Researchers say that the sexual area of a gay man's brain works a lot like that of a woman when exposed to a particular stimulus.
During the experiment, men and heterosexual women sniffed a chemical from the male hormone testosterone. The homosexual men's brains responded differently from those of heterosexual males - in a similar way to the women's brains.
"It is one more piece of evidence ... that is showing that sexual orientation is not all learned,'' 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.
Witelson, who was not part of the research team, said the findings clearly show a biological involvement in sexual orientation.
The studies findings, published in Tuesday's issue of Proceedings of the National Academy of Sciences, was found by researchers at the Karolinska Institute in Stockholm, Sweden.
The experiment 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.
Despite the debates over whether humans respond to pheromones, in 2000 American researchers reported finding a gene that they believe directs a human pheromone receptor in the nose.
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.
According to the study, 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 - portions of the brains active in sexual activity were activated in straight women and in gay men, but not in straight men.
When the female hormone estrogen was used, the women and homosexual men had reactions in similar areas of the brain, as opposed to the heterosexual men who reacted in the opposite area.
Savic said the group is also doing a study involving homosexual women, but those results are not yet complete.
