It turns out that sniffing a chemical from testosterone, the male sex hormone, causes a response in the sexual area of gay men's brains, just as it does in the brains of straight women, but not in the brains of straight men.
"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 that conducted the study, said the findings show a biological involvement in sexual orientation.
The study, published in Tuesday's issue of Proceedings of the National Academy of Sciences, was done by researchers at the Karolinska Institute in Stockholm, Sweden.
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 sex in many animals.
Whether humans respond to pheromones has been debated, although in 2000 American researchers reported finding a gene that they believe 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 straight women and in gay men, but not in straight men, the researchers found.