Brains of lesbian girls reacted somewhat, though not totally, like those of heterosexual guys, a team of Swedish scientists told in Tuesday's edition of Proceedings of the National Academy of Sciences. A year ago, the same researchers reported discoveries for homosexual men that displayed their brain response to hormones was identical to that of heterosexual girls.

In both cases the discoveries were important for the idea that homosexuality has a physical basis and isn’t learned behavior. Sandra Witelson, an expert on brain anatomy and sexual orientation at the Michael G. DeGroote School of Medicine (McMaster University in Hamilton, Ontario) reported, it displayed sexual orientation may very well have a various basis between guys and girls…this wasn’t just a mirror picture situation.

Witelson who wasn’t a member of the study team added, the important thing was to be open to the likely situation that there were biological factors that contributed to sexual orientation. The study team held by Ivanka Savic (Stockholm Brain Institute) had participants sniff chemicals taken from female and male sex hormones. These chemicals are considered to be pheromones – molecules known to trigger responses such as sex and defense in many animals.

No matter if people respond to pheromones has been argued, although in 2000 American scientists reported discovering a gene that they believe directs a human pheromone receptor in the nose. The same team said last year when comparing a response of homosexual guys to heterosexual girls and men. They discovered that the brains of homosexual men reacted more like those of girls than of heterosexual guys.

The new research displays an identical, but weaker, relationship between the response of lesbian women and heterosexual men. Straight girls found the female and male pheromones about equally pleasant, while heterosexual guys and lesbian women liked the female pheromones more than the male ones. Guys and lesbian girls also discovered the male hormone more irritating than the female one, while heterosexual girls were more likely to be irritated by the female hormone than the male one.

The whole three groups rated the male hormone more familiar than the female one. Heterosexual girls considered both hormones about equal in intensity, while lesbian women and heterosexual men found the male hormone more intense than the female one. The brains of all three groups were scanned when sniffing female and male hormones and a set of 4 usual odors. Usual odors were processed in the brain circuits connected with smell in all the participants.

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

In lesbian women, both female and male hormones were processed the same, in the basic odor processing circuits, Savic and her colleagues reported. Each of the three groups of subjects consisted of 12 healthy, right-handed, unmedicated and HIV-negative persons.