They don’t.
According to the American Psychological Association, “There is no evidence that any pattern of physiological reactions is unique to deception. An honest person may be nervous when answering truthfully and a dishonest person may be non-anxious.”
It has never actually been shown in a controlled test that a polygraph machine can detect lies. Even the creator of the machines stated that they were bunk. They’re purely a prop for interrogators.
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