Why You Love the Way Your Partner Smells

The Science of Sex is a column from Broadly exploring the tech behind the complicated and fantastic ways we get off—because sex is sexy, but science is sexier. This week, we learn how a little-known genetic mechanism is the reason we don't all accidentally have sex with our brothers.

Humans, explains Dr. Claus Wedekind of the University of Bern, have been genetically programmed to discern—from smell alone—whether potential partners may be closely related to them, and reject or accept them on this basis. Wedekind, an expert on human sexual selection, specializes in analyzing a cluster of genes called major histocompatibility complex (MHC) genes. These genes play a critical role in the immune systems of vertebrates, and are often tested to determine how suitable individuals are for organ transplants. If your organ donor isn’t from a similar MHC pool, there’s a higher chance the transplant won’t work. In humans, pheromones secreted from the body carry markers that help identify each individual's MHC genes.

And importantly, MHC genes can help to demonstrate the genetic makeup of an individual. Someone with very similarly coded MHC genes to you may well be a relative; someone with differently coded genes will likely not share any family members with you.

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“I read an old scientific paper from 1976 about how the MHC gene influences urinary odors in mice,” Wedekind remembers. “It found that male and female mice prefer mice who smell differently, because they have different MHC genes.” The report’s authors suggested that this might be a potential mechanism to avoid inbreeding in mice.

The suggestion stuck with Wedekind. “I thought it made perfect sense. Because if you’re worried about mating with individuals of a similar genotype, then [such a mechanism] would automatically avoid in-breeding.”

Inspired, he decided to test his hypothesis on humans in 1995. Does the way we smell indicate how closely we may be related to people—and how does this affect our romantic choices? “I wanted to find out whether MHC is important when it comes to avoiding inbreeding," he explains.

This specific function of MHC genes, Wedekind says, are an ancient evolutionary hang-up. “We’re looking at a very old mechanism that evolved at a time when humans lived in tribes of maybe 100 people, and the genetic relationship between individuals in the tribe wasn’t always clear."

While individuals would know who their mother was, they might not necessarily know their father or the genetic origins of their extended family. “It’s possible that in your hundred-strong tribe, possible mating partners could be half-siblings, and you wouldn’t know," he explains. The result? Inadvertent incest.

Some in incestuous relationships have argued that there's nothing wrong—from an ethical point of view, at least—about consensual sex with a relative. But one thing is for certain: genetically speaking, incest is a terrible idea when it comes to procreating. Inbreeding dramatically increases offspring’s chance of contracting a range of genetic diseases, as well as dying early. So it makes sense that humans would have evolved a way to avoid inbreeding as much as possible when choosing future mating partners.

As part of an experiment, Wedekind recruited 49 female students and 44 male students to part in a seminal research project that became widely known as the “sweaty T-shirt study.” Female participants were given T-shirts that had been worn by six male participants, and asked to evaluate them by smell. Half of the T-shirts assessed had been worn by men that had similar MHC genes to the women, and half worn by men who had different MHC genes to the women.

“The females preferred MHC-dissimilar males as mating partners,” Wedekind explains of his finding. In essence, we’re evolutionarily programmed to prefer sexual partners who don’t share similar genes to us—and to dislike people who may be related to us.

Fascinatingly, women who were on the pill had the exact opposite response; they preferred the T-shirts of the men who smelled like they could be related to them. “As the contraceptive pill assimilates to some degree the effects of pregnancy on a woman’s body,” Wedekind explains, with the caveat that this is speculative, “it’s possible that pregnant women prefer the odors of MHC similar individuals because they prefer to be around relatives that would support them during pregnancy.” However, he adds, more research is needed to definitively establish a link.

Dr Wedekind's smelly t-shirt study has subsequently been supported by a generation of researchers, although there is still dispute on the details. For example, a debate continues around whether we prefer to find partners that are merely MHC dissimilar, or as different as possible from our own MHC types. "Some time ago it was realized that, at least in humans, optimal difference, rather than maximal, may be the best strategy," says Professor Charles Wysocki of the University of Pennsylvania via email.

However, the evidence that MHC genes play a powerful role in mate selection is compelling: one 2015 evidence review of 34 scientific studies found that "MHC is likely to be involved in mate-choice decisions for many populations," and that, leaving aside socio-economic factors that complicate things (such as modern hygiene routines that mask our natural smell), "human populations do show evidence for MHC-mediated mate selection."

So if you really like the way your fuck buddy smells—great! But since we don't live in such small, isolated communities as we once did, you're most likely not at risk of being related anyway.

“Our ecology nowadays doesn’t fit the ecology that existed when this evolutionary mechanism evolved. If you meet someone with a similar MHC type to you, they’re very likely not your relative,” Wedekind says. “So this mechanism has lost its evolutionary function, but it’s still there.”