[INTRO ♪].

One of our species' trademark expressions is the smile. And smiling is a form of communication—it's not just about how you feel, but also how you want to make others feel.

We use smiles to put each other at ease, to bring people together. So it may seem a little weird that we show off our teeth when we want to be friendly, because teeth are for biting. And many other mammals do bare their teeth as a way to make a threat.

Just think about the snarl of a wolf or the open-mouth display of a baboon. It's a shorthand way of saying, like, “Back off if you don't want these to go inside of you!” It's pretty unusual that our smile evolved as a sign of friendliness. But there may be some clues to why it happened in the behavior of our primate cousins.

See, many primates show off their teeth for nonaggressive reasons. For example, rhesus macaques make what's called a silent bared-teeth expression as a sign of submission. When a dominant or aggressive opponent is threatening them, they'll show off their teeth as a signal that they don't want any trouble.

In other primates, including some macaques and baboons, the same toothy expression shows up while the monkeys are greeting, grooming, and embracing each other. And one of our closest cousins, chimpanzees, show a clear connection between the silent bared-teeth expression and social bonding: the more they flash their teeth at each other, the better they get along. If that sounds familiar, it's because it is also true in humans!

Smiles make us feel more comfortable with each other. Studies have found that if you show people pictures of faces, they'll rate smiling faces as more generous and agreeable. A study in 2001 had 120 participants pair up to play a simple game of trust.

The players could choose to be selfish and win some money, or work together and win even more money. The catch was that if either player chose to be selfish, they could end the game early and take their payout while the other player got much less. But they'd both win more money if they trusted each other to make the right moves and not be selfish.

The players never met each other, though; they were just shown a picture of their partner and had to decide if they trusted them. And the study found that people were more likely to trust a smiling face. Along those same lines, a 2007 study took a look at how people smile when they share.

In the case of that study, the experimenters gave pairs of people money to share between them—because, like, I guess involving money is a good way to get genuine behavior out of people. And they found that people generally smiled more when they were sharing than when they were just talking. What's really cool is that the participants were also asked to report on their own emotions during the study.

And surprisingly, the amount that people smiled was mostly unrelated to their feelings of happiness, anger, or other emotions. It wasn't that sharing made people so happy that they couldn't help smiling, it was that smiling was a signal of cooperation. So smiling really is about more than just feeling happy.

It's about how we feel around each other and how we want others to feel about us. So it seems like, over our evolution, this toothy expression may have gone from a threat to a signal of non-aggression to the friendly, cooperative smile that we know and love today. Thanks for watching this episode of SciShow!

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As always, thank you for watching. [OUTRO ♪].