By Randolph E Schmid, AP Science Writer 

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.

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8 May 2006
NewScientist.com  Breaking news

Lesbian and heterosexual women respond differently to specific human odours, a brain-scanning study has found. The homosexual women showed similar brain activity to heterosexual men when they inhaled certain chemicals, which may be pheromones, the researchers say.

"But our study can't answer questions of cause and effect," cautions lead researcher Ivanka Savic at the Karolinska Institute in Stockholm, Sweden. "We can't say whether the differences are because of pre-existing differences in their brains, or if past sexual experiences have conditioned their brains to respond differently."

Savic and her colleagues asked 12 lesbian women and 12 heterosexual women to breathe concentrated samples of two steroids EST, which is derived from oestrogen and found in the urine of pregnant women; and AND, which is derived from progesterone and found in men’s armpit sweat.

Positron emission tomography (PET) scans revealed which regions of the women's brains responded to each chemical. When the heterosexual women smelled AND their brains showed activity in the anterior hypothalamus, a region of the brain thought to process sexual cues. But EST only produced activity in the olfactory region of their brains, the area that processes smells.

The lesbians, however, only showed activity in the olfactory region whichever odour they smelled.

Sexual preference

The researchers conclude that these "pheromone-like stimuli" produce different responses in the anterior hypothalamus of women of different sexual orientations, and that their research supports the idea that the anterior hypothalamus plays a role in sexual preference.

The pattern of activity in the brains of lesbians for both chemicals was similar - though not identical - to the pattern for heterosexual men. In May 2005, the same team showed that the brains of heterosexual women and homosexual men reacted in the same ways when they smelled either AND or EST.

However, George Preti and Charles Wysocki, who have performed related research at the Monell Chemical Senses Center in Philadelphia, Pennsylvania, US, have some criticisms of the new study. They told New Scientist that the Swedish researchers go too far by implying that the two chemicals they used are pheromone-like. "Although [they're] calling them putative pheromones, there really isn't any strong evidence in the biomedical literature that these compounds are pheromones," Wysocki says. In fact, experts debate whether human sex pheromones exist at all. Humans do have other kinds of pheromones chemicals we release when we are frightened, or that allow a mother to identify her baby by smell, or that cause female roommates' menstrual cycles to synchronise.

Inconclusive evidence

Circumstantial evidence suggests that humans also have sex pheromones and so scientists in the field are open to the possibility, Savic says. But to date, Wysocki notes, no research has conclusively shown the existence of pheromones that influence people's sexual behaviour.

Little is known about EST, the chemical derived from oestrogen. Evidence that the second, progesterone-derived compound might be a sex pheromone is stronger, Savic says. But while it is found in men's sweat, Preti notes that it is also present in the sweat of women, making its role as a sexual attractant for heterosexual women uncertain.

He also points out that the researchers used the purified, crystalline forms of these two chemicals, making them roughly a million times more concentrated than when they are produced naturally by the body. Greater concentrations can sometimes cause not merely a stronger response, but a completely different one.