True smiles are brief – between two-thirds of a second and four seconds. That’s why a held smile begins to look menacing. A true smile is the one expression that we cannot fake. As the French anatomist G.-B. Duchenne de Boulogne noticed as long ago as 1862, a genuine, spontaneous smile involves the contraction of the orbicularis oculi muscle in each eye and we have no independent control over those muscles.11 You can make your mouth smile, but you can’t make your eyes sparkle with feigned joy.
