Chemical Communication is Nothing to Sniff At

The highs and lows of chemical communication • In love with the smell of you • Creating New Smell Stories in Virtual Reality

When we call to mind the people we love, it’s common to conjure the image of a smiling face or the comforting sound of someone’s voice. But what about their smell? Humans’ sense of smell, supported by 6 million olfactory receptors, may seem measly compared to the 100 million to 300 million receptors that dogs possess, but researchers are quickly finding that our noses tell us much more about our social lives than once thought. 

“The role of the human sense of smell in social interactions has been underestimated for a long time,” said Helene M. Loos (Friedrich-Alexander-Universität Erlangen-Nürnberg), who studies the perceptual effects and chemical basis of human body odor, in an interview with the Observer. “Nowadays we know that body odors convey information about our conspecifics, for instance their health or emotional state.”

Noted academic figures such as Charles Darwin, Paul Broca, and Sigmund Freud are all documented as disparaging our sense of smell as “of extremely slight service, if any” as early as the 1870s. During this time, scholars labeled humans “microsmatic” in a bid to align the scientific understanding of humans’ sensory abilities with elitist Western social standards that equated odor with uncleanliness, wrote Loos and colleagues in a 2023 article for Perspectives on Psychological Science. 

“Intellectual figures disparaged olfaction as a source of rational knowledge, aiming ultimately to differentiate humans from reeking and snuffling animals,” Loos and colleagues explained, but “‘unintentional and unnoticed’ does not equate to ineffective and nonfunctional.” 

Like other animals, humans have evolved to adapt our thoughts, feelings, and behaviors in response to the scents emitted by the people around us. But this kind of chemical communication often depends more on our social environment than once thought, the researchers wrote. Certain animals, such as insects, produce chemical compounds known as pheromones that compel the receiver to perform behaviors like mating, but even these seeming stereotyped responses can be influenced by environmental conditions, Loos and colleagues explained. Humans’ olfactory responses are even more context-dependent, with variation across relationship categories, age, sex, and cultures. 

The highs and lows of chemical communication 

Smell can influence humans’ sexual preferences by encouraging us to: 

• pursue certain mates while avoiding incestuous relationships, 
• limit the spread of infectious diseases by warning us when someone is sick, and 
• prepare us for threats by alerting us to other people’s fear. 

This kind of chemical communication often offers benefits to both the sender and the receiver, Loos and colleagues noted. For example, fear-induced sweat may serve as both a silent cry for help and a warning to others. In a 2021 study in Psychological Science, Jasper H. B. de Groot, Peter A. Kirk, and Jay A. Gottfried (University of Pennsylvania) found that smelling another person’s fear-induced sweat indeed alters our perception, triggering brain activity that causes us to see fearful expressions in otherwise neutral faces. 

Related content from ICPS 2023: Blood, Sweat, and Tears: Human Social Chemosignaling in Health and Disease

The researchers collected sweat samples from 36 men who wore absorbent pads in their armpits while watching fear-provoking horror clips and neutral nature scenes. During this stage of the study, de Groot and colleagues found that the men produced more sweat and volatile odorant molecules, the substances responsible for body odor, when they reported feeling fearful and exhibited physiological signs of tension, including increased heart rate and breathing.

Later, in an fMRI experiment, the researchers exposed 32 women—who are generally more responsive to scents than men—to the participating men’s sweat pads while they rated the expressions of ambiguously fearful and disgusted faces. This resulted in a pair of congruent findings. The female participants who smelled the male fear-generated sweat showed more activity in the amygdala, a brain region associated with perceiving fearful faces, than the women who smelled the neutral sweat. When they were exposed to fear-induced sweat, the female participants also reported perceiving ambiguous faces as more fearful. Moreover, they perceive this fear in an “all-or-none” manner, with the sweat from low, medium, and highly fearful men having the same perceptual effect. 

“These combined findings suggest that the human sense of smell engages a binary on/off mechanism for identifying body odor as a fearful stimulus,” de Groot and colleagues wrote. “This ensures that the receivers of these fear odors are safe rather than sorry.” 

Fear isn’t the only feeling that can be communicated through body odor, however. de Groot’s research suggests that we can smell other people’s happiness, too. In an article published in Psychological Science, de Groot and colleagues reported results from a similar experiment to their study on fear. This time, nine men watched a combination of film clips found to induce feelings of happiness, fear, or neutrality while sweat was collected from their armpits. Soon after, 36 women were exposed to these sweat samples while their facial expressions were monitored with electrodes as they completed tasks on a computer. 

The sweat samples had mixed effects on the women’ task performance. During a figure-selection task, the women exhibited a global processing style, which is associated with positive mood, after smelling happy sweat samples but not fearful ones. On the other hand, in another task the sweat samples were found to have no effect on how pleasant or unpleasant they perceived a set of neutral symbols to be. 

