Among my girlfriends, who are as a rule strong-minded and opinionated, and range from long-time wives and moms to freedom-loving creative types to serial monogamists, when the conversation turns to sex, we can all agree on one thing: We love the way our men smell, whether he is a two-time dalliance or a going-on-two-decades partner.

So much so that when our boyfriend/husband/long-term hookup is away from us, we like to keep a bit of something they wore around (or wear it ourselves); T-shirts are an almost-universal favorite. And this is not just silly pining — though surely the guys are missed — it has to do with the fact that smell conjures up more than just pleasant memories of the beloved.

For years, it wasn’t clear whether or not human beings produced (and responded to) pheromones, as we knew unequivocally that animals did. Pheromones are how most animals communicate their readiness to mate, as well as other information, and aren’t just ‘smells’ but specific chemical signals that are picked up by specialized receptors. Numerous studies over the last 15 years have proved that humans also exude and pick up on these signals (but we don’t ‘smell’ them in our nose, we process them in one of the oldest parts of our brain, the hypothalamus).

Not only do pheromones exist, but they are actually incredibly complex. Turns out that we can smell all kinds of details about someone, especially someone of the opposite sex (but only if we’re heterosexual; homosexuals generally respond to sex signals and information from members of their own sex).

According to ABC News: “Women’s hypothalami are activated when they smell the chemical similar to testosterone but not to the estrogen-like substance, whereas men’s hypothalami have the opposite response: They are turned on only by the estrogen-like chemical and not the testosterone-like one. There is also sexual disparity between the specific sub-regions of hypothalamus that are activated.”

One of the things we can smell on our partners — or even randoms that we get close enough to — is whether they have had sex recently. (Irresistibly attracted to that hottie next to you in yoga class? He may have just had a roll in the hay and neglected to shower). A Journal of Neuroscience study backs up what cuckolded spouses have long known: Humans emit specific pheromones when they have sex, and they can be detected. Take note, cheating partners:

Here, we use functional magnetic resonance imaging to show that the right orbitofrontal cortex, right fusiform cortex, and right hypothalamus respond to airborne natural human sexual sweat, indicating that this particular chemosensory compound is encoded holistically in the brain. Our findings provide neural evidence that socioemotional meanings, including the sexual ones, are conveyed in the human sweat.

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