WASHINGTON – Lesbians’ brains react differently to sex hormones than those of heterosexual women, new research indicates.

That’s in line with an earlier study that had indicated gay men’s brain responses were different from straight men – though the difference for men was more pronounced than has now been found in women.

Lesbians’ brains reacted somewhat, though not completely, like those of heterosexual men, a team of Swedish researchers said in Tuesday’s edition of Proceedings of the National Academy of Sciences.

A year ago, the same group reported findings for gay men that showed their brain response to hormones was similar to that of heterosexual women.

In both cases the findings add weight to the idea that homosexuality has a physical basis and is not learned behavior.

“It shows sexual orientation may very well have a different basis between men and women … this is not just a mirror image situation,” said Sandra Witelson, an expert on brain anatomy and sexual orientation at the Michael G. DeGroote School of Medicine at McMaster University in Hamilton, Ontario.

“The important thing is to be open to the likely situation that there are biological factors that contribute to sexual orientation,” added Witelson, who was not part of the research team.

The research team led by Ivanka Savic at the Stockholm Brain Institute had volunteers sniff 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 same team reported last year on a comparison of the response of male homosexuals to heterosexual men and women. They found that the brains of gay men reacted more like those of women than of straight men.

The new study shows a similar, but weaker, relationship between the response of lesbians and straight men.

Heterosexual women found the male and female pheromones about equally pleasant, while straight men and lesbians liked the female pheromone more than the male one. Men and lesbians also found the male hormone more irritating than the female one, while straight women were more likely to be irritated by the female hormone than the male one.

All three groups rated the male hormone more familiar than the female one. Straight women found both hormones about equal in intensity, while lesbians and straight men found the male hormone more intense than the female one.

The brains of all three groups were scanned when sniffing male and female hormones and a set of four ordinary odors. Ordinary odors were processed in the brain circuits associated with smell in all the volunteers.

In heterosexual males the male hormone was processed in the scent area but the female hormone was processed in the hypothalamus, which is related to sexual stimulation. In straight women the sexual area of the brain responded to the male hormone while the female hormone was perceived by the scent area.

In lesbians, both male and female hormones were processed the same, in the basic odor processing circuits, Savic and her team reported.

Each of the three groups of subjects included 12 healthy, unmedicated, right-handed and HIV-negative individuals.

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

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Proceedings of the National Academy of Sciences: http://www.pnas.org