and 20 gay members of each sex. They found that the straight men and gay women had asymmetrical brains; that is, the cerebrum (the largest part of the brain, which is responsible for thought, sensory processing, movement and planning) was larger on the right hemisphere of the brain than on the left. In contrast, they found that women and gay men had symmetrical cerebrums.

The team next used PET (positron emission tomography) scans to measure the blood flow to the amygdala, that part of the brain controlling emotion, fear and aggression. The images showed how the amygdala connects to other parts of the brain, giving them clues as to how this might influence behavior. They scanned subjects' brains when they at rest and did not show them photos or introduce other behavior that might have been learned.

They found that in gay men and women, the blood flowed to areas involved in fear and anxiety, whereas in straight men and lesbians it tended to flow to pockets linked to aggression.

Robert Epstein, emeritus director of the Cambridge Center for Behavioral Studies in Concord, Mass., agrees that the study offers compelling evidence that sexual orientation is a biologically fixed characteristic. But he cautions that these findings may vary in different people whose sexual orientation is not that clear-cut, which his own research shows includes a majority of the population.