How can a machine tell if a person is lying? The key to a lie
detector is that people's bodies give off signals when they try to
tell lies. If you're not a very good liar, then you know what some of
these signals are - you get sweaty, and your heart beats faster.
Professional liars can hide some of it, but their bodies still give
off subtle signals.
A lie detector is a machine that records things like heart rate,
breathing rate, skin resistance (which tells you how much the person is sweating) and blood pressure. The examiner asks the subject a few simple questions to find the baseline (the standard or normal state).
Then, additional, more difficult questions are asked. The expert
looks at how the person's body responds. The key, or at least the
hope, is that even someone trying very hard to cover something up, won't be able to hide the physical signs of distress that go with lying.
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