PET imaging verifies 'don't ask, don't smell' hypothesis

Researchers in Sweden using PET imaging and human pheromones have discovered biological differences between the brains of homosexual and heterosexual men. The study did not determine whether these differences are a cause or effect of sexual orientation.

Dr. Ivanka Savic, an associate professor of neuroscience at the Karolinska Institute in Stockholm, and colleagues subjected heterosexual and homosexual men and women to two human pheromones: a testosterone derivative found in male sweat (AND) and an estrogen derivative found in female urine (EST).

Using PET imaging, the researchers found that the male pheromone activated the hypothalamus in homosexual men and heterosexual women, but not heterosexual men. EST activated the hypothalamus only in heterosexual men.

Data on homosexual women are more complicated and need further analysis, Savic said.

The anterior hypothalamus showed maximal activation, which animal studies have shown to be highly involved in sexual behavior. Subjects in the three groups excepting homosexual women processed other common odors using only the olfactory brain.

The findings suggest that a link may exist between sexual orientation and hypothalamic neuronal processes. The study, which appeared in the May 10 Proceedings of the National Academy of Sciences, does not determine if homosexuality is innate or learned, Savic said.

"Additional studies with other techniques and in other populations of subjects would be needed to definitively say that sexual orientation is biological," she said.

Savic and colleagues had previously found differences between heterosexual men and women with respect to pheromone activation of the hypothalamus.