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emotional data funnels to the brain, exciting the left anterior temporal region in particular, then smolders to the surface of the face, where two muscles, standing at attention, are roused into action: The zygomatic major, which resides in the cheek, tugs the lips upward, and the orbicularis oculi, which encircles the eye socket, squeezes the outside corners into the shape of a crow’s foot. The entire event is short — typically lasting from two-thirds of a second to four seconds — and those who witness it often respond by mirroring the action, and smiling back.

For decades, many psychologists agreed that smiles reflected a vast array of emotions rather than a universal expression of happi­ness. This belief persisted until the 1970s, when Paul Ekman and Wallace Friesen, psychologists at the University of California at San Francisco, captured the precise muscular coordinates behind 3,000 facial expressions in their Facial Action Coding System, known as FACS. Ekman and Friesen used their system to resurrect Duchenne’s distinction, by that time forgotten, between genuine smiles of enjoyment and other types of smiles.

Some researchers now believe that genuine smiles are not transient sparks of emotion but rather clear windows into a person’s core disposition.