A real smile is measurable — and it's not about the mouth

A 19th-century neurologist electrically stimulated facial muscles and found genuine joy fires a muscle around the eyes that's hard to trigger on command. Fake smiles use the mouth alone. A century later psychologists confirmed the eye-crinkle predicts real positive emotion better than the grin.

— Guillaume Duchenne, Mécanisme de la Physionomie Humaine — 1862; later confirmed by Paul Ekman