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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; Q 2 : 0.76; R 2 : 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’.

Chemical communication is common among animals. In humans, the chemical basis of social communication has remained a black box, despite psychological and neural research showing distinctive physiological, behavioral, and neural consequences of body odors emitted during emotional states like fear and happiness. We used a multidisciplinary approach to examine whether molecular cues could be associated with an emotional state in the emitter. Our research revealed that the volatile molecules transmitting different emotions to perceivers also have objectively different chemical properties. Chemical analysis of underarm sweat collected from the same donors in fearful, happy, and emotionally neutral states was conducted using untargeted two-dimensional (GC&times, GC) coupled with time of flight (ToF) MS-based profiling. Based on the multivariate statistical analyses, we find that the pattern of chemical volatiles (N = 1655 peaks) associated with fearful state is clearly different from that associated with (pleasant) neutral state. Happy sweat is also significantly different from the other states, chemically, but shows a bipolar pattern of overlap with fearful as well as neutral state. Candidate chemical classes associated with emotional and neutral sweat have been identified, specifically, linear aldehydes, ketones, esters, and cyclic molecules (5 rings). This research constitutes a first step toward identifying the chemical fingerprints of emotion.

Humans use multiple senses to navigate the social world, and the sense of smell is arguably the most underestimated one. An intriguing aspect of the sense of smell is its social communicative function. Research has shown that human odors convey information about a range of states (e.g., emotions, sickness) and traits (e.g., individuality, gender). Yet, what underlies the communicability of these states and traits via smell? We fill this explanatory gap with a framework that highlights the dynamic and flexible aspects of human olfactory communication. In particular, we explain how chemical profiles, associative learning (i.e., the systematic co-occurrence of chemical profiles with state- or trait-related information), and top-down contextual influences could interact to shape human odor perception. Our model not only helps to integrate past research on human olfactory communication but it also opens new avenues for future research on this fascinating, yet to date poorly understood, field.

Most everyday smells, from lavender to body odors, are complex odorant mixtures that “host” particular compounds that guide (social) behavior and motivation (biomarkers). A key element of social behavior is interpersonal trust, and building on previous research showing that (i) lavender odor can enhance trust, and that (ii) certain compounds in body odor can reduce stress in mice and humans (called “social buffering”), we examined whether a grassy-smelling compound found in both body odors and lavender, hexanal, would enhance interpersonal trust. Notably, we applied odor masking to explore whether trust could be influenced subconsciously by masked (i.e., undetectable) hexanal. In Study 1 (between-subjects), 90 females played a Trust Game while they either smelled hexanal (0.01% v/v), clove odor (eugenol: 10% v/v), or hexanal masked by clove odor (a mix of the former). As a sign of higher trust, participants gave more money to a trustee while exposed to masked hexanal (vs. the mask: eugenol). In Study 2 (within-subjects, double-blind), another sample of 35 females smelled the same three odors, while they rated the trustworthiness of a spectrum of faces that varied on trustworthiness. Controlling for subjective odor intensity and pleasantness and substantiating that masked hexanal could not be distinguished from the mask, faces were perceived as more trustworthy during exposure to masked hexanal (vs. the mask: eugenol). Whereas non-masked hexanal also increased face trustworthiness ratings, these effects disappeared after controlling for the odor’s subjective intensity and pleasantness. The combined results bring new evidence that trust can be enhanced implicitly via undetected smells.

Our eyes and ears mainly register the continuous stream of social stimuli impinging on us; vision and hearing are the typical modalities one would associate with social communication. However, the current dissertation is concerned with a particular branch of social communication, namely that of emotional states, and it examines the role of a potentially neglected modality in human communication: the sense of smell. As there is substantial overlap in limbic brain regions processing emotional and olfactory information, olfaction is often considered to be the “most emotional” sense. Indeed, odors can easily become associated with emotional experiences (e.g., fear). This dissertation constitutes a systematic examination of whether fear can be communicated from sender to receiver via body odor, by testing hypotheses derived from an embodied social communication framework. Chapter 3 showed that receivers exposed to the body odor of fearful individuals (vs. neutral, disgust) showed signs of emotional contagion evidenced by fearful facial expression and vigilant behavior (more effective eye scanning, increased sniffing behavior, enhanced accuracy during visual search). Chapter 4, 5, and 6 were aimed at examining the boundary conditions of the chemical communication of fear. More specifically, Chapter 4 examined whether olfactory information (fear-related and neutral) would be overridden by (in)congruent, simultaneously presented audiovisual information. Interestingly, fear-related information was communicated to receivers regardless of whether fear was communicated via the audiovisual or olfactory modality. Chapter 5 showed that women who are generally more sensitive to odors and emotions emulated the fearful state of the senders, whereas men did not. Chapter 6 showed that the capability to communicate by means of olfactory signals may not only be limited to negative emotions, but could be extended to (high arousal) positive ones as well. Chapter 7 closed in on the mechanisms of the chemical communication of fear. As sweat glands in the armpit region have receptors for adrenalin, it was expected that “fear odor” would be produced as a function of adrenalin release. Indeed, what drove the chemical transfer of fear from sender to receiver was the product of the rapid stress system, adrenalin, rather than cortisol resulting from the slow stress system. Overall, the combined set of studies indicated that humans have the capacity to become emotionally affected by body odors produced during distinctive emotional states. Hence, human odors can serve as a communicative medium by transferring dynamic emotional states from a sender to a receiver.