The next time someone smiles at you, your face will probably smile back before you even realize it. Within 300 to 400 milliseconds — about the blink of an eye — your facial muscles begin to mirror theirs.
Psychologists call this automatic response emotional mimicry, and new research suggests it may shape one of the most important decisions we make about other people: whether we trust them.
This involuntary mirroring serves a purpose. "We are mimicking each other because it helps us to affiliate," says Michał Olszanowski, a psychology researcher at SWPS University.
Scientists have long known that copying someone's expressions makes them like and trust you more. But Olszanowski and his colleagues wondered whether the opposite might also be true: could mimicking someone else's smile change how you feel about them?
In a 2026 study published in Emotion, they found that the stronger a person unconsciously mirrored a smile, the more trustworthy they rated the smiling face, and the more resources they were willing to share in a trust-based game. Trust ratings rose when participants' smile imitation was enhanced, and fell when their ability to mimic was blocked.
Those findings suggest that one of our most important social judgments may begin with something as simple as a shared smile.
How can copying someone else's smile make them seem more trustworthy? Researchers say the answer begins with a split-second chain reaction in the brain.
Smile mimicry is rooted in the brain's mirror neuron system, says neuroscientist Marco Iacoboni of UCLA. Mirror neurons fire both when you act and "just by seeing the action that someone else is doing."
As your face smiles back, it also nudges your brain in a more positive direction — a phenomenon known as the facial feedback hypothesis. A 2017 study found that smiling can lift mood. Smiling also activates the brain's reward circuits, says Paula Niedenthal, a social psychologist at the University of Wisconsin-Madison. "One possibility is that we trust people whose smiles we mimic because that mimicry produces positive feelings."
When we copy someone's smile, our faces briefly move in sync, and that tiny coordination creates what Piotr Winkielman, a professor of psychology at the University of California San Diego, calls a "feeling of groupiness, of being a unit." He likens it to the "dance crush" felt by strangers who move in rhythm together.
"Once you synchronize with someone on the biological level," he says, "then your brain perceives some sense of an overlap of being in one group." Synchronized behavior syncs people's brains too, driving prosocial behavior, and "the sense of bonding increases trust," says Iacoboni.
Trust doesn't just shape social interactions; it also has measurable health consequences. A 2025 study published by the American Psychological Association found that people who trust others more report higher subjective well-being — a predictor of longevity.
The amount of trust a smile generates depends on the type being mimicked. In a 2017 review, Niedenthal and her colleagues identified three distinct types: reward smiles that express positive emotion, affiliation smiles that signal non-threat, and dominance smiles that assert status. "All are 'true' smiles, but the mimicry of each will not necessarily generate positive feelings," she says.
How genuine the smile appears also matters. In Olszanowski's study, posed smiles were sufficient to trigger mimicry and boost trust, suggesting a polite smile goes a long way. But Winkielman suspects that genuinely felt smiles, which typically raise the cheeks and crinkle the eyes, may have an even stronger effect.
Smile mimicry isn't universal, however. People with certain neurological conditions that affect facial movement, or those who process social cues differently, may mimic facial expressions less consistently. Cultural norms can also shape when people smile and how those smiles are interpreted, meaning the same expression may not inspire the same level of trust everywhere.
In the age of digital communication, emojis stand in for real smiles. And to a degree, the brain accepts the substitution: research shows that emojis have effects on the brain similar to face-to-face smiles, and that people mimic the emojis they receive, says Niedenthal.
But Winkielman is skeptical they can build trust the way real smiles do, partly because they are static: moving expressions elicit stronger mimicry than still ones. Emojis also focus mainly on the mouth, losing the eye component that Winkielman considers "quite important for the perception of facial expression." And ultimately, emojis aren't embodied, just text. "It functions more as a communicative thing, rather than grabbing us by the power of emotion induction or synchrony induction."
All the effects of a smile — the mirror neurons firing, the facial muscle movement, the mood shift, the sense of being in sync — are much stronger when two people are in the same room.