In a series of studies in the early 1990s, psychologists Nalini Ambady and Robert Rosenthal showed participants brief, silent video clips — as short as 10 seconds — of teachers they'd never met, and asked them to rate the teacher's effectiveness. Those snap judgments, from strangers with no context, correlated strongly with the same teachers' actual end-of-semester evaluations from students who'd spent a full term in the room.
Ambady and Rosenthal called this thin-slicing: the ability to find meaningful patterns in very limited information, very quickly, often below conscious awareness. It isn't magic and it isn't always accurate — but it's a real, repeatedly replicated finding that a surprising amount of judgment gets locked in within seconds, well before any conscious evaluation begins.
The catch is that thin-slicing works well for some things — warmth, competence cues, teaching effectiveness — and badly for others, like honesty or long-term compatibility, where snap reads are no more reliable than a coin flip. The speed of the judgment doesn't track its accuracy, which is exactly what makes it easy to trust more than it's earned.