Several people have said essentially this. The polygraph can measure your pulse and respiration rates, your blood pressure, your galvanic skin response, and from this data it can determine your pulse and respiration rates, your blood pressure and your galvanic skin response. No machine exists that can detect a lie. If a polygraph test uncovers any information at all it's because the person being tested offered it up.
Anyone can "beat" a polygraph test simply by stating that they've told the truth as many times as necessary. A person's behavior during the test might raise concerns about other things, though.
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