How Lie Detectors Work
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Polygraph
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The basic idea behind a lie detector is that people experience subtle physiological changes when they lie or become nervous. If an interrogator can detect these changes, they have a good chance of identifying when the person is telling the truth and when they are lying. The technical name for a lie detector is a polygraph, because it simultaneously charts several physiological processes. Traditionally recorded by a polygraph are the subject's heart rate, breathing rate, blood pressure and sweat on the fingertips (also called galvanic skin response, or GSR), but more modern machines can also detect movements in the arms and legs. In the old analog machines, sensors attached to the subject in the appropriate places sent their readings to be graphed by very sensitive needles over constantly rolling graph paper. Today, digital machines using complex algorithms creates graphs on a computer screen.
Interrogation
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Just as important as the machine itself is how the lie detector is used. The information provided by the polygraph must be interpreted by a technician to be of any value. There are three major approaches to effective interrogation with the polygraph, the most reliable of which is called the guilty knowledge test. This method monitors the subject's physiological responses when asked multiple choice questions of which some of the answers contain information only the criminal would know. More classic approaches involve asking the subject a series of both neutral and emotionally charged control questions to calibrate their responses before asking the important questions. A similar method of calibration is to ask the subject to deliberately lie, which is compared to test questions answered truthfully.
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Controversy
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The major problem with the lie detector test is that it does not directly measure truthfulness. The aspects it does measure, heart rate, breathing and such, can vary for a number of reasons not related to lying. In many cases, the lie detector is used not so much to determine the truthfulness of the subject's answers, but to intimidate them into a confession. During the confirmation hearings of Supreme Court Justice Clarence Thomas, Senator Orrin Hatch dismissed polygraph results that supported claims against Thomas, saying that anyone who believes their lies could pass a polygraph. In 1998, the Supreme Court officially ruled that lie detector results are too controversial to be used as evidence (even in defense) or to replace the judgment of a jury.
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