Faster than a blink of an eye.

A new study has shown that people judge attractiveness, likeability, trustworthiness, competence, and aggressiveness of a facial expression within 100ms, i.e., a tenth of a second. Talk about first impressions!

The research was quite clever: if you show unfamiliar faces to people very quickly (fractions of a second), and ask them to judge some particular trait of that face, you can then compare the judgement to one made without any time constraints. What they found that is that judgements made with 100ms exposures highly correlate to those made without any time limits, suggesting that decisions are made in a tenth of a second.