Psychologist Paul Ekman's cross-cultural research, including studies with isolated communities in Papua New Guinea with minimal outside media exposure, found that seven facial expressions — happiness, sadness, anger, fear, surprise, disgust, and contempt — are produced and recognized the same way across radically different cultures, suggesting they're biologically hardwired rather than learned. A microexpression is a fragment of one of these seven expressions that flashes across the face involuntarily, typically for less than half a second, often when someone is suppressing or masking their actual emotional reaction.
The practical value of knowing the seven is narrower than popular 'human lie detector' claims suggest: microexpressions reveal that a genuine emotional reaction occurred and is being masked, but they don't tell you why, and they're notoriously easy to over-interpret without training (Ekman's own training programs take substantial practice to apply reliably). The most defensible everyday use isn't diagnosing deception — it's noticing a flash of a suppressed reaction as a cue to ask a genuine follow-up question, rather than treating the person's stated reaction as the whole story.