The brains of gay men resemble those of straight women, according to research published today that provides more evidence of the role of biology in sexual orientation.
Using brain-scanning equipment, researchers said they discovered similarities in the brain circuits that deal with language, perhaps explaining why homosexual men tend to outperform straight men on verbal skills tests -- as do heterosexual women.
The area of the brain that processes emotions also looked much the same in gay men and straight women -- and both groups have higher rates of depressive disorders than heterosexual men, researchers said.
The study in Proceedings of the National Academy of Sciences, however, found that the brain similarities were not as close in the case of gay women and straight men.
Previous studies have found evidence that sexual orientation is influenced by biological factors. More than a decade ago, neurobiologist Simon LeVay reported that a key area of the hypothalamus, a brain structure linked to sexual behavior, was smaller in homosexual men than in heterosexual men.
The latest study, led by Dr. Ivanka Savic of the Karolinska Institute in Stockholm, was significant in that it looked at areas of the brain that have nothing to do with sexual behavior, suggesting that there was a basic biological link between sexual orientation and a range of brain functions.
"The question is, how far does it go?" said Dr. Eric Vilain, who studies human sexual development at UCLA and was not involved in the study. "In gay men, the brain is feminized. Is that limited to particular areas, or is the entire brain female-like?"
Vilain said his hunch was that the entire brain was not feminized because "gay men have a number of masculine traits that are not present in women." For example, he said, men regardless of sexual orientation tend to be interested in casual sex and are stimulated by sexually suggestive images.
Savic and her colleagues used magnetic resonance imaging to measure brain volumes of two groups, each divided evenly between men and women: 50 heterosexuals and 40 homosexuals. They knew going into the study that in men the right cerebral hemisphere is larger than the left, but in women the hemispheres are of equal size.
The results showed that gay men had symmetrical brains like those of straight women, and homosexual women had slightly asymmetrical brains like those of heterosexual men. Symmetry is thought to favor verbal skills, the report said.
The differences were pronounced. For example, the right cerebral hemisphere in heterosexual men was 624 cubic centimeters -- 12 cubic centimeters greater than the left side. In homosexual men, the right hemisphere was 608 cubic centimeters -- 1 cubic centimeter smaller than the left.
In heterosexual women, there was no volume difference between right and left hemispheres. But in homosexual women, their right hemisphere was 5 cubic centimeters larger than the left.
Next, researchers used positron emission topography to measure blood flow in the amygdala, a brain area involved in processing emotions. The circuitry of the amygdala in gay men more closely resembled that of straight women than straight men, researchers said. The amygdalas of gay women looked more like those of straight men, the report said.
Savic said she thought the brain differences originated in the womb or infancy, probably as a result of genetic or hormonal factors. She said she could not explain why the differences were more pronounced in homosexual men than in homosexual women.
S. Marc Breedlove, a Michigan State University neuroscientist who studies sexual development, said that in his studies with rats, changes in prenatal levels of testosterone caused the sort of brain alterations that Savic observed.