How messages are scent

The idea that humans release chemical signals – pheromones – that affect people around them is controversial. If pheromones do exist, however, two chemicals that might fulfil this role are the testosterone derivative 4,16-androstadien-3-one (AND), found in men’s sweat, and the oestrogen-like steroid estra-1,3,5(10),16-16-tetraen-3-ol (EST), found in female urine. As well as activating smell-related brain regions, these chemicals also have a different effect on a small, frontal brain region, the anterior hypothalamus, depending on the sex of the person doing the inhaling. AND triggers activity in this region of a woman’s brain but not a man’s, whereas EST triggers strong activity here in a man’s brain but not a woman’s. However, Ivanka Savic and colleagues wondered whether the different brain activity caused by EST and AND might depend not so much on a person’s sex, but rather on their sexual orientation.

Savic’s team used positron emission tomography to scan the brains of 12 heterosexual men, 12 heterosexual women and 12 homosexual men while they inhaled AND, EST, a range of odours including lavender oil, and odourless air.

The testosterone derivative AND found in men’s sweat, caused maximum activation in the same specific region of the brains of women and homosexual men, but not in the brains of the heterosexual men. This region, the preoptic area of the hypothalamus, is known to be involved in sexual behaviour. In contrast, it was the chemical EST that led to strong activation in the hypothalamus of the heterosexual men. The ordinary, control odours affected all of the participants’ brains similarly.

So, why does this brain region in homosexual men respond to AND in the same way as it does in the brains of women? First, it’s possible that this brain region develops differently in homosexual men’s brains than it does in heterosexual men’s brains. “Alternatively”, the authors explained “it could reflect an acquired sensitisation to AND stimuli in the hypothalamus (of homosexual men)…due to repeated sexual exposure to men. A third possibility is that heterosexual women and homosexual men associated (the smell of) AND with sex, whereas heterosexual men made a similar association with EST”.
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Savic, I., Berglund, H. & Lindstrom, P. (2005). Brain response to putative pheromones in homosexual men. Proceedings of the National Academy of Sciences, 102, 7356-7361.

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