As far as your nose is concerned, the old adage “opposites attract” couldn’t be further from the truth, a new study suggests. The research, published today (June 24) in Science Advances, suggests that people whose natural body odors are chemically similar are more likely to “click” and form fast friendships.

Through a series of experiments designed to uncover the effects of body odor on social dynamics, researchers from the Weizmann Institute of Science in Israel found that existing pairs of same-sex, nonromantic friends who reported having clicked when they first met emitted similar odors. Additionally, a sensor that the study describes as an “electronic nose” was able to predict above chance levels whether strangers would feel a click upon meeting, based on the chemical signature of their natural body odors.

“I feel comfortable to conclude now that there is chemistry in social chemistry,” study coauthor and Weizmann neurobiologist Inbal Ravreby tells The Scientist.

Sniffing out friends over foes

In the study, the researchers first polled people online for how they would describe the initial spark preceding a fast friendship—which, according to Ravreby, is often intuited but doesn’t exist in the scientific literature. As she describes it, common themes kept emerging, such as being on the same wavelength as someone, feeling as though there is “chemistry” between them, and experiencing a sense of immediate bonding and understanding. Using those definitions, the team recruited 20 existing pairs of same-sex friends between the ages of 22 and 39—half of whom were men and half women—in which both members reported having clicked upon meeting.

In order to harvest participants’ body odor, each was provided with unscented soap and a cotton T-shirt along with instructions to avoid all scented products, such as other soaps, perfumes, and lotions, before wearing the shirt for at least six hours overnight for two nights. Participants were also asked to keep pets and other people out of their bed for those nights, and to avoid eating body odor-influencing foods such as curry, asparagus, and garlic.

A woman sits at a desk, using electronic equipment to analyze a T-shirt stored in a glass jar with an attached hose.
T-shirts containing each participant’s scent were frozen overnight and then stored in a glass jar for chemical analysis.

The odors trapped in the shirts were then evaluated by a commercially available electronic nose, which uses an array of chemical sensors to determine the components of a gas. In order to compare the device’s performance to that of a human nose, which experts note is far more sensitive, other participants were presented with the odors of each person from either a click pair or a random pair as well as a third “distractor” odor, according to the paper, and were asked to guess which two, if any, were from those who had clicked. While there were differences between human and electronic perception, the test ultimately helped validate the device’s predictions.

See “Malarial Mosquitos Drawn to Human Smell

Finally, the authors wanted to test whether odor similarity predicted clicking among strangers meeting for the first time. For this experiment, the team assigned pairs of strangers who had followed the same T-shirt protocol to stand facing each other from a short distance and mirror each other’s movements without speaking for two minutes. All of the participants then repeated the process with all the participants of the same gender, resulting in 22 pairings among men and 45 among women, and rated how well they meshed with each partner, answering questions including whether they felt connected to the partner, whether they wanted to meet them again, and whether they felt comfortable with them, on a scale of 1 to 100.

Across all tested cases, there was a subtle correlation between body odor similarity and clicking that exceeded what would be expected by chance, and the difference in body odors between click friends was smaller than the difference in odor between other pairs, the paper reports.

“The finding that it could predict clicking by body odor similarity alone—this was really cool,” says Ravreby. “We were really excited to find this.”

Still, experts caution that the effects were weak and there was considerable overlap in the data. Leslie Kay, a neurodynamics researcher who studies olfaction at the University of Chicago and didn’t work on the study, notes that data from both clicking and nonclicking pairs often appeared similar, and that the correlations only emerged when calculating the data of each group as a whole.

Kay tells The Scientist that the results aren’t strong enough to suggest “you can just walk up and sniff strangers and say ‘Oh, you’re going to be a really good friend.’ You’d probably be really bad at that, and get punched in the nose or something.”

Faint, friendly smells

Kay and other experts also note the myriad possible confounding factors that complicate social dynamics in any setting outside of a highly controlled experiment.

“It is worth noting that this paper has several experiments, but the overall number of people involved is rather limited, and limited to a specific case” designed by the study authors, Valentina Parma, a psychologist studying olfaction at the Monell Chemical Senses Center in Philadelphia who didn’t work on the study, tells The Scientist over email. “These results provide a good starting point to answer additional questions.”

Essentially everything that the study controlled for—lifestyle factors including diet, scented products, pet ownership, and exposure to different people—as well as genetics and even mood, can influence the odor one emits.

See “The Genetics of Friendship

“If you were to tell somebody, ‘Here’s how you need to sniff somebody to see if they’re going to be your friend,’ you probably couldn’t because they’re always eating whatever food they eat, they’re always wearing whatever scents they normally wear,” says Kay. “They’re always going to have those odors that could certainly mask or change the key features of the body odor that contribute to that sense of familiarity.”

Ravreby, however, points to the effects of those confounders as possible support for the study’s findings. After all, the existing click pairs still bonded when those variables were at play, indicating that the effect of their similar odors may not have been entirely masked. However, Kay notes that it’s also possible people became good friends because of their lifestyle similarities, which could lead to the emission of similar scents—meaning smell itself may not have factored into their clicking after all.

The finding that it could predict clicking by body odor similarity alone—this was really cool.

—Inbal Ravreby, Weizmann Institute of Science

Ravreby readily allows that “there are other factors [in clicking], and we know many of them. For instance, visual similarity. . . . We wanted to add body odor similarity to this list.”

“Remember that this is, if at all, one factor that is important for click friendship and it certainly isn’t the most important thing or maybe even a very big thing,” says Jessleen Kanwal, who studies olfaction in insects at Caltech and didn’t work on the study. In addition to scent, bonding is “probably very much going to depend on other inputs that we are getting through other sensory streams, like what we are seeing about a person [and] what we experience from communicating with them, that are going to be integrated in the brain and allow us to make that click friendship decision or not.”

Still, Kanwal and others tell The Scientist that the study is likely correct in identifying body odor as a factor influencing friendships and social connections—scent is important to interactions among nonhuman animals, so it’s reasonable to assume it would be to humans as well, they say.

“It’s really important to remember that we are mammals as well, and we have some shared mechanisms with other animals,” Ravreby says.

Animal instincts, human relationships

Ravreby says that her study can’t explain why body odor seems to influence friendship in humans, but that she’s now designing a follow-up study that will attempt to reveal the underlying mechanisms. In it, she says, she and her colleagues will manipulate the body odor of study participants, put them in an fMRI machine, and scan the activity of brain regions associated with olfaction and social behaviors while exposing them to body odors that either match or differ from their manipulated odors.

Speculating on what that study might reveal, Ravreby suggests that perhaps people use their own body odor as a sort of template against which they compare that of other humans, not unlike how other animals use odor information “to decide who is a friend or a foe,” she says.

By remembering that humanity’s advanced cognitive abilities don’t necessarily place us on a pedestal, unaffected by the kinds of chemical communication prevalent among other mammals, “we can learn about how we as humans work and what are the underlying mechanisms of our behavior,” she adds. “I think we should appreciate the science of body odor at least a little more.”