Science of KISSING

Why do we kiss? Well, easy, because it feels good. But why? Kissing is weird. We love doing it, but let's face it, it's weird. You're rubbing your open mouth on another human being's open mouth. So there must be a good reason that we do it.

According to scientists, there's actually many good reasons, and they start with the eyes. And those eyes are looking at lips. Human lips are unique in the animal world - they are "everted" or exposed outward. Today, 8 out of 10 women paint their lips, often some shade of red. And men, well they say they're more attracted to fuller, redder lips. Our primate relatives are attracted to rosy colors too, just in a different place. As we evolved to walk upright, we began to advertise our fertility face-to-face.

As you start to kiss, you engage 5 of your 12 cranial nerves, and more than a dozen facial muscles. One of those, your orbicularis oris, allows you to make this puckering shape, the same shape as a nursing baby. That's our first clue to kissing's origins.

Breastfeeding is an incredibly neurologically pleasurable experience, it releases waves of the hormone oxytocin, which promotes bonding and comfort in mother and  child. This bond is so strong that there's a two-thirds chance that when you kiss, you tilt your head to the right. This might be because 80% of mothers hold their babies to the left, so we're used to turning our heads to the right for comfort, and puckering up.

As you get closer, it's time to engage your nose. Our oldest references to kissing come from Vedic Sanskrit texts from 1500 BC, which refer to it as a "sniff" or a "smell". And for many cultures, kissing is still a primarily nasal experience. Breath can be an indicator of health in a potential mate, we might even be able to smell a good genetic match. In one famous experiment, woman smelled t-shirts worn by different men, and then rated their smell preferences. They overwhelmingly preferred the smell of men with different immune system genes from their own. It's almost like we're testing compatibility with a kiss.

Finally, contact. Our lips are some of the thinnest and most nerve-rich skin in our bodies, and our brain's somatosensory cortex devotes more neural real estate to our lips than even our genitals. The sensation of kissing sends signals directly to the brain's pleasure and reward centers, and unleashes a spectrum of neurotransmitters and hormones. While no one brain chemical can be responsible for something as complex as kissing, we can feel a few of them at work.

A first kiss brings on a rush of novelty, as a flood of dopamine acts on the same brain reward centers triggered by drugs like cocaine. It can even bring on feelings of withdrawal and addiction. Thanks to epinephrine and norepinephrine, your heart beats faster, you get a wave of oxygenated blood to your brain, and your pupils dilate, and maybe that's why we close our eyes.

The pituitary gland and hypothalamus can release waves of endorphins, bringing on feelings of euphoria. Despite all that action on the microsecond scale, it can feel like time is standing still. As the minutes of kissing turn to days and weeks, the body produces less of the stress hormone cortisol. It seems like kissing can actually be good for your long-term health.

In surveys women consistently rate kissing as more important in relationships than men do. Maybe that's because they actually have to physically carry the children, and they're a little more biologically invested in all those things that come after kissing. As important as kissing is to sex, the two can be and often are completely separated. Not only do people often view kissing as a more intimate act than sex itself, researchers say that people can usually remember more about their first kiss than they can about their first time going all the way. Clearly, people take smooching very seriously.

Romans would certify the sharing of property in marriage based on whether the betrothed had shared a kiss, and in the Middle Ages, men who couldn't read or write would seal a contract by kissing a written "x", a symbol we still use today.

Even though kissing, in an evolutionary sense, isn't required to reproduce, more than 90% of human cultures do it in some way. We call it locking lips, making out, playing tonsil hockey, snogging, pecking, even osculation. Maybe we have so many words for kissing because kissing can be so many things.

Kissing has evolved from its biological origins into a complex, diverse human behavior, that often doesn't mean the same thing to any two people, even when those people are kissing each other. While we understand bits and pieces of the science of kissing, it's dangerous to make generalizations about something so diverse, or as diverse as the people who do it.

Most research on kissing has centered around heterosexual, cis-gendered couples, and usually college students, but there's so many more different types of kissing out there. Like most science, there's a lot left to learn about the science of kissing. Maybe that's why we keep doing it.