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"You can tell a lie but you will give yourself away. Your heart will race. Your skin will sweat ... I will know. I am the lie detector." Thus began each episode of Lie Detector, a strange cross between a relationship counseling session and an episode of the Jerry Springer Show that ran on British daytime television last year. Against a backdrop of flashing computer screens and eerie blue light, participants--usually feuding couples but sometimes warring neighbors or aggrieved business partners--sat on a couch and were quizzed by the program's host. A frequent topic of discussion was one guest's suspicion that his or her partner had been unfaithful. The person suspected of infidelity denied it, of course, and the object of the show was to find out--through cross-examination and computer analysis--whether that person was telling the truth.

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However much we may abhor it, deception comes naturally to all living things. Birds do it by feigning injury to lead hungry predators away from nesting young. Spider crabs do it by disguise: adorning themselves with strips of kelp and other debris, they pretend to be something they are not--and so escape their enemies. Nature amply rewards successful deceivers by allowing them to survive long enough to mate and reproduce. So it may come as no surprise to learn that human beings--who, according to psychologist Gerald Jellison of the University of South California, are lied to about 200 times a day, roughly one untruth every five minutes--often deceive for exactly the same reasons: to save their own skins or to get something they can't get by other means.

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But knowing how to catch deceit can be just as important a survival skill as knowing how to tell a lie and get away with it. A person able to spot falsehood quickly is unlikely to be swindled by an unscrupulous business associate or hoodwinked by a devious spouse. Luckily, nature provides more than enough clues to trap dissemblers in their own tangled webs--if you know where to look. By closely observing facial expressions, body language and tone of voice, practically anyone can recognize the telltale signs of lying. Researchers are even programming computers--like those used on Lie Detector--to get at the truth by analyzing the same physical cues available to the naked eye and ear. "With the proper training, many people can learn to reliably detect lies," says Paul Ekman, professor of psychology at the University of California, San Francisco, who has spent the past 15 years studying the secret art of deception.

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In order to know what kind of lies work best, successful liars need to accurately assess other people's emotional states. Ekman's research shows that this same emotional intelligence is essential for good lie detectors, too. The emotional state to watch out for is stress, the conflict most liars feel between the truth and what they actually say and do.

Even high-tech lie detectors don't detect lies as such; they merely detect the physical cues of emotions, which may or may not correspond to what the person being tested is saying.