Human beings smile for a number of reasons. You may smile when you spot your long-lost bestie in baggage claim, when you engage your co-workers during a presentation, or when you imagine your ex’s lawyer tripping on the way into the courthouse.

People are fascinated by smiles — all of them. From Mona Lisa to the Grinch, we’re captivated by those both genuine and fake. This enigmatic facial expression has been the subject of hundreds of studies.

Here’s what we know about the 10 different types of smiles, what they look like, and what they mean.

One of the most useful ways to categorize smiles is according to their social function, or the purposes they serve in groups of people.

Broadly speaking, there are three kinds of smiles: smiles of reward, smiles of affiliation, and smiles of dominance.

A smile may be among the most instinctive and simple of expressions — just the hoisting of a couple of facial muscles. But as a form of social interaction and communication, a smile is complex, dynamic, and powerful.

Studies have shown that people are incredibly perceptive when it comes to reading and recognizing these smiles in social situations.

Many people are able to correctly identify which kind of smile they’re witnessing, and seeing certain kinds of smiles can have powerful psychological and physical effects on people.

Here are the 10 most common types of smiles:

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Many smiles arise from a positive feeling — contentment, approval, or even happiness in the midst of sorrow. Researchers describe these as “reward” smiles because we use them to motivate ourselves or other people.

Reward smiles involve a lot of sensory stimuli. Muscles in the mouth and cheeks are both activated, as are muscles in the eye and brow areas. More positive input from the senses increases the good feelings and leads to better reinforcement of the behavior.

For example, when a baby unexpectedly smiles at their mother, it triggers the dopamine reward centers in the mother’s brain. (Dopamine is a feel-good chemical.) The mother is thus rewarded for her baby’s apparent happiness.

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People also use smiles to reassure others, to be polite, and to communicate trustworthiness, belonging, and good intentions. Smiles like these have been characterized as “affiliation” smiles because they function as social connectors.

A gentle smile is often perceived as a sign of compassion, for example.

These smiles involve the upward pull of the lips, and according to researchers, often trigger dimpling in the cheeks.

According to research, affiliative smiles can also include a lip pressor, where the lips remain closed during the smile. Keeping the teeth hidden might be a subtle reversal of the primitive tooth-baring aggression signal.

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People sometimes smile to show their superiority, to communicate contempt or derision, and to make others feel less powerful. You might call it a sneer. The mechanics of a dominance smile are different than reward or affiliative smiles.

A dominance smile is more likely to be asymmetrical: One side of the mouth rises, and the other side remains in place or pulls downward.

In addition to these movements, dominance smiles may also include a lip curl and the raising of an eyebrow to expose more of the white part of the eye, both of which are powerful signals of disgust and anger.

Studies show that the dominance smile works.

Researchers tested the saliva of people on the receiving end of a dominance smile and found higher levels of cortisol, the stress hormone, for up to 30 minutes after the negative encounter.

The study also found that the sneer raised heart rates among the participants. This kind of smile is a nonverbal threat, and the body responds accordingly.

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If you’re looking for a foolproof lie detector, the face isn’t it. According to research, even the most experienced law enforcement officials only spot liars about half the time.

Nevertheless, there have been studies that revealed smile patterns among people who were actively trying to deceive others in high-stakes situations.

A 2012 study conducted a frame-by-frame analysis of people filmed while publicly pleading for the return of a missing family member. Half of those individuals were later convicted of killing the relative.

Among the deceivers, the zygomaticus major muscle — the one that pulls your lips into a smile — repeatedly fired. Not so with those who were genuinely grief-stricken.

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Anyone who has seen the 1989 movie classic “Steel Magnolias” will recall the cemetery scene when M’Lynn, played by Sally Fields, finds herself laughing raucously on the day she buries her daughter.

The sheer dexterity of human emotion is astonishing. So, we’re able to smile in the midst of both emotional and physical pain.

