We do it automatically. As soon as we observe another person, we try to read his or her face for signs of happiness, sorrow, anxiety, anger. Sometimes we are right, sometimes we are wrong, and errors can create some sticky personal situations. Yet Paul Ekman is almost always right. The psychology professor emeritus at the University of California, San Francisco, has spent 40 years studying human facial expressions. He has catalogued more than 10,000 possible combinations of facial muscle movements that reveal what a person is feeling inside. And he has taught himself how to catch the fleeting involuntary changes, called microexpressions, that flit across even the best liar's face, exposing the truth behind what he or she is trying to hide.

Ekman, 72, lives in Oakland, Calif., in a bright and airy house near the bay. As I talked with him there, he studied me, his eyes peering out from under bushy brows as if they were registering each brief facial tic I unknowingly exhibited. Does his talent make him a mind reader? "No," he says candidly. "The most I can do is tell how you are feeling at the moment but not what you are thinking." He is not being modest or coy; he is simply addressing the psychological bottom line behind facial expressions: "Anxiety always looks like anxiety," he explains, "regardless of whether a person fears that I'm seeing through their lie or that I don't believe them when they're telling the truth."

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