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Sure, kissing feels good. But it’s also an evolutionary advantage. Now a new book explains the science behind passionate lip-locks, and what they tell us about how men and women approach romantic relationships.

The precise origins of kissing are unknown. But some scientists hypothesize that the practice evolved from feeding rituals between animal mothers and their young, wherein mothers would chew and break down food before passing it directly to their offspring by mouth. Out of that gesture grew a universal sign of love and affection.

In the new book The Science of Kissing: What Our Lips Are Telling Us, author Sheril Kirshenbaum cites the work of Rutgers University anthropologist Helen Fisher, who says kissing evolved to fulfill three essential needs: sex drive, romantic love and attachment. Romantic kissing is a part of more than 90% of human cultures, and its role, as Kirshenbaum puts it, is to help us “find partners, commit to one person and keep couples together long enough to have a child.” (More on Time.com: Why Women Are Attracted to Guys Who Play Hard to Get)

How? The cascade of biological reactions during a passionate kiss plays a role. Research shows that kissing boosts levels of neurotransmitters like dopamine (which is involved in craving and desire) and serotonin (which elevates mood and can help spark obsessive thoughts about a partner). It also causes a jump in oxytocin, the so-called “love hormone,” whose release during orgasm triggers attachment between couples; high levels of it in new parenthood also foster lifelong attachment between mother and child.

“What I found so fascinating is that the chemicals in our bodies are responsible for the so-called symptoms we associate with falling in love,” Kirshenbaum says. “I don’t think it takes the romance out of the equation, but it gives us a better scientific understanding of how our bodies are behaving.”

Of course, sometimes a kiss doesn’t go well, which turns people off to one other upon first contact. Pointing to research by evolutionary psychologist Gordon Gallup of the University of Albany, Kirshenbaum writes on the University of Texas website:

Fifty-nine percent of men and 66 percent of women say they have ended a budding relationship because of a bad kiss. It turns out that our sense of smell may be partially responsible as we pick up subconscious clues about the other person’s DNA or reproductive status. Biologist Claus Wedekind found that women are most attracted to the scent of men who have a very different genetic code immune system than their own in a region known as the major histocompatibility complex. This may be because potential children would have a higher level of genetic diversity, making them healthier and more likely to survive.

Men also use kissing to weed out unsuitable mates — or, more often, to make themselves attractive to desirable ones. “Males tend to kiss as a means of gaining sexual favors, or as a means of affecting reconciliation,” says Gallup. “Females kiss more as a mate-assessment device.”

Indeed, in studies of smooching styles in men and women, Gallup found that men were more likely to initiate tongue contact, preferring French kissing to closed-mouth pecks. One theory behind that finding is that men have small amounts of testosterone in their saliva. “In saliva exchange — whether that’s over weeks or months or even years — trace amounts of testosterone in male saliva could raise testosterone levels in the female and therefore increase her receptivity and her libido,” Gallup says. “In long-term relationships, the frequency of kissing is a good barometer of the health and well-being of that particular bond.” (More on Time.com: Does Men’s ‘Bond’ with Porn Ruin Them for Real-Life Sex?)

In longer, committed relationships, Gallup found, women also continued to use kissing — to evaluate the state of their partnership. They were also far more likely than men to insist on kissing before, during and after sex.

Those differences are in line with what we know more generally about men and women when it comes to sex and intimacy. “Women put a lot of weight into the act of kissing itself,” says Kirshenbaum. For them, it’s “nature’s ultimate litmus test”— a way to probe the potential of a future partner. And for men? Well, I think we all know what men want.