When someone smiles at us, we tend to smile back without thinking. Three experiments now suggest that this small act of copying shapes a first impression: the more strongly people mirrored a smile, the more they trusted the person behind it.
The work comes from Michal Olszanowski and Aleksandra Tołopiło of SWPS University in Warsaw and Ursula Hess of Humboldt University in Berlin. It was published online in Emotion on September 29, 2025. The materials, data and preregistration are posted on the Open Science Framework.
Electrodes on the face and a trust game
Participants watched six-second video clips in which a neutral face turned happy, sad or angry. The faces were built from a Polish set of emotional expression photos, four men and four women.
In the first experiment, 71 students were recruited and the muscle data of 62 were analysed. Electrodes over the smiling and frowning muscles measured how much they mimicked each face while they rated it for trustworthiness, confidence and attractiveness. Before that, a questionnaire made some of the faces seem socially similar to them.
The second experiment tested cause and effect. Forty-six participants rated trustworthiness while being told to make facial movements that either matched or worked against the expression on screen. The task was presented as a test of face-tracking software.
The third experiment, with 64 analysed participants, swapped ratings for money-like stakes. In each round of an adapted trust game, players got 6 points and decided how many to send to the person in the video, knowing the points would be tripled for that player.
Smiles win, sadness costs trust
In all three studies, stronger smile mimicry went with higher trust. In the second experiment, making facial movements that matched a smile led to higher trustworthiness ratings than movements that worked against it, which points to a causal role for the face itself.
Sadness behaved differently. In the first and third experiments, more mimicry of a sad face was linked to lower trust, but forcing sad-face movements in the second experiment did not change ratings. Anger was mimicked far less than either happiness or sadness, and happiness was the expression people copied most.
Mimicry mattered most for judgments tied to social connection, such as trustworthiness, and less for confidence. The authors called the drop in trust after sadness mimicry “intriguing”, since sadness is usually seen as an appeal for support.
A lab, not a first date
The team notes that facial electrodes can make people more aware of their own expressions, and that two muscles capture only part of an expression. Watching recorded faces in a lab, and playing a trust game with no feedback on whether points came back, is also far from meeting a live person.
Still, across ratings and a game with points at stake, the same pattern held: a face that is smiled back at is a face that gets trusted.
Study Details:
- Title: Smile and the world smiles (and trusts) with you: Happiness mimicry shapes first impressions
- Authors: Michal Olszanowski, Aleksandra Tołopiło, Ursula Hess
- Journal: Emotion
- Publication Date: September 29, 2025
- DOI: 10.1037/emo0001586