We know that variables (age, gender, culture, and social setting, among them) influence the frequency and character of a grin, and what purpose smiles play in the broader scheme of existence. We know that some smiles - Duchenne’s false friends - do not reflect enjoyment at all, but rather a wide range of emotions, including embarrassment, deceit, and grief. We now know that genuine smiles may indeed reflect a “sweet soul.” The intensity of a true grin can predict marital happiness, personal well-being, and even longevity. Psychological scientists no longer study beheaded rogues - just graduate students, mainly - but they have advanced our understanding of smiles since Duchenne’s discoveries. “Its inertia, in smiling,” Duchenne wrote, “unmasks a false friend.” (The technique hurt so much, it’s been said, that Duchenne performed some of his tests on the severed heads of executed criminals.) In his 1862 book Mecanisme de la Physionomie Humaine, Duchenne wrote that the zygomatic major can be willed into action, but that only the “sweet emotions of the soul” force the orbicularis oculi to contract. The name is a nod to French anatomist Guillaume Duchenne, who studied emotional expression by stimulating various facial muscles with electrical currents. Psychologists call this the “Duchenne smile,” and most consider it the sole indicator of true enjoyment. Other muscles can simulate a smile, but only the peculiar tango of the zygomatic major and the orbicularis oculi produces a genuine expression of positive emotion. 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. This 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 hand feels the pressure of another hand. The eyes spot an old friend on the station platform.

Still Smiling at 99 by Michael Levy