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Can Science Explain Why Long-Term Couples Stop Kissing?

What’s the real reason so many couples halt the oral spelunking when the relationship gets serious?

At first, deep kissing — what is also shudder-inducingly sometimes called “oral spelunking” — is crucial, wonderful and frequent, an act so potent in the early stages of courtship that you could swear it generates actual electricity. But after you’ve secured your partner and speplunked yourselves into the easy-chair phase of the relationship, it turns more perfunctory and far less titillating — more like a handshake than a firework. Is that, though, a death sentence, or a perfectly ordinary, if boring, stage of attraction that in no way signals the end of love?

Like anything else, to understand why we stop kissing, you first have to understand why we start kissing in the first place. Science doesn’t completely know — including whether or not we learn to kiss or are instinctive kissers, since about 10 percent of the world doesn’t smooch, and some people just hate kissing — but there are a few theories, some of which are a lot more appealing than others. They are, roughly, as follows:

To Sniff Out a Mate. This theory posits that we kiss to get a better smell of the person we might end up breeding with, because human scent tells us whether our immune systems are different enough to produce a more optimal human who will survive diseases, bouncy houses and public school lunches. It’s a kind of “primal interview” for assessing suitable mating, in other words. And given that kissing for a few seconds transfers 80 million bacteria, it’s also an act of colonization.

So You’ll Screw Each Other. Another theory is we kiss to get hornier so we’ll knock those boots straight into Baby Town.

Because of Mommy. An older theory is that we kiss now because early human mothers used kissing to feed their babies when it was time to wean off the boob. Basically, moms chewed up food for babies and pushed it into their mouths using lip contact, i.e., kissing. The baby’s “searching tongue movements” found that food to eat. (Alicia Silverstone was widely mocked when she admitted she did this mouth-to-mouth or kiss-feeding, or baby bird style, with her kid, who is also named Bear.) When adult human beings kiss erotically now, it’s considered by anthropologists to be a “relic gesture” of that behavior. Gross. (I warned you!)

As Relationship Maintenance. Bonobos, for instance, use kissing to bond and make up after conflicts. People do the same sometimes, and it may be for the same reasons. 

That’s what scientists think. But what about what we think? As kissing humans, we have our own theories about why we need to wiggle around inside each other’s mouths. In 2013, researchers at Oxford University surveyed more than 900 men and women aged 18 to 63 about their attitudes toward kissing (kissing meaning “French” kissing here) to see if they could figure out why we really kiss.

Generally, those surveyed said kissing was most important before sex, and not as much during or after the deed. This was particularly true if the person was a woman, someone who considered themselves “highly attractive” or someone who liked casual sex. Because of this, researchers concluded that kissing is in fact for the same reasons the first theory up top suggests: as a biological interview to assess mating potential (which is generally the most widely accepted theory about why we kiss).

All this helps explain why we might stop kissing after a while, too. Because if kissing is to assess a potential mate, we might stop doing it so much after a while because it may be less necessary to keep kissing after you’ve committed. Essentially, kissing has done its job, and it can now lie back and rest on its laurels. Mostly.

At first, “kissing is a commitment device — a signal to your partner that you’re going to stick around,” Valerie Curtis, a doctor at the London School of Hygiene and Tropical Medicine, told the Telegraph about the Oxford study. She also wrote a book about revulsion, Don’t Look, Don’t Touch, Don’t Eat: The Science Behind Revulsion, and told the paper that kissing evolved out of our natural disgust for other people, a way to get past being grossed out by doing a feel-good thing, basically.

Per Curtis: “One of the biggest dangers we faced wasn’t from large predators, but from invisible predators inside: the worms, the scabies, the parasites. So we all have ancestral voices telling us to avoid them. If you see someone walking down a street, they’re a seething mass of parasites. You certainly don’t want to kiss them. On the other hand, humans are deeply social, so we have to deal with this problem. … We have to prove we can get over the disgust. Kissing is the first sign that you’re taking a risk.”

While that makes sense, it doesn’t explain why some people are so disappointed when kissing starts fading out of a relationship, and takes it as a sign the bloom might be off the rose. If it’s not as useful anymore, why do we miss it? Therein lies the kissing conundrum: At first, kissing is a commitment device that signals getting over disgust. Later, when you’re no longer kissing, it may be a sign that disgust has taken back over.

“Relationship psychology shows that one of the first signs that your marriage is in danger is when the disgust starts to get the upper hand — when you start squirming at his smelly feet or her nasty socks,” Curtis has said. “It’s the first sign that you no longer love this person.”

The conclusion for us then is, if you’re not kissing someone because you’re repulsed by them, pack it up. Because if you’re not kissing and you’re also not having any sex whatsoever, you’re not in a relationship, you’re in a dead bedroom. Sympathies.

But if you’re not kissing as much because you’re in a secure relationship that’s far past the early courtship, it’s logical for kissing to slow down. One study found that one of every five couples go an entire week without kissing at all. And some eight out of 10 couples don’t kiss each other before going to bed. People end relationships over bad kissing, but they also sometimes stay together when kissing isn’t off the charts either, indicating it isn’t the most important thing in the world to everyone. In fact, some 40 percent of couples who still kiss don’t even kiss longer than five seconds. 

If that sounds sad, it simply means you’re unhappy with your own kiss quotient. It’s an easy fix, though. (There are good reasons to up it, too, as other research has found that frequent kissing later in a relationship is a good indication of relationship satisfaction and lower conflict.) Sex therapist Vanessa Marin says it’s about recreating the early courtship energy of the relationship: Things like going back to places you made out and incorporating teasing and challenges to kissing. Other therapists suggest trying to only kiss your partner for an entire week to get that early kiss mojo back.

That might not feel like the first time, but that sort of effort guarantees it certainly won’t be the last.