Data from the facial electrodes revealed that the women participants’ expressions changed significantly depending on the type of sweat they were smelling. Compared to the neutral condition, sweat produced during a happy film led to a “Duchenne smile,” characterized by the presence of wrinkles around the eyes, and sweat from a fear-inducing film was more likely to lead participants to raise their eyebrows into a fearful expression.

Furthermore, participants’ facial expressions showed no relation to how pleasant or intense they perceived the sweat sample to be. And although they could distinguish happy and fearful samples from neutral ones, they could not reliably tell happy and fearful samples apart.

Related content: A Sniff of Happiness: Chemicals in Sweat May Convey Positive Emotion

Together, these findings suggest that human body odor can transmit feelings of happiness or, at least, positive affect. Measures that go beyond participants’ descriptions of their feelings may better reflect the effects of this chemosignaling, the researchers wrote (de Groot et al., 2015). 

Though fear often captures the spotlight because of its role in preparing us for potential threats, happiness can counter the damage of such negative emotions by restoring our cardiovascular, neuroendocrine, and immune systems, the researchers noted. 

“Humans are a social species with the capacity to share these positive effects, using not only modalities such as vision, hearing, and touch, but also—as this exploratory study indicates—the sense of smell,” de Groot and colleagues wrote. 

In love with the smell of you 

Chemical communication is processed not only in the amygdala but in other parts of the brain’s emotional limbic system, including the fusiform gyrus and insula, Loos and colleagues noted. Although scents may activate the thalamus, a region associated with attention and language, and hypothalamus, an area that regulates hormonal functions, social scents can skip over these regions entirely, allowing them to more directly influence our feelings and memories through the limbic system and hippocampus, the researchers said. 

This suggests that the human brain evolved to glean social information not just from other peoples’ appearance or behavior, but from their smell, Marlise K. Hofer, Frances S. Chen, and APS Fellow Mark Schaller (University of British Columbia) wrote in Current Directions in Psychological Science (Hofer et al., 2020). This form of chemical communication provides cues about other peoples’ emotional states, health, and identity, Hofer explained in an interview with the Observer. 

“Human smell communication is largely unconscious, and because of this, it is often surprising to find out how much humans learn about one another via odor,” Hofer said. 

Research has shown that this sensitivity to scents is intertwined with some of our most intimate relationships, the researchers continue. In one study, for example, premature infants were discharged from the hospital 4 days sooner when they were exposed to their mother’s scent.

Later in life, we may also find comfort in the aroma of our romantic partners, which has been found to reduce stress, improve sleep quality, and even reduce the perceived painfulness of electric shocks. 

In a study involving 155 couples, Hofer and Chen found that sleeping with a T-shirt that carried a romantic partner’s scent improved participants’ sleep as much as taking melatonin. As part of this study, one member of each couple served as a scent donor by wearing a T-shirt for 24 hours. The other member of the couple was then tasked with sleeping with one of two T-shirts on their pillow for four nights; they spent two nights with their partner’s shirt and two nights with a shirt that was worn either by a stranger or no one at all. During this time, sleeper participants reported when they went to bed and woke up in the morning using both a sleep diary and a wearable wrist monitor, which also recorded their sleep/wake activity throughout the night. 

On nights when participants slept with their partner’s shirt on their pillow, they spent 2% more of the time they were in bed asleep, Hofer and Chen reported in Psychological Science (Hofer & Chen, 2020). 

Research on scents and sleep could help provide simple interventions to improve sleep quality, particularly when we are away from our homes and loved ones, the scientists wrote.

“Unlike sights and sounds, smells can linger long after a person is gone. This means that they could be used to promote the positive effects of a loved one even when they are not physically present,” Hofer said. 

More research and methodological advances are needed to uncover other ways that body odor may contribute to relationships, Hofer and colleagues noted. Researchers have already begun analyzing the chemical compounds groups of people release into entire rooms, paving the way for more precise measurement of our responses to specific events, the researchers wrote. This and other advances could allow researchers to identify and manipulate these compounds and may ultimately lead to applications such as diagnosing diseases from a patient’s odor, they added (Hofer and Chen, 2020). Answering lingering questions about chemical communication will require international collaboration between psychologists and chemists, microbiologists, medical researchers, and anthropologists, among other scholars, Loos and colleagues wrote in their Perspectives article. 

“Developing such interdisciplinary programs is no mean feat,” the researchers said. “It will require international consortia conjoining disciplines, with a strong focus on building systematic, open-access databases and innovative methods for the analysis of complex datasets, including machine- and deep-learning approaches.” (Loos et al., 2023) 

Understanding how our sense of smell informs our social experiences is more important than ever as people spend increasing time communicating online instead of in person, Loos and colleagues wrote. The smell loss associated with viral infections such as COVID-19 could also negatively impact people’s social relationships, Hofer said. 

Although we may finally be ready to accept humanity’s place in the olfactory animal kingdom, we must now confront how these newfound sources of nose-blindness could be influencing our relationships and how to support people who may be missing out on social scents. 

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