Witelson, who was not part of the research team, said the findings clearly show a biological involvement in sexual orientation.
The study, published in today's issue of Proceedings of the National Academy of Sciences, was done by researchers at the Karolinska Institute in Stockholm.
They exposed heterosexual men and women and homosexual men to chemicals derived from male and female sex hormones.
These chemicals are thought to be pheromones, molecules known to trigger responses such as defense and sexual interest in many animals.
Whether humans respond to pheromones has been debated, although in 2000 American researchers reported finding a gene that they think directs a human pheromone receptor in the nose.
The Swedish study was one of a series looking at whether parts of the brain involved in reproduction differ in response to odors and pheromones, lead researcher Ivanka Savic said.
The brains of different groups responded similarly to ordinary odors such as lavender, but differed in their response to the chemicals thought to be pheromones, Savic said.
The Swedish researchers divided 36 subjects into three groups -- heterosexual men, heterosexual women, and homosexual men. They studied the brain response to sniffing the chemicals, using PET scans. All the subjects were healthy, unmedicated, right-handed, and HIV negative.
When they sniffed smells like cedar or lavender, all of the subjects' brains reacted only in the olfactory region that handles smells.
But when confronted by a chemical from testosterone, the male hormone, portions of the brains active in sexual activity were activated in heterosexual women and in gay men, but not in heterosexual men, the researchers found.
The response in gay men and heterosexual women was concentrated in the hypothalamus with a maximum in the preoptic area that is active in hormonal and sensory responses necessary for sexual behavior, the researchers said.
And when estrogen, the female hormone was used, there was only a response in the olfactory portion of the brains of heterosexual women.
Homosexual men had their primary response also in the olfactory area, with a very small reaction in the hypothalamus, while heterosexual men responded strongly in the reproductive region of the brain.
