The polygraph lie detector works on the same principles as detecting behavioral betrayals of deceit, and it is vulnerable to the same problems.
The polygraph exam does not detect lies, just signs of emotion. Wires from the polygraph are attached to the suspect to measure changes in sweating, respiration, and blood pressure.
Increases in blood pressure or sweating are not in themselves signs of deceit. Hands get clammy and hearts beat faster when emotion is aroused.
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