Smelling a male pheromone prompts the same brain activity in homosexual men as it does in heterosexual women, a new study has found. It did not excite the sex-related region in the brains of heterosexual males, although an oestrogen-derived compound found in female urine did.

The testosterone-derived chemical AND is found in male sweat and is believed to be a pheromone. It activated the anterior hypothalamus and medial preoptic area of gay men and straight women alike. Researchers led by Ivanka Savic at the Karolinska University Hospital in Sweden believe this brain region integrates the hormonal and sensory cues used in guiding sexual behaviour.

The research demonstrates a likely link between brain function and sexual orientation, Savic suggests. But she told New Scientist that the study “does not answer the cause-and-effect question”.

So the brain-activation of gay men by AND may contribute to sexual orientation of those men, or simply be the result of their orientation and sexual behaviour. She added that the brain scans revealed no anatomical differences between any of the participant’s brains.


Lavender and cedar

The team observed 36 healthy men and women, who were exposed in turn to AND, the oestrogen-derived compound EST and other odours, including lavender oil, cedar oil, eugenol and butanol.

While the subjects were consciously aware of each unidentified smell as it was presented, Savic does not believe the reactions in the subjects brains were intentional in any way: “The pattern of activation does not suggest cognitive processing,” she says.

PET and MRI scans revealed that the ordinary odours activated parts of the brain associated with smelling in all test subjects. But in addition to that activation, AND excited the brain areas associated with sexual behaviour for female and gay male participants, as did the EST for straight men.

Open minded

Ada Frumerman, a psychotherapist based in New York, US, who has presented papers on related topics, says sexual orientation is probably determined by a mixture of biological and psychological influences.

“I think we should be open minded,” she says. “It would not be advisable to focus solely on biological causes. Similarly, it would do a disservice to only look for psychodynamic causes.”

Savic’s team has also conducted similar experiments with gay women and the researchers are currently analysing the results.

Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.0407998102)