Out of the long list of strange things women are attracted to -- a list that includes chocolate, new shoes and man buns -- our boyfriend's body odor is probably the strangest.
We can't figure out why we love it so much, but we do. There are few things sexier than watching our guy remove his shirt after a day at the gym, or after playing football with his friends, or after going for a run and catching a whiff of his scent: a combination of deodorant, rugged masculinity and, our favorite, his natural musk (better known as body odor).
People spend hundreds of dollars trying to cover that pesky BO, layering on powders and perfumes and colognes to get rid of it.
Any traces of it on our own bodies leave us feeling dirty and untouchable, and send us spiraling into tunnels of self-consciousness, frantically texting our friends to ask if they have any Secret in their purse. Truthfully, most BO smells like onions.
However, as much as we might hate body odor, we unconsciously love it, too; if you've ever worn your boyfriend's old t-shirt and found yourself getting weirdly turned on, you've experienced this phenomenon.
The natural odor soaked into the shirt is faint enough where it's not overly powerful, but powerful enough where you can't help but feel an instant attraction to it.
A large bundle of molecules, called the major histocompatibility complex (MHC), influences the genetic makeup of our immune system, including the smell of our body odor.
In a famous 1995 study, researchers discovered that women preferred the body odors of men whose MHC compositions differed from their own, suggesting that body odor actually plays some kind of role in the mating process.
Forty-four male students were asked to wear a t-shirt for two nights in a row and to avoid anything that might interfere with their natural scent. Researchers even supplied them with unscented soap and aftershave. After two nights, each woman was asked to judge six t-shirts.
Consistently, the women said that the scents of the t-shirts worn by men whose MHC compositions differed from theirs were more "pleasant." Men whose MHC compositions were similar had less pleasantly-smelling shirts.
Of course, there are a variety of factors that influence sexual attraction, but it's definitely not farfetched to think that biology plays a part. And our noses are really good at picking people who are genetically compatible with us.
Geneticist Carol Ober from the University of Chicago studied members of the religious Hutterite clan: a closed, Amish-like society consisting of 40,000 members that extends through the rural Midwest.
Hutterites only marry other members of their clan, so the variety in their gene pool is pretty low.
Despite this, Hutterite women still managed to find partners whose MHC composition differed from theirs. The couples who had similar MHC compositions tended to have higher rates of miscarriage.
Even more so, similar MHC profiles mean difficulty in maintaining sexual attraction, suggests a study by psychologist Christine Garver-Apgar at the University of New Mexico: The more similar the MHC genotype between heterosexual couples, the lower their sexual responsiveness to each other.
Researchers Charles Wysocki, a behavioral neuroscientist, and George Preti, an organic chemist, both at the Monell Chemical Sense Center in Philadelphia, PA say that women are more affected by odors than men because women are able to sense emotions (like fear, after participants in a study had just watched a scary movie and removed their shirts) in BO and consistently score higher on smell-sensitivity tests.
And there's a lot of evidence that shows the important role smell plays in the mating selection, so this ability is certainly useful.
Women might prefer men whose immune systems are genetically different from theirs in order to give their children's immune system more genetic variation to protect from a maximum number of diseases.
It's also possible that the aversion to a similar MHC genotype protects women from inbreeding -- that is, from having sex with their cousin by accident.
So, is there an ideal smell? Is a Casanvoa a Casanova because he has a perfect genetic makeup? Psychologist Rachel Herz says no: "There's no Brad Pitt of smell.
Body odor is an external manifestation of the immune system, and the smells we think are attractive come from the people who are most genetically compatible with us."
You can't predict your preferred MHC makeup, either. MHC profiles are as diverse and distinct as fingerprints, so there are millions upon millions of combinations to choose from.
Let's take this as a sign of romantic hope: somewhere, by probability alone, someone's MHC combination will align with yours, increasing your likelihood of producing genetically healthy babies and of maintaining a healthy level of chemically-induced sexual attraction.
Now, isn't that romantic?