BY ROB STEIN, Washington Post Service
Is there such a thing as a "gay brain''? And, if so, are some people born with brains that make them more likely to be homosexual? Or do the brains of gay people develop differently in response to experiences?
Those are some of the thorny questions that have been raised by a provocative new study that found striking differences between the brains of homosexuals and heterosexuals in both men and women.
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."
Others, however, argue that such research is far from conclusive.
"I remain skeptical," said William Byne, a professor of psychiatry at the Mount Sinai School of Medicine in New York. "There's been a history of jumping to conclusions and over-interpreting findings in this field."
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.
But such research is fraught with uncertainty, and could not rule out that the findings were the result of changes that occurred in response to experiences and behaviors, rather than being inborn.
"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.
Savic and Lindstrom stressed that their findings need to be confirmed by additional research and that it remains unclear how the differences might affect behavior.
While other researchers agreed, some said the findings about the amygdala could help explain why gay men tend to respond to emotional situations more like women and gay women more like men, and could even play a role in their sexual orientation.
"This ancient structure is involved in `orienting' our attention to biologically important stimuli in our environment (such as attractive partners ... )," Qazi Rahman, who studies sexual orientation at Queen Mary, University of London, wrote in an e-mail.
Others said that that interpretation was highly speculative, but at the very least the findings support the idea that there tend to be fundamental differences in brain structure, supporting the idea that sexual orientation is inborn.
"This suggests that there's something going on during development that influences sexuality and the brain," LeVay said. "It points more persuasively to some early biological difference."
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.
"You get a sort of global shift in gender traits in gay people and straight people that affects not only their sexual orientation but other things as well," LeVay said.
Many researchers suspect that changes may be the result of the levels of hormones, such as testosterone, that fetuses are exposed to in the womb.
"We see the same asymmetries in the brains of rats and mice, and in rats and mice testosterone seems to be controlling it prenatally," said Marc Breedlove, a neuroscientist at Michigan State University.
But researchers say many questions remain about all this research. And there are as many differences within groups individuals of the same sexual orientation as between those of different orientation. Moreover, the new work involved adults, meaning there is no way to know with certainty when the structures and connections formed and why.
"It takes a snapshot of a group of people at a particular age," said Anne Fausto-Sterling, a professor of biology and gender studies at Brown University. "Even if there are reliable brain differences, it doesn't tell you anything about how those brain differences came into being."