Earlier work had found that body odor from frightened people produces implicit fear in whoever smells it. What nobody had pinned down is the plumbing: which stress system actually releases the stuff. The armpit glands responsible for chemical signals carry receptors for , which points at the fast system rather than the slow one. So sweat was sampled from people preparing a speech. Which system delivered?
The fast system did it, and the smell carried across. Participants in the fast stress condition had higher heart rates and produced more armpit sweat than at baseline or in the slow stress condition. Receivers exposed to that sweat showed a fearful facial expression on and became more vigilant, classifying emotional facial expressions faster. A state in the sender reappeared as a in the receiver.
People register fear in others through eyes and ears, and also, it turns out, through the nose. Previous research had shown that exposure to body odors from fearful individuals elicits implicit fear in whoever smells them. The odor of a frightened person has a distinctive signature, produced fairly quickly.
What had gone unexplored was the physiology behind that speed. The apocrine sweat glands in the armpit that produce contain receptors for adrenalin, which suggests the release is driven by the rapid sympathetic-adrenal medullary system rather than by the slower hypothalamus-pituitary-adrenal axis. Those two systems run on different clocks, so they can be separated experimentally.
Sweat was sampled while eight participants prepared to give a speech. Those in the fast stress condition had higher heart rates and produced more armpit sweat than at baseline and than in the slow stress condition, which is the prediction confirmed.
Then the sweat was given to 31 receivers. Exposure to the fast stress samples induced a simulacrum of the sender’s state: a fearful facial expression recorded by facial electromyography, and vigilant behavior, shown as faster classification of emotional facial expressions. Weigh the numbers before the story, though. Eight donors is a very thin base for a mechanistic claim, and the receiver effects are a muscle signal and a reaction time rather than anybody saying they felt afraid.
wat. Sweat produced under fast stress pulled a flicker of fear onto the faces of strangers who only smelled it.
wat wat
It supplies a mechanism rather than another demonstration. Tying fear chemosignals to the rapid sympathetic response explains why they can be produced in the seconds that a genuine alarm would allow.
How do we know?
De Groot, Smeets and Semin sampled sweat from eight participants preparing a speech under fast or slow stress conditions, then exposed 31 receivers to it while recording facial muscle activity and expression-classification speed.
What is still unknown
Eight sweat donors is a very small sample for a claim about a physiological mechanism, and the receiver effects are a facial muscle signal and a reaction time rather than reported fear.
Learn something?
Where to next
Can you smell that somebody has come down with something?smell
Can you tell where a smell is coming from with your nose alone?smell
Can you tell hot water from cold just by the sound of it pouring?senses