In Western cultures, we mark the beginning of romantic entanglement by touching lips. Few actions are as fraught with anxiety and symbolism as that first kiss—and it’s no exaggeration to say that some kisses feel like life or death.

Sticking your tongue in the mouth of another person for ten seconds can transmit 80 million bacteria.

Indeed, a kiss can kill, in the most medical, unromantic, nonmetaphorical sense. Sticking your tongue in the mouth of another person for ten seconds can transmit 80 million bacteria, says one 2014 study. As if to drive this point home, last week the Brazilian government warned pregnant women to refrain from kissing for fear of contracting the Zika virus.

But romantic kissing isn’t universal, not even close. In fact, many cultures consider kissing on the lips to be repulsive—a perfectly sound conclusion, given how much disease can pass between mouths. The most authoritative cross-cultural analysis of kissing, published last year in the journal American Anthropologist, reviewed studies of 168 societies and found that less than half showed evidence of “sexual-romantic kissing,” as the authors call it.

Who are the kissing cultures? According to the meta-analysis, couples in economically developed and socially stratified cultures are almost three times more likely to kiss on the lips than those who live in tribes—who are almost four times more likely to never kiss on the lips than counterparts in complex societies. So, lovers in sub-Saharan tribes tend not to kiss, at least in front of European ethnographers, but sophisticated New Yorkers who can’t put down their smartphones seem to love a good smooch, in private and in public.

What’s up with that? We can’t say for sure, but the cross-disciplinary evidence to date suggests the rise of romantic kissing is linked to the changing roles of women. Studies show, pretty conclusively, that kissing is critical to how modern women choose a sexual partner.

“Women—in the West anyway—have gained far more autonomy in mate choice, and they are freer to kiss, and kiss whom they will,” says Sarah Blaffer Hrdy, a famed anthropologist and author of the 1999 bestseller Mother Nature . “Hard to say, though, what women in other cultures would wish to do, if they dared?”

That’s a question some researchers are now tackling—including through a survey of Muslim-majority societies, many of which are in the midst of a transition from arranged to love-based marriages. The results point to a conclusion that studies confirm again and again- Kissing is much more important to women than men—and it’s women who appear to have driven the rise of kissing in romantic life.

Why do we kiss?

Everyone knows the first kiss can launch a thousand ships—or sink every single one of them.

“There was one guy I dated who seemed so great,” says Veronica (not her real name), a 40-something single woman in Berkeley, California. “But when we finally kissed, it was like kissing my brother.”

How could Veronica know enough from that one kiss to give up on a relationship that could have lasted a lifetime?

When people push their lips together, they appear to exchange an enormous amount of biological information.

When people push their lips together, they appear to exchange an enormous amount of biological information—and, according to a 2014 paper, their brain activity spikes and harmonizes. In fact, the degree of synchronization between smooching brains correlates with the self-reported quality of the kiss.

“If I would speculate about the role of kissing in human life,” says Viktor Müller, the study’s co-author, “I would suggest that kissing synchronizes our brains to produce a state or conditions for a better understanding of each other—or for getting in the right mood for partner-oriented behavior.”

Meanwhile, research says there is a very good chance that women are covertly picking up pheromones and genetic information that might inform their decision. In a landmark 2000 study, for example, Claus Wedekind of the University of Lausanne in Switzerland found that women prefer the scents of men whose MHC (major histocompatibility complex) genes are different from their own—which would produce offspring with stronger immune systems. From this perspective, kissing is just sniffing in disguise.

As Veronica discovered, sometimes these brainwaves and olfactory signals combine to say, “Stop!”—and other times they scream “GO!”

Even societies that don’t encourage mouth-to-mouth kissing often provide for some kind of up-close snuffling. “Nearly all cultures practice a courtship behavior analogous to kissing, which involves close face-to-face proximity and may involve licking, biting, rubbing, or merely sniffing,” points out Rafael Wlodarski, a University of Oxford post-doctoral researcher who studies mating behavior. The ubiquity of romantic sniffing suggests that it has some evolutionary function.

80 percent of hunter-gatherers think it’s disgusting for two humans to shove their heads together and start licking the inside of each other’s oral cavities.

But science hasn’t yet determined exactly what that function is, especially when it comes to kissing. For decades, researchers assumed that evolution hard-wired human lovers to kiss on the lips. In the 1960s, British zoologist Desmond Morris suggested that kissing might have arisen from the primate practice of a mother chewing food and then pushing it with her tongue into the mouth of an infant. Others have suggested that the template for the lover’s kiss is a baby’s mouth on a mother’s breast.

But if that were the case, why don’t friends in France brush each other’s nipples in greeting, instead of grazing each other’s cheeks with their lips? There are plenty of ways for courting lovers to synchronize brain activity and collect subterranean genetic information. Dogs sniff each other’s butts; why can’t we?

If all that sounds gross to you, remember that more than 80 percent of hunter-gatherers think it’s disgusting for two humans to shove their heads together and start licking the inside of each other’s oral cavities. Given that humans have spent most of our history in such groups, it’s more logical to see them as normal and the rest of us as the sexual deviants. Last year’s study of 168 societies overturned decades of scientific speculation by discovering that there is nothing natural or inevitable about kissing on the lips. It’s a learned behavior, and one that appears to emerge only in specific social conditions.

What are those conditions? That’s a matter of debate—but much of the evidence so far suggests that it might be women who turned kissing from a disgusting practice to a desirable one. There’s little question, at this point, that kissing is more important to women than to men in assessing the suitability of a romantic partner.

