A new study continues to tell us that the brains of gay people are different from those of straight people. In fact, the brains of gay people are more similar to those of straight people of the opposite sex than to straight people of the same sex.
Some scientists say the new findings are part of an increasingly convincing body of evidence that suggests sexual orientation results from fundamental developmental differences that are probably caused by hormonal exposures in the womb.
"This research is pointing to basic differences in the brain between homosexual and heterosexual people that are likely there right from the beginning," said Sandra F. Witelson, a professor of psychiatry and behavioral neurosciences at McMaster University in Ontario. "These could be reflecting some genetic or hormonal factors that predetermine your sexual orientation."
Several earlier studies have found what appear to be differences between the brains of gay and straight people. In 1991, brain scientist Simon LeVay reported that the hypothalamus, which is involved in sexual behavior, tended to be smaller in gay men. Other researchers subsequently showed that the brains of gay and straight people appeared likely to respond differently to sexual images. The researchers who conducted the new study previously reported that the brains of gay and straight men seemed to react differently to suspected pheromones -- odors thought to be involved in sexual arousal.
"The next question was 'If there is a difference, could there be differences in parts of the brain that have nothing to do with sexual behaviors?' " said Ivanka Savic of the Karolinska Institute in Stockholm, who led the new research published online last week by the Proceedings of the National Academy of Sciences.
So Savic and her colleague Per Lindstrom first used magnetic resonance imaging, or MRI, to compare the symmetry of the brains of 25 straight men and 25 straight women with those of 20 gay men and 20 gay women.
Gay men tended to have brains that were more like those of straight women than of straight men -- the right and left sides were about the same size, the researchers found. Gay women's brains tended to be more like those of straight men than of straight women -- the right side tended to be slightly larger than the left.
Next, the researchers used positron emission tomography (PET) scans to examine how a part of the brain involved in processing emotions -- the amygdala -- was connected to other brain regions. Again they found that gay men tended to be more like straight women, with a stronger link between the amygdala and regions involved in emotions. Gay women tended to be more like straight men, with stronger connections to motor functions.
LeVay and other researchers said the findings fit with studies that found gay people tended to have different ratios in the lengths of their fingers and in the frequency of imperceptible clicking sounds in the ear.
"There's this cluster of interrelated findings," said Richard A. Lippa, a professor of psychology at California State University at Fullerton, who has found evidence that in gay men, the hair on the back of the head is more likely to curl counterclockwise than in straight men. "These are all biological markers that something must have gone on early in development."
These findings also fit with studies showing gay men tend to choose professions that typically attract women, such as teaching and social work, and have verbal and other cognitive skills that tend to be more like women's, he said.