Researchers in Sweden have found that a compound taken from male sweat stimulates the brains of gay men and straight women but not heterosexual men, which raises the possibility that homosexual brains are different.
The findings supports other evidence that humans respond to pheromones, compounds which are known to affect animal behaviour, especially in mating, but whose role in human activity is unclear.
According to the researchers the pheromone in question, a derivative of testosterone, is called 4,16-androstadien-3-one, AND, and is detected primarily in male sweat.
A previous study, by Ivanka Savic of Karolinska University Hospital in Stockholm and colleagues found that the hypothalamus region of the brain became activated when women smelled AND and men smelled a corresponding compound in female urine called EST.
This time they compared the reactions of 12 women, 12 heterosexual men and 12 homosexual men, letting them smell EST, AND, and ordinary odours such as lavender, and they then used positron emission tomography to watch their brain responses.
Savic's team say that in contrast to heterosexual men, and in congruence with heterosexual women, homosexual men displayed hypothalamic activation in response to AND, and a region of the brain called the anterior hypothalamus responded most strongly, this is an area that in animals "is highly involved in sexual behaviour". Other smells were processed the same in all three groups.
Savic says the findings show that our brain reacts differently to the two putative pheromones compared with common odours, and suggest a link between sexual orientation and hypothalamic neuronal processes.
In the majority of animals, pheromone signals go to the hypothalamus region of the brain via a pit-like structure in or near the nose called the vomeronasal organ. People have a vomeronasal pit but there are no nerves connecting it to the brain, leading biologists to question whether humans respond to pheromones.
The research is published in the current issue of the Proceedings of the National Academy of Sciences.