Experts at the National Institutes of Health think that the ability to smile and laugh during the grieving process protects you while you recover. Interestingly, scientists think we might smile during physical pain for protective purposes, too.

Researchers monitored the facial expressions of people who were undergoing painful procedures and found that they smiled more when loved ones were present than when they were alone. They concluded that people were using smiles to reassure others.

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You dispense a polite smile surprisingly often: when you first meet someone, when you’re about to deliver bad news, and when you’re concealing a response you believe someone else won’t like. The list of social situations requiring a pleasant expression is a long one.

Most of the time, a polite smile involves the zygomaticus major muscle, but not the orbicularis oculi muscle. In other words, your mouth smiles, but your eyes don’t.

Polite smiles help us maintain a kind of discreet distance between people. Whereas warm smiles sparked by genuine feeling tend to draw us closer to others, that closeness isn’t always appropriate.

Lots of social situations call for trustworthy friendliness but not emotional intimacy. In those situations, researchers have found the polite smile is as effective as a heartfelt one.

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Dating, psychology, and even dental websites offer advice on how to use your smile to flirt with someone.

Some tips are subtle: Keep your lips together and lift an eyebrow. Some are coy: Smile while tipping your head down slightly. Some are downright comical: Smile with a little whipped cream or coffee froth on your lips.

While there’s a lot of cultural influence on these tips and comparatively little evidence to back their effectiveness, there’s proof that smiling makes you more attractive.

One study found that attractiveness is heavily influenced by smiling, and that a happy, intense smile can “compensate for relative unattractiveness.”

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An oft-quoted 1995 study found that a smile provoked by embarrassment is often accompanied by a downward tilt of the head and a shifting of the gaze to the left.

If you’re embarrassed, you’ll probably touch your face more often, too.

A 2009 study on embarrassed smiles did confirm the head movements. However, it didn’t confirm that people who are embarrassed usually smile with their mouths closed. Their smiles tend to not last as long as amused or polite smiles.

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This smile gets its name from the Pan Am flight attendants who were required to keep smiling, even when customers and circumstances made them want to throw peanut packets across the cabin.

Widely regarded as forced and fake, the Pan Am smile might have appeared extreme.

Studies show that when people are posing, they use extra effort to yank on their zygomaticus major muscle.

As a result, the corners of the mouth are extra high, and more of the teeth are exposed. If a posed smile is asymmetrical, the left side of the mouth will be higher than the right side.

If you’re one of the nearly 2.8 million people employed in the customer service industry, or if your job requires you to interact regularly with the public, you might want to reconsider relentlessly deploying the Pan Am smile, as it could affect your health.

A recent study published in the Journal of Occupational Health Psychology found that people who have to fake happiness regularly at work often end up drinking off the stress after they clock out.

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This one is the gold standard. The Duchenne smile is also known as the smile of genuine enjoyment. It’s the one that involves the mouth, the cheeks, and the eyes simultaneously. It’s the one where your whole face seems to light up suddenly.

Authentic Duchenne smiles make you seem trustworthy, authentic, and friendly. They’ve been found to generate better customer service experiences and better tips. And they’ve been linked to longer life and healthier relationships.

In a 2009 study, researchers looked at the intensity of smiles in college yearbook photos and found that women who had Duchenne smiles in their photos were more likely to be happily married much later.

In another study published in 2010, researchers examined baseball cards from 1952. They found that players whose photos showed intense, authentic smiles had lived much longer than those whose smiles looked less intense.

Smiles vary. Whether they express genuine bursts of feeling or they’re intentionally created to suit a specific purpose, smiles serve important functions in systems of human interaction.

They may reward behavior, inspire social bonding, or exert dominance and subservience. They can be used to deceive, to flirt, to maintain social norms, to signal embarrassment, to cope with pain, and to express rushes of sentiment.

In all their ambiguity and variety, smiles are one of the most powerful means we have of communicating who we are and what we intend in social contexts.