Chances are, you can recall a time when you passed by a stranger, found their scent to be particularly delightful, and perhaps felt a fleeting spark of instant attraction. Or maybe you’ve stepped into a room that a partner or close friend just left only to catch a whiff of their unique smell and feel totally comforted. While these conscious smelling experiences nod to the connection between odor and compatibility, according to recent research, they’re just brushing the surface. When it comes to the different ways that body odor can influence a platonic or romantic connection, there’s likely far more than meets the nose.

What does it mean to be compatible with someone from a B.O. perspective?

A recent study on friendship and odor found that humans may pick up on certain elements of others’ scents that subconsciously draw them toward those folks or even help create a “clicking” feeling of connection. In analyzing the body odors of 20 pairs of friends who mutually agreed that they “just clicked” when they met, the researchers found that the smells of each pair were significantly more similar to each other than they were to random others. To ensure that these smell similarities were actually reflective of the peoples’ true B.O., the researchers used T-shirts that they’d slept in for two consecutive days, during which time they also didn’t use perfumes, deodorants, or scented soaps, and stayed away from pungent foods (yep, there are plenty of foods that cause body odor) and drinks—leaving just pure B.O. behind (yum).

“We suspect that having similar body odor is a relevant factor for people who experience a clicking [friendship] phenomenon with someone else.”  —Inbal Ravreby, social-sciences researcher

What makes that particularly interesting is, even though these people met their “click” friends in real life—where any number of those lifestyle elements could have masked their real smell—they still somehow gravitated toward people whose underlying natural odor was more similar to theirs than would be expected by chance, says Inbal Ravreby, lead author on the study and graduate student in the Department of Neurobiology at the Weizmann Institute of Science. “That’s why we suspect that having similar body odor is a relevant factor for people who experience a clicking phenomenon with someone else, even in a natural setting where you might not be able to detect it consciously.”

To test that theory further, the scientists also gathered a bunch of strangers, collected their smells via slept-in T-shirts, and had them play a game where pairs of people stood close to each other and mirrored each other’s body movements. Afterward, the participants filled out a questionnaire, rating various aspects of how much they felt a connection with their game partner. And 71 percent of the time, strangers who reported “clicking” also had more similar smells than random duos, suggesting a tendency for folks to subconsciously gravitate toward and feel more comfortable with others who share some components of their own B.O.

Crucially, the friend and stranger pairs in this study all included two heterosexual people of the same sex, both because people of different sexes have distinctive kinds of smells, and the biology of sexual compatibility and odor has been shown to take on a different kind of tinge than that of platonic compatibility. Prior research has found that females tend to rate the body odor of males as more sexually attractive when it reflects a Human Leukocyte Antigen (HLA, aka an underlying gene complex) that’s dissimilar from their own—particularly when they’re not taking hormonal birth control (though the same thing doesn’t happen in the other direction with males).

So, does body odor have a different effect on initial attraction or “click” compatibility depending on whether you see the person as a potential friend or sex partner? Probably. “Whereas mate selection might inherently have something to do with our ability to detect genetic diversity, friendship doesn’t require that same level of diversity,” says olfactory scientist and experimental psychologist Pam Dalton, PhD, MPH, who conducts research on scent at the Monell Chemical Senses Lab. “The idea that we naturally choose to be with different people for different reasons, like camaraderie versus procreation, and that smell plays a role in both, is quite compelling when you think about it.”

Why does body odor affect compatibility with platonic friends and romantic partners?

The short answer for why we may be wired to suss out romantic and platonic partners via scent: Evolution.

In terms of romantic partnership (at least for heterosexual partners), the evolutionary link between smell and compatibility springs from the above-mentioned research showing that females tend to gravitate toward males with a different HLA genetic code from their own, as signaled by certain smelly cues. The reason why? The HLA is a component of the immune system, and procreating with someone who has a different HLA from yours means that your child will get an immune system that’s better able to cope with a wider variety of pathogens, says Ravreby. It’s also true that a person with a different HLA from yours is unlikely to be your kin—and it’s always a good thing for a potential future kid’s genetics to have two parents who aren’t, well, related. (Again, this theory only applies to heterosexual couplings; other research suggests that “factors independent from reproduction” play a role in how smell impacts queer attraction.)

As for platonic compatibility and odor, the “why” behind peoples’ apparent tendency to choose friends who smell similar to them is a little less clear, though Ravreby still posits an evolutionary advantage.

“We know from previous research that friends are often genetically more similar than random dyads, which raises the question of, ‘How do they know?’” she says. “You don’t meet someone and say, ‘Sorry, I need to sequence your genes, and then I will decide whether I want or intend to be friends.’ But somehow this genetic similarity does happen among friends, and one potential route is through olfactory signals.”

In the same way that the particular smells linked with a person’s HLA can offer clues about their genetic dissimilarity from you—and in turn, their qualifications as a potential sex partner—these smelly signals can hint at genetic similarity, too. And choosing to befriend folks who are genetically similar to you could be of evolutionary benefit, given that they may share other key qualities with you, too, says Dr. Dalton, like cultural background, upbringing, or way of life.

How important is “odor compatibility” with friends and romantic partners, both initially and long-term?

Even though someone’s odor might make you more or less likely to instantly vibe with them as a friend (see: all the subconscious motivations above), it should be noted that it’s still just one factor of initial compatibility among many. “People have clicked with other people via a video conference, where there’s no smell involved, so we know that body odor is not a necessary factor for that,” says Ravreby. And, of course, just because someone smells good to you now—whether consciously or subconsciously—doesn’t mean that they have other traits necessary for a healthy, lasting friendship (like, say, communication skills).

As a friendship goes on, it’s also likely that odor compatibility could become less important or even less relevant. Not only are people generally less attuned to the odor of their friends than they are to that of a potential or current romantic partner (more on that below), but also, it’s the case that being friends with someone could start to make you smell similar, anyway, says Ravreby. “Friends tend to lead similar lifestyles, living nearby and eating the same types of things, and it’s possible that this influences their body odors in similar ways.”

In the case of a sexual or romantic partner, sharing some degree of “odor compatibility”—in this case, likely having genetically dissimilar odors—might matter a bit more. “We know that people aren’t very tolerant of what they sense as an unpleasant odor in a romantic partner, and it can be a contributing factor in a break-up,” says Ravreby.

And on the flip side, loving a partner’s smell could be just as positively associated with relationship longevity. A recent study on body odor among couples identified a potential “positive feedback loop” between how much a person likes their partner’s B.O. and how much they’re exposed to it, which researchers suspect may actually increase relationship commitment.

All that said, remember that Ravreby’s study was on a small group of people (and most of the other studies cited also have small sample sizes)—so, more research is needed to understand all the facets of odor compatibility IRL. As a result, much of the implications of how body odor affects connection are still well beyond what the nose knows.