Basic Instinct: The Smell of Fear and Sex

Rice University psychologist Denise Chen has rapidly established herself as the leading expert on the communication of emotion through body odor. It might sound like a narrowly defined research topic, but it has enormous implications for understanding the social aspects of smell. Chen, along with her colleague Wen Zhou, has just published a pair of papers that show how BO (underarm odor) can signal fear and sexual arousal.

Both studies collected BO from male donors who wore 4 x 4 inch cloth pads in their armpits while watching 20-minute videos. In one study the videos were happy (slapstick comedy), scary (horror movies) or emotionally neutral (an educational documentary). Based on heart rate and high levels of self-reported happiness and fear during viewing, eight men were selected from a larger group to be the sweat donors. Their pads provided the “fear sweat” and “happy sweat” for the actual experiment.

In the other study, 20 hetero guys watched the educational documentary (snore . . .) and a porno video (hot!). Based largely on self-reported sexual arousal, three guys were selected as the “sexual sweat” donors. Needless to say, all donors followed meticulous protocols that avoided colognes and scented soaps as well as odiferous foods like garlic.

Who was smelling these samples of emotional sweat? In both experiments, it was young women.

In Chen’s fear study, published this month in Psychological Science, the women were shown faces and had to decide as quickly as possible whether the facial expression was happy or fearful. The faces were from a computer generated continuum in which an actor’s face was morphed from a happy expression to a fearful expression. The women judged the faces while smelling the various sweat samples. Happy sweat and the smell of clean pads had no effect on how women read the faces. Fear sweat, on the other hand, caused women to perceive otherwise neutral faces as fearful. It appears that the BO of fear has no effect on the perception of unambiguous facial expressions; however it can put a negative emotional spin on ambiguous expressions.

Chen’s study of sex sweat, published on New Year’s Eve in the Journal of Neuroscience, examined brain response to smells using fMRI scans. While in the scanner, women smelled four different odors: sex sweat, neutral sweat, phenylethyl alcohol (rose), and androstadienone (a chemical thought to be a human sex pheromone). Chen and Zhou found two brain areas that responded to sex sweat but not to the other three odors. One area was the right orbitofrontal cortex, a multisensory area known to be involved in smell judgments. The other was the right fusiform cortex, an area involved in the recognition of faces and voices, but not previously known to respond to smells.

These experiments, along with earlier work by Chen, establish that BO can function as a chemosignal that conveys the emotional state of the odor donor. Having ruled out counter-explanations—the intensity or pleasantness of the BO doesn’t matter, for example—Chen proposes that BO chemosignals are produced and perceived as a consequence of human evolution.

I think she’s on to something.