Humans can 'smell' each other's emotions — but we don't know how

After a viral infection robbed Chrissi Kelly, an American archeologist living in the U.K., of her sense of smell, she no longer felt like herself. It was as if she were "floating away," untethered from the rest of the world. Smell, she says, is something that binds us to nature and to our family, and without it, we cannot fully participate in everyday life. She missed the social part of scents: the deep joy of hugging a loved one and taking in their personal aroma. "I found living without the sense of smell profoundly disorienting," she says.

Kelly felt so strongly about what happened to her that she started a charity called AbScent to help people with smell loss. Kelly's perception that smell forms part of a person's identity is now receiving confirmation from recent research findings. A 2023 study from European researchers found, for instance, that not only can we pick up the scent of other people's fear or anxiety, but such emotions affect how we feel, too. Another study from China showed that people with better olfaction have more friends. "We see all kinds of behavioral effects," says Shani Agron, a neurobiologist at the Weizmann Institute of Science in Israel.

Humans have a long history of disregarding our noses — even Darwin claimed that the sense of smell is of "extremely slight service" to people. According to Bettina Pause, a biological psychologist at Heinrich Heine University Düsseldorf in Germany, one reason may be that social olfaction happens outside of our conscious attention. "The only thing I might know about this conversation is that my body feeling changes," she says. Yet humans seem quite able to pick out someone else's body odor. One study found that after shaking hands with people of the same gender, people reflexively sniffed their right hand more than twice as often as they did before the greeting.

Related: Why can't we smell ourselves as well as we smell others?

We pick up quite a lot of information from sniffing the body odor of people around us: we can recognize our kin, tell who is genetically related and pinpoint potential friends (we tend to choose friends who are genetically similar to us and have similar body odor). In one study, most new mothers were able to identify their baby by its smell after spending as little as 10 minutes together, and newborns can recognize their mother, too.

Adult human sniffers, meanwhile, can match pairs of identical twins by their body odor, even if the siblings live apart. In a 2022 study, researchers at the Weizmann Institute of Science managed to predict which volunteers would bond together simply by comparing their body odor — a task performed both by human smellers and an electronic nose (a device that looks like an old CB radio with a hose). The scientists discovered that people who smelled similar to each other were more likely to enjoy chatting and report that they felt instant chemistry. This goes along with earlier research showing that we subconsciously choose friends who share some of the same genes.

What's more, if we were to chat with someone feeling happy, chances are we would detect their current emotional state through smells that reach the nose. In one experiment conducted in the Netherlands, volunteers watched cheerful videos while holding absorbent pads in their armpits. Later, when another group sniffed the pads, measurements of their facial muscles' activity revealed that their mood improved, too: their smile muscles moved more.

Yet it's not only happy feelings that can be communicated through body odor. A 2020 study by Pause and colleagues showed that women's brains reacted more strongly when they smelled the sweat of men who had played an aggressively competitive game compared with the odors of men who had just enjoyed a calm construction game. It turns out that women also proved to be particularly sensitive to odors that signaled male anxiety. On picking up such odors, they became more risk-avoidant and less trusting. "Anxiety is a signal of, 'Please, I need help,'" Pause says. This, she believes, may explain why women appear more attuned to the smell of anxiety — historically, in distressing situations, it was women that cared for the young and the feeble. Such evolutionary links could also explain why women with more discerning noses perform better at tests of empathy, as revealed in a small 2022 study carried out by Pause and her colleagues.

In general, a sensitive nose seems to be an asset that enhances our deeply social life. Those who could better tell apart everyday odors also reported less loneliness, a 2020 study of 221 volunteers concluded. In other experiments, people with a better sense of smell had a larger social network and more friends, and they met with those friends more often. Functional magnetic resonance imaging of the brain, meanwhile, revealed that the same brain circuits may be involved in both our sense of smell and the size of our social circle.

For now, however, the mechanisms of how exactly humans pick up body odors and translate them into changes in our behaviors remain largely a mystery. "It's a multifaceted problem that we have yet to really begin to tease apart," says Johan Lundström, a neuroscientist at the Karolinska Institute in Sweden. Scientists are also just beginning to pinpoint which chemicals in body odor may be responsible for influencing social connections. One such molecule may be hexanal, which gives off a pleasant whiff of freshly cut grass — and appears to boost trust in people. Yet we still don't know if those who have more hexanal in their body odor are perceived as more trustworthy, says Monique Smeets, a social psychologist at Utrecht University in the Netherlands.

More research will likely follow because, as Agron says, "The pandemic really put a spotlight on the sense of smell." Even though omicron appears to be less damaging to our noses than previous COVID variants were, a 2023 study estimated that 11.7% of adults of European ancestry who have been infected with omicron have had some amount of olfactory dysfunction. People with smell loss may end up missing out on important but subconscious ways of communicating with others. And smell should be valued because olfaction is the most honest of our senses — something that, unlike our words or facial expressions, we just can't fake. "I can laugh even though I'm sad or aggressive, but I cannot intentionally change my chemical messages," Pause says. "It's kind of the only information which you can trust."

This article was first published at Scientific American. © ScientificAmerican.com. All rights reserved. Follow on TikTok and Instagram, X and Facebook.