Science has taught us a lot about a smooch.

Researchers have discovered kissing helps you choose the right mate and helps you live longer. They have found you use 146 muscles when you pucker up and swap 80 million new bacteria when you lock lips. And you will spend some 20,000 minutes — or two weeks — of your lifetime doing it.

But the cultural significance of a kiss may not be that widely shared, according to new research published in American Anthropologist.

Researchers at the University of Nevada and Indiana University found fewer than half of the world’s cultures kiss in a romantic way. Although many societies consider kissing to be a romantic or erotic activity, others have gone as far as to call it “gross” and ask why anyone would “share their dinner.”

The researchers studied 168 cultures over the past year and found evidence of romantic kissing in 77 societies, or 46 percent, but none in 91 others.

“It’s a reminder that behaviors that seems so normative often do not occur in rest of the world. Not only that, but they might be viewed as strange,” Justin Garcia, the study’s co-author who teaches gender studies at Indiana University, told The Washington Post. “It’s a reminder of romantic and sexual diversity around the world. It shows how human biology interacts with different cultures to explain various behaviors humans engage in.”

The researchers found romantic kissing to be the norm in the Middle East, with the practice established in 10 out of 10 cultures studied. In Asia, 73 percent enjoyed romantic kissing; in Europe, 70 percent; and in North America, 55 percent. No smoochers were found in Central America.

“No ethnographer working with Sub-Saharan Africa, New Guinea, or Amazonian foragers or horticulturalists reported having witnessed any occasion in which their study populations engaged in a romantic-sexual kiss,” the researchers wrote in the study.

Garcia said he and his teammates, University of Nevada anthropology department chair William Jankowiak and graduate assistant Shelly Volsche, set out last year to find evidence to support or contradict long-established claims that romantic kissing was a cross-cultural expression. The researchers searched standard data sets on world cultures to find examples of kissing and wrote to ethnographers to find data on cultures that were not available.

In the end, they found “no evidence that the romantic-sexual kiss is a human universal or even a near universal,” according to the study. “Moreover, there is a strong correlation between the frequency of the romantic-sexual kiss and a society’s relative social complexity: The more socially complex the culture, the higher frequency of romantic-sexual kissing.”

Indeed, necking can be traced to primates. Chimpanzees and bonobos are known for kissing on the lips. Bonobos use tongue. But such gestures are a way for them to reconcile — not a form of foreplay, BBC News reported.

Humans, too, have been known to “kiss” in a nonsexual way. History documents mouth-to-mouth “kiss-feeding” — a way parents fed children when baby food wasn’t an option, the way a mother bird feeds her young.

Charles Darwin wrote about kissing in his 1872 book called “The Expression of the Emotions in Man and Animals.” He talked about “kissing-like behaviors,” including rubbing noses and patting arms, which he thought might be the primitive kiss, according to the Guardian. He thought these gestures showed an instinctual urge to get “pleasure from close contact with a beloved person” and that the kiss and its related forms of expression could therefore be considered universal. But the lip-to-lip action, it seems, is not.

Across Europe, a peck on the cheek is a common cultural greeting; one on the lips is indeed a romantic gesture. In India, Bangladesh and Thailand, it’s a private practice. Still, some societies do not consider kissing romantic at all.

The Oceanic kiss, for example, involves passing open mouths over each other — without actual contact, according to news.com.au. It’s not that these cultures aren’t sexual, the researchers said, but that the kiss is not seen as a sexual expression. For instance, some consider smelling a partner’s face to be sexual because it allows them to learn more about each other.

“The Aka pygmies talk about their ‘night’s work,” researcher Volsche told news.com.au. “This is the euphemism they use for sexual contact. They admit that it is enjoyable, the main purpose is to conceive a child. Where we in the West may brag about the quality of foreplay or the length of an individual interaction, the Aka focus on how many times in a night they ‘worked.'”

So even though the kiss may, in fact, be an evolutionary adaptation, it doesn’t appear to be a cross-cultural one, Garcia said.

“It’s only in those societies that have come to see the erotic kiss as part of larger of romantic and sexual repertoire,” he said.