It takes just a tenth of a second for people to make judgements about you based on your facial appearance.

Subjects were asked to rate attractiveness, likeability, competence, trustworthiness, and aggressiveness of faces after looking at their photos for just 100ms.

The ratings they gave the faces correlated strongly with ratings given by other students who were allowed as long as they wanted to rate the faces. The strongest correlation was for trustworthiness.
“Maybe as soon as a face is there, you know whether to trust it”, the researchers surmised.

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