We make snap judgments of others based not only on their facial appearance, but also on our pre-existing beliefs about how others’ personalities work, finds a new study by a team of psychology researchers.

Its work, reported in the journal Proceedings of the National Academy of Sciences, underscores how we interpret others’ facial features to form impressions of their personalities.

“People form personality impressions from others’ facial appearance within only a few hundred milliseconds,” observes Jonathan Freeman, the paper’s senior author and an associate professor in NYU’s Department of Psychology and Center for Neural Science. “Our findings suggest that face impressions are shaped not only by a face’s specific features but also by our own beliefs about personality—for instance, the cues that make a face look competent and make a face look friendly are physically more similar for those who believe competence and friendliness co-occur in other people’s personalities.”

“Although these impressions are highly reliable, they are often quite inaccurate,” Freeman adds. “And yet they are consequential, as previous research has found face impressions to predict a range of real-world outcomes, from political elections, to hiring decisions, criminal sentencing, or dating. Initial impressions of faces can bias how we interact and make critical decisions about people, and so understanding the mechanisms behind these impressions is important for developing techniques to reduce biases based on facial features that typically operate outside of awareness.”

The paper’s other authors included Ryan Stolier, lead author of the paper and doctoral candidate in NYU’s Department of Psychology, Eric Hehman of McGill University, and Matthias Keller and Mirella Walker of the University of Basel in Switzerland.

We have long known that people make some personality impressions of others based merely upon their facial appearance. For instance, we see those with babyish features as agreeable and harmless and those with faces that resemble anger as dishonest and unfriendly.

What’s less clear is how widespread this process is and how, precisely, it transpires.

In their PNAS study, the researchers explored these questions through a series of experiments, specifically seeking to determine whether our own pre-existing beliefs about how personality works affect the way we “see” it on others’ faces.