Abstract
Humans, like other animals, have an excellent sense of smell that can serve social communication. Although ample research has shown that body odours can convey transient emotions like fear, these studies have exclusively treated emotions as categorical, neglecting the question whether emotion quantity can be expressed chemically. Using a unique combination of methods and techniques, we explored a dose–response function: Can experienced fear intensity be encoded in fear sweat? Specifically, fear experience was quantified using multivariate pattern classification (combining physiological data and subjective feelings with partial least-squares-discriminant analysis), whereas a photo-ionization detector quantified volatile molecules in sweat. Thirty-six male participants donated sweat while watching scary film clips and control (calming) film clips. Both traditional univariate and novel multivariate analysis (100% classification accuracy; Q2: 0.76; R2: 0.79) underlined effective fear induction. Using their regression-weighted scores, participants were assigned significantly above chance (83% > 33%) to fear intensity categories (low–medium–high). Notably, the high fear group (n = 12) produced higher doses of armpit sweat, and greater doses of fear sweat emitted more volatile molecules (n = 3). This study brings new evidence to show that fear intensity is encoded in sweat (dose–response function), opening a field that examines intensity coding and decoding of other chemically communicable states/traits.This article is part of the Theo Murphy meeting issue ‘Olfactory communication in humans’.
Highlights
Humans use multiple sensory modalities to express themselves in a social world
The three donors were representative of the larger male donor sample (N = 36) in terms of age, their adherence to a scent-free protocol, and amount of sweat produced in the lower fear intensity condition
These methodologies had never been applied to the field of human chemical communication; yet, through this unique combination, we could identify a dose–response function between experienced fear intensity and intensity encoding in fear sweat
Summary
Humans use multiple sensory modalities to express themselves in a social world. An important element to everyday social interactions is the expression of our emotions, which can be conveyed through our face, body, speech, touch and even our smell. Studies capitalized on the idea of emotional contagion, as might be induced by watching scary videos [16], skydiving for the first time [17], participating in a high rope course [18] or awaiting an important academic examination [19] In these studies, successful fear induction was based on self-report questions (e.g. more reported fear, anxiety) typically combined with one physiological endpoint, including—in the order of frequency—higher heart rate (HR), more armpit sweat production, higher cortisol, higher skin conductance levels (SCL) and higher respiratory rate (RR) (see [12], for a list of studies). We made the following predictions: (i) using scary film clips, participants would experience discrete fear (beyond high negative arousal per se); (ii) capitalizing on individual response variability, participants’ fear experience will be classified into different intensities (low, medium and high); (iii) stronger fear experiences will translate into more produced (armpit) sweat; and (iv), higher doses of fear sweat will be linked to more volatile molecules being emitted by the sweat, providing decoders with a quantity cue/signal
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