"When I wish to find out how wise, or how stupid, or how good, or how wicked is any one, or what are his thoughts at the moment, I fashion the expression of my face, as accurately as possible, in accordance with the expression of his, and then wait to see what thoughts or sentiments arise in my mind or heart, as if to match or correspond with the expression." – Edgar Allen Poe, "The Purloined Letter"

The above quotation from a Poe short story is attributed to a clever schoolboy who uses his empathetic skills to win guessing games. It's also the epigraph of a review article, published today in Trends in Cognitive Sciences, by University of Wisconsin psychologist Paula Niedenthal and colleagues about how mimicking facial expressions helps us to read emotions.

The schoolboy's strategy works, it turns out, and is something that humans do unconsciously when we interact. We mimic the facial expressions of others, and these movements call up the emotions that we feel when we make the same faces ourselves, the article explained. This sensorimotor simulation, as it's called, helps us to feel what another person might be feeling. For this reason, facial mimicry helps us to read others' emotions. (Our tendencies to empathize and mimic may also be the reason why emotions seem to be contagious.)

And the opposite may be true as well. Some evidence suggests that impairing the ability to mimic decreases our ability to discern emotions. For example, one study by Niedenthal and others found that the longer infant boys used pacifiers, which prevent emotional expressions of the mouth, the less they mimicked others' facial expressions and the worse they scored on a test of emotional intelligence as older children. (This effect was not seen in girls.) Another study found that college students wearing a boil-and-bite mouth guard, which hampered their ability to mimic, were worse at distinguishing between genuine and fake smiles than those who did not wear the guard.

How it works

The faces we make are external representations of what we feel inside. There's something to the idea that this also works in reverse and if you make an emotional expression, the associated emotions will follow. For example, if you force yourself to smile you might actually feel better. Conversely, in people who have facial paralysis, the degree to which the zygomatic muscle, which lifts the corners of the mouth, is immobilized predicts severity of depression.

That's why when we mimic the countenance of another person, the same internal link to the associated feelings gets switched on and we also mimic those feelings—at least, our versions of them.

You might wonder why such machinations—unconscious, remember—are necessary to understand that a smile is happy or a frown is sad. It's because most expressions are not so stereotypical. "They are instead fleeting, subtle, and somewhat idiosyncratic facial gestures shaped by learning and culture," Niedenthal and her colleagues wrote. "Different emotions, attitudes, and intentions can be communicated with the slight changes in eyebrow position, head tilt, onset dynamic, or lip press." Through mimicry, we can understand these emotional expressions without naming or even thinking about them at all.

Yet facial mimicry is not absolutely required for understanding emotions. For example, people with Moebius syndrome, a weakness or paralysis of the facial muscles that prevents the formation of facial expressions, can still read others' emotions, Niedenthal told Braindecoder. Scientists aren't completely sure what comes first in the mental processing of another's expression—facial mimicry or emotional identification. One hypothesis is that rather than mimicry triggering emotions, facial mimicry could be a "spill-over" from an emotional identification with another person's expression. It's also possible that the motor and mental responses mutually influence each other, Niedenthal said.

What we mimic may affect what we see

The ability to mimic affects emotional perception not only in the more abstract, empathetic sense; it also seems to affect visual perception. In a study, led by Niedenthal, wearing a gel-based face mask that hindered their facial movements seemed to prevent participants from distinguishing between photos that varied subtly in emotional expressions. In the study, participants were shown a single photo on a continuum from angry-to-sad. Then they were shown a pair of photos, the one they had just seen, and another on the continuum, and asked to identify the photo they had already viewed. Those who wore the mask were less able to distinguish between the two photos than mask-less control participants, suggesting that mimicking an expression may be important for seeing it clearly.

Don't try this at home

If you get excited to try out mimicry during your next conversation, you may quickly find that it's hard to talk while mimicking someone who is listening. While conversations involve their own mimicry systems, Niedenthal said, these dynamics of facial mimicry apply more to situations where people are not talking—for example, an encounter with a stranger. In a scenario like this, the ability to read emotions is less about intellectual understanding or responding empathetically but about behavior: for example, approaching another person or not.

"When someone is showing you with a smile that they are approachable and that they are not dangerous, and/or that they're sort of a member of the same category and similar to you, the most important thing that you get from that is not the label, 'oh, this is an affiliative kind of smile.' What's the most important thing is that you approach them," Niedenthal said. "Whereas a different kind of smile may push you away."

This kind of emotional understanding may be particularly useful, say, in a country where you don't speak the language, Niedenthal explained. "If you're in a different country where you can't communicate otherwise, then simulating the person's facial expression will give you that really subtle information about 'what does this really mean?'" She continued, "You don't say, 'that's a smile so that means we're gonna be friends.' That would be a kind of stupid thing to do, right? Instead, you adjust your behavior according to the meaning of the smile as you've simulated it."

An outstanding question, the authors wrote, is whether people could learn to improve their emotional understanding by improving their mimicry skills or by consciously mimicking others, as Poe's schoolboy character does.

"In that quote, he's talking about something that's done consciously," Niedenthal said. "In our research and in our theory, while somebody may try to do that consciously, we largely think of it as spontaneous or automatic and not intentional." Spontaneous and intentional expressions involve different areas of the brain, the authors wrote, so the effects of concerted and unconscious mimicry might not be the same.

So don't try facial mimicry at home. Or rather, don't worry: you're probably doing it already.