A tenth of a second
In a 2006 experiment, people were shown unfamiliar faces for 100 milliseconds - shorter than a blink. Right after the flash, they were asked to decide: Is this person trustworthy? attractive? competent? aggressive? Another group rated the same faces separately, calmly and with no time limit.
The ratings were almost identical, especially for trustworthiness. Extra time did not reverse the conclusion: it added confidence in it and made the rating slightly more extreme. Your judgment of a person forms not in conversation, but before it - and afterward, you mostly look for evidence to support it.
What you read fastest is not beauty but safety: first comes the question, "one of us or dangerous?" One explanation is a simple match of shapes. A neutral face with furrowed brows and downturned mouth corners resembles an angry one - and reads as untrustworthy. Soft features read as friendliness. There is no knowledge of the person in it, only a resemblance to expressions you have seen thousands of times.
Why the first seconds matter most
An unfamiliar face is an attention-demanding event. In the first seconds, it gets the most resources: you examine it, compare it, look for familiar features. Then the face is tagged as "seen before," your brain processes it more efficiently, and you do not examine it as closely again.
And the ceiling comes quickly. In the same data, ratings after a 1-second exposure explain about three quarters of what people say about a face when they have all the time they want. There is almost nothing left to add.
Ratings after a 1-second exposure explain about 75% of what people say with no time limit
That is why first impressions are so sticky. Not because they are accurate, but because they form when you are being examined most closely. Everything a person learns afterward lands on an already prepared frame.
How accurate is a quick judgment of a face?
Here is the honest answer. People agree with each other remarkably well: most will call the same face trustworthy or unpleasant. But agreement is not accuracy. The link between "looks trustworthy" and "behaves trustworthily" is weak and inconsistent in research: some studies find it, some do not - if the effect were robust, it would be found consistently. A quick judgment is a well-shared stereotype, not an X-ray.
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Sources
- J. Willis, A. Todorov. "First Impressions: Making Up Your Mind After a 100-ms Exposure to a Face," Psychological Science, 2006. On agreement and accuracy in face-based judgments - a review by A. Todorov and colleagues in Annual Review of Psychology, 2015