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Polygraph Tests: The Lie Detector

Learn how polygraph tests work, as well as what supporters and detractors have to say about them.

See also Justice by Machine: Living with Lie Detector Tests

The theory underlying a lie detector test — or a polygraph test, in more scientific terms — is that lying is stressful, and that this stress can be measured and recorded on a polygraph machine. Lie detectors are called polygraphs because the test consists of simultaneously monitoring several of the suspect’s physiological functions — breathing, pulse and galvanic skin response — and printing out the results on graph paper. This printout shows exactly when, during the questioning period, the biologic responses occurred. If the period of greatest biologic reaction lines up with the key questions on the graph paper — the questions that would implicate the person as being involved with the crime — stress is presumed. And along with this presumption of stress comes a second presumption — that the stress indicates a lie.

Supporters of lie detector tests claim that the test is reliable because:

  • very few people can control all three physiological functions at the same time, and
  • polygraph examiners run preexamination tests on the suspect that enable the examiners to measure that individual’s reaction to telling a lie.

On the other hand, critics of polygraph testing argue that:

  • many subjects can indeed conceal stress even when they are aware that they are lying, and
  • there is no reliable way to distinguish an individual’s stress generated by the test and the stress generated by a particular lie.

The courts in most jurisdictions doubt the reliability of lie detector tests and refuse to admit the results into evidence. Some states do admit the results of polygraph tests at trial if the prosecution and defendant agree prior to the test that its results will be admissible.

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