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 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 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.

Some researchers 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. Others called that interpretation highly speculative but agreed the findings support 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."

The 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.