Does homosexuality have a physical basis or is it learned behavior? In order to find out, a team of Swedish researchers studied how the brains of straight males, straight females and lesbians respond to male and female pheromones as well as to usual odors.

Last year, the same team compared in a similar study male homosexuals to heterosexual men and women and found that the brains of gay men reacted more like those of women than of straight men. Now it was discovered that lesbians' brains also reacted somewhat, though not completely, like those of heterosexual men.

The research team led by Ivanka Savic at the Stockholm Brain Institute had three groups of 12 volunteers each sniff chemicals derived from male and female sex hormones. While heterosexual women found the male and female pheromones about equally pleasant, straight men and lesbians liked the female pheromone more than the male one.

Interestingly all of them rated the male hormone more familiar than the female one. But while straight women perceived both hormones as having the same intensity, lesbians and straight men perceived the male hormone more intensely.

The scientists scanned the brains of the participants while sniffing the pheromones and a set of four ordinary odors. The question posed by researchers was whether some of the hormones are perceived by the same part of the brain as the ordinary odors, while the hormone that causes attraction is processed by the hypothalamus, which is related to sexual stimulation.

Researchers found that in case of straight women and men the opposite sex hormone was processed in the hypothalamus, while the same sex hormone was perceived by the same area of the brain that deals with regular odors. On the other hand, in case of lesbians, both male and female hormones were processed the same, in the basic odor processing circuits.

This discovery is surprising because it raises the question why do lesbians feel sexual attraction toward other women. And why do they like female hormones better than male ones. It thus seems that female homosexuality has a less precise physical basis than male homosexuality.