[Music]

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 an 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. 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. Stay curious.

If you want to know more about this amazing science of kissing, check out the book "The

Science of Kissing" by my friend Sheril Kirshenbaum, link down in

the description.