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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.

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.

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.

Accumulating evidence has pointed to a human capacity to communicate emotions to others via sweat. So far, these studies have relied exclusively on Western Caucasian samples. Our aim was to test whether the chemosensory communication of emotions extended beyond ethno-cultural boundaries, from Western Caucasians (N = 48) to East Asians (N = 48). To test this, we used well-validated materials and procedures, a double-blind design, a pre-registered analysis plan, and a combination of facial electromyography (EMG) and continuous flash suppression techniques to measure unconscious emotions. Our results show that East Asian (and Western Caucasian) female receivers exposed to the sweat (body odor) of fearful, happy, and neutral Western Caucasian male senders emulate these respective states based on body odors, outside of awareness. More specifically, East Asian (and Western Caucasian) receivers demonstrated significantly different patterns of facial muscle activity when being exposed to fear odor, happy odor, and neutral odor. Furthermore, fear odor decreased the suppression time of all faces on an interocular suppression task (IST), indicating subconscious vigilance, whereas happy odor increased the detection speed of happy faces. These combined findings suggest that the ability to perceive emotional signals from body odor may be a universal phenomenon.

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.