You're scared - I can smell it.

In January, scientists published a paper which found that women's brains reacted differently to sweat from aroused men. This another study showing the mounting evidence that human beings, although unconsciously, communicate information via olfactory cues that our brains are able to interpret. Now another study has shown that such olfactory cues can actually affect our interpretation of other people. The study, published in Psychological Science, has found that women who smelled 'fearful sweat' actually perceived neutral facial expressions as more fearful.

People have thrown around the phrase to "smell" fear for centuries. Some attribute the phenomenon of animals "smelling" fear to an acute ability to read body language or even hear a racing heartbeat, not to an actual olfactory cue. However, there is reason to believe that scent cues do exist. When human beings are scared, they increase production in their apocrine glands. These are the specialized sweat glands which produce body odor. When we're strongly stimulated (sexually, emotionally), the glands produce chemicals that are broken down by bacteria, which we smell as body odor. Some have suggested that this response could reveal the intensity of emotion, but not specific details about which emotion. It's also possible that we produce pheromones which are detectable by other members of our own species, but these are hard to isolate and thus tough to show scientifically.

The researchers from Rice University have been looking at the effects of scent cues in sweat for years. In 2006, they found that exposing women to sweat produced by fearful men enhanced their cognitive performance on a set of tests. Now, they sought to determine if the fear chemosignals could affect how we perceive the world around us.

The idea that we might be able to pick up on fearful cues in human sweat isn't new. In 2002, researchers from the University of Vienna found that women could distinguish and identify the scent of "fear" sweat from neutral sweat and scent controls when told to rate them by intensity, pleasantness, and how much it smelled like "sex", "aggression" or "fear". However, no study has shown whether these cues influence our own perceptions of fear in others. In other words, are we more likely to think someone is afraid if we can smell fear?

In two separate experiments, Rice researchers had men watch scary movies with absorbent pads under their arms to capture the 'smell of fear', or fearful sweat. Afterwards, women were shown faces that morphed from happy to neutral, and finally to fearful expressions while smelling neutral and fearful sweats. They were asked to ID the faces as "happy" or "fearful". When smelling the fearful sweat, the participants interpreted the faces as more fearful when the expressions were ambiguous or neutral, but not when the faces were clearly happy. Thus the chemosignals in the men's fearful sweat modulated the women's visual emotional perception - the first time such an effect has been documented.

This means that scent might help us understand each other when our other senses are unsure of the situation. Evolutionarily, this makes sense. Being able to pick up on someone's fear in a situation when visual cues are ambiguous could give someone the edge in avoiding danger. Future research will look into exactly how we process the smell of fear and which compounds might be involved in the emotional response. I think it would be particularly interesting to see if women are affected the same way by aggressive or sexual sweat - that is, do we think faces look angrier when smelling angry sweat or think a person is into us when smelling sexual sweat. At any rate, studies like this one are opening our eyes - and our noses - to the complex world of olfactory signaling in humans. Who knows? Perhaps, in the future, we will be able to pick our scent signals like we do perfume.

Zhou, W., & Chen, D. (2009). Fear-Related Chemosignals Modulate Recognition of Fear in Ambiguous Facial Expressions Psychological Science, 20 (2), 177-183 DOI: 10.1111/j.1467-9280.2009.02263.x