| " Mona Lisa's smile is mysterious, the Cheshire Cat's is devious, the Joker's is mischievous and Buddha's beatific. Humans probably have been smiling for as long as they've been around. But despite the long history of smiles, scientists still haven't figured out exactly how or why the brain tells the lips to curve, the nose to wrinkle, the eyes to twinkle and the cheeks to lift." " Babies generally start smiling at about six to eight weeks. Throughout childhood, boys smile just about as much as girls. That changes soon after puberty. Grown women smile more than men, and they also smile wider. Smiling, studies suggest, makes people appear more attractive, kinder and, by some accounts, easier to remember." " Smiles carry myriad meanings: joy, amusement, politeness, mockery, disdain, lechery and deceit, to name a few. But no matter the emotion, all smiles call on many muscles and nerves, starting with one called cranial nerve seven." " Cranial nerve seven leaves the brain and heads for the face, and at the point where the jawbone meets the skull, it branches off. Some of its tributaries travel to the muscles of the forehead, some to the eyes, some to the nose and others to the cheeks, lips and chin. When cranial seven sends its message to the face, the face will smile." " All smiles share something else in common: an emotional foundation. But there's subtlety here. Depending on what the emotion is, the brain sends different instructions to the face ? such was the conclusion of a young, 19th century French doctor named Guillaume Duchenne." " In the 1840s, Duchenne went from hospital to hospital in Paris carrying a box-like contraption of his own making. Using the coil and electrodes in the box, he applied volts of electricity to the faces of his patients. As their faces contorted, he took notes, ultimately creating a map of the face's muscles and nerves." " In the process, Duchenne noticed that the range of human facial expressions includes two kinds of smiles: one that stops at the lips, and one that extends across the face, to the eyes. A smile engaging the eyes, he concluded, was a genuine smile, one that is technically called, to this day, a "Duchenne smile."" " A century after Duchenne, scientists studying facial expressions began applying electrical currents directly to the brain. They found that stimulating certain areas could induce a smile, and that stronger stimulation could make a person laugh." " But not all scientists got the same results. In one experiment,
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