The finding, published in Tuesday's issue of Proceedings of the National Academy of Sciences, shows differences in physiological reaction to sex hormones.

Researchers led by Ivanka Savic at the Karolinska Institute in Stockholm, Sweden, 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 sex in many animals.

Whether humans respond to pheromones has been the subject of debate, although in 2000 American researchers reported finding a gene that they believe directs the human pheromone receptor in the nose.

In the Swedish study, when sniffing a chemical from testosterone, the male hormone, portions of the brains involved in sexual activity were activated in gay men and straight women, but not in straight men, the researchers found.

When they sniffed smells like cedar or lavender, all of the subjects brains reacted only in the olfactory regions that handles smells.

The result clearly shows a biological involvement in sexual orientation, said Sandra Witelson, an expert on brain anatomy and sexual orientation at the Michael G. DeGroote School of Medicine at McMaster University in Ontario, Canada.

The research was funded by the Swedish Medical Research Council, the Karolinska Institute and the Megnus Bergvall Foundation.

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