Lesbians and straight men have similar reactions to sexual hormones

The study was conducted by a team of Swedish researchers, and published in Tuesday's issue of Proceedings of the National Academy of Sciences,  Xinhua reports.

"The data suggest that there is a difference between male and female sexuality," said Savic, who, with her colleagues, examined the brains of 12 lesbian women, using positron emission tomography (PET), to evaluate their brains' responses to potential sex pheromones.

The new work builds on previous research by Savic and her colleagues, in which they found the same brain region - the anterior hypothalamus - was activated in homosexual men and heterosexual women when exposed to the progesterone derivative 4,16-androstadien-3-one (AND). AND is found in human sweat, and occurs in concentrations 10 times higher in men than in women. But the anterior hypothalamus region in the brains of heterosexual men was activated in response to the female pheromone estra-1,3,5(10),16-tetraen-3-ol (EST), the researchers also found. EST is an estrogen-like substance, found in the urine of pregnant women, according to Forbes.

The hypothalamus regulates metabolic process and links the nervous system to the endocrine system by secreting brain hormones. It also responds to odor stimuli, including pheromones.|

When Savic's team looked at the brain activity of the 12 lesbian women, it found the lesbians responded to both compounds in a similar way. And they processed them in a way more like heterosexual men than heterosexual women. But the relationship to the opposite sex was not as strong as the researchers found it to be in a previous study between homosexual men and heterosexual women. This lends support to the idea, the researchers said, that AND and EST are pheromones involved in sexual preferences. And it lends credence to the theory that homosexuality is different in men than in women, the researchers said.

The finding builds on previous research that suggest that gay men responded in a way more similar to heterosexual women than heterosexual men when exposed to a synthetic chemical.

 "Both studies … indicate that the physiological response in brain regions associated with reproduction are different in homo- and heterosexual persons," said Ivanka Savic, a neuroscientist at the Karolinska Institute in Stockholm, Sweden.

Savic, who is the lead author of both studies, cautioned that neither study proves people are born gay. The response could be biological or learned. Determining an answer will require further study.

Savic added that the similarity between homosexual men and heterosexual women seems stronger than the similarity between lesbians and heterosexual men, highlighting the notion that male homosexuality is quite different than female homosexuality.