How do lie detectors work?

From Caroline Jackson

Lie detectors, or polygraphs, monitor the body’s stress responses to try and identify if someone is lying. The subject is hooked up to several sensors to detect responses such as heart rate, blood pressure, breathing rate and sweating. They are asked some simple questions at the beginning of the test so that their normal levels are recorded, then the real questions follow. If the person is lying, the theory is that they will feel nervous, and spikes in their stress responses will give them away. However, polygraph results are not reliable enough to use in courts of law.

Answered by Katy Sheen. Originally featured in How It Works issue 109

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