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Polygraph machines, or "lie detectors," can't empirically determine whether or not someone is telling the truth. They can only measure physiological changes in someone's breathing rate, pulse, blood pressure, and perspiration level. When people lie, these measurements tend to rise. At the start of a polygraph test, the investigator asks a few simple questions with known answers in order to establish a physiological "baseline." Later answers with a marked deviation from the baseline are usually interpreted as lies. But the lines between truth and fiction are often pretty murky, and lots of factors can trigger an increase in heart rate -- the stress of submitting to an
employer-mandated polygraph test, for instance. The ACLU offers an interesting brief noting that over 300,000 American workers have been fired during the last 10 years for failing a polygraph test. Robert Todd Carroll's excellent Skeptic's Dictionary also mentions that the notorious American spy Aldrich Ames passed several polygraphy tests while working for the CIA. The polygraph test is rarely admissible in court. Incidentally, the word "polygraph" comes from the number of sensors used (many, or "poly"), and the fact that the reactions are recorded on paper ("graph").
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