For example, one 2007 study of 1,041 heterosexual college students found “that females place more importance on kissing as a mate assessment device.” Another published in 2013 got a very similar result—a survey of 308 men and 594 women found that, just like Veronica, women often made a snap judgment about a relationship based on the quality of the first kiss. (There are no similar studies of same-sex couples kissing.)

A 2014 study ran two experiments to find out how important a kiss was to deciding whether to move ahead with a partner. The conclusion?

The positive impact of purported “kissing quality” on a participant’s willingness to have casual sex with a potential partner was significantly greater for women than it was for men, suggesting that women may be particularly influenced by this factor. When examined in light of previous findings that women are the more selective sex during the mate assessment process, and are particularly attuned to, and discriminating about, cues signaling superior genetic fitness, this result is highly suggestive of the conclusion that kissing may convey some mate quality information.

“If kissing is used in mate assessment, then it is not surprising that females place more value on it,” says Wlodarski, the paper’s lead author. “Since the negative consequences of making a ‘poor’ mating decision are more severe for the female, females are typically more selective and utilize more cues to make mate choice decisions—including, potentially, kissing.”

Kissing as empowerment

So how the heck did women make romantic decisions before kissing was invented?

While humans seem to have always had ways to get in close and sniff prospective partners, many pre-kissing cultures did not give brides a whole lot of choice. Parents decided whom they’d marry. For much of human history, people didn’t pick their own mates and divorce wasn’t an option. This is still the case in many parts of today’s world, which allows researchers to see the transition unfold in real time.

“Kissing is much more important to women than men.”

University of Nevada, Las Vegas, ethnographer William Jankowiak, who led the study of 168 cultures, points out that the rise of dating as courtship ritual coincides with “embracing pleasure as a tacit or explicit expectation.” Whenever people start pursuing pleasure, he says, “anything that heightens that state will find willing participants.”

But Jankowiak doesn’t think pleasure alone can account for the shift from mere sniffing to French kissing. Affectionate smooching is strongly associated with the turn away from a social organization based on arranged marriages that “are about the family and never the couple” toward a couple-centered ideal, which suggests there must be some connection. Today, “so strong is this ideal that many women (still in the minority) do not want children so they can focus on the couple,” he says.

Amy Parish, an anthropologist based at the University of Southern California, Los Angeles, thinks that kissing might be one way to hold on to mates in a voluntary—and therefore unstable—family situation. “I would say that women may need to kiss more often in more stratified/complex societies because in those societies, women are more dependent on pair bonds with men and have less support from their communities and kin groups than one finds in egalitarian societies,” she says. (When anthropologists refer to “egalitarian societies,” they’re talking about small bands without formal pathways to power and wealth.)

In this view, choice triggers anxiety, especially when other people are freer to make their own choices about you, both before and during marriage. Kissing—with the multilayered sensory experience it offers—is a behavioral adaptation to stimulate desire and loyalty in a socially powerful male.

But that perspective alone can’t account for the recent psychological studies that show how critical kissing is to helping a woman decide, on her own, whether or not she will pursue a sexual relationship. For many modern women, it seems, kissing is an expression of power, not of powerlessness.

When kissing is dangerous

Two researchers are watching the transformation of sexual and marital relationships in the Middle East. Janet Afary and Roger Friedland—both religious studies professors at the University of California, Santa Barbara—have surveyed 18,000 people in seven Muslim-majority countries about marriage and romance. Most of the respondents were gathered through Facebook and ranged in age from 18 to 40 years.

Averaged across all seven countries, the overwhelming majority—60 percent—still believe kissing between unmarried people is wrong. In Pakistan, the most conservative of the countries surveyed, 71 percent say it’s always wrong. Throughout the Muslim world, on average, only 18 percent approve of kissing between courting men and women. It is still the norm for parents to play a role in arranging marriages, and the union is still considered one of two families, not of individuals.

The overwhelming majority—60 percent—still believe kissing between unmarried people is wrong.

But here’s where things get interesting- Younger people are dramatically more likely than older ones to think kissing is good, and young women are the ones who approve the most of kissing, by a large margin. In the survey results, kissing is linked to belief in voluntary, love-based, dyadic marriage, though “this does not mean that family arrangement is necessarily excluded,” Friedland adds. “The two can coexist and interplay.” Many respondents still want their parents involved in the match—but the young women just want a chance to sniff and nuzzle their prospective groom before they spend the rest of their lives with him.

In short, the results of this anonymous survey suggest there’s an under-the-radar, woman-led sexual revolution underway in the Middle East, one that’s totally illegal in many places. Friedland emphasizes that this isn’t just a matter of private belief—young women are voting with their lips, even in regions where kissing between unmarried people is against the law.

“The big story here is that women are daring to kiss,” says Friedland. “Love is dangerous in this world, particularly for women—but women are going after it. It indicates an incredible daring on their part.” This right to kiss is intrinsically tied to a right that seems fundamental to women in many Western countries- to choose your own mate.

“Pakistani women who believe in love are more likely to be OK with kissing their boyfriends than Pakistani men who believe in love,” confirms Sahar Habib Ghazi, a native of Islamabad, Pakistan, and managing editor of the news site Global Voices. In an exchange over Facebook, she adds-

They want to fall in love to bring more agency and control in their lives and they want to test that love with physical contact. They have more at stake if they fall in love with the wrong person than a man would in a predominantly patriarchal society like Pakistan, so it makes sense to me that they would be more willing to test that love. When the stakes are higher, the risks people will take are higher.

So why do couples kiss? For pleasure, sure, but there’s more to kissing than what meets the lips. From an evolutionary perspective, it seems, women kiss for freedom and control. If men seem to enjoy it, too—well, that might be just a happy accident.