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How Lie Detectors Work

Lie Detector Test


­ Undergoing a lie detector test can be an intimidating experience that can challenge the nerves of even the most stoic person. You are sitting there with wires and tubes attached to and wrapped around your body. Even if you have nothing to hide, you could be afraid that the metal-box instrument sitting next to you will say otherwise. Fittingly, undergoing the uncomfortable experience of a polygraph test is often referred to as "going on the box."


Mouse-over the wire colors to see where they lead.

A polygraph exam is a long process that can be divided up into several stages. Here's how a typical exam might work:

  • Pretest - This consists of an interview between the examiner and examinee, where the two individuals get to learn about each other. This may last about one hour. At this point, the examiner gets the examinee's side of the story concerning the events under investigation. While the subject is sitting there answering questions, the examiner also profiles the examinee. The examiner wants to see how the subject responds to questions and processes information.

  • Design questions - The examiner designs questions that are specific to the issue under investigation and reviews these questions with the subject.

  • In-test - The actual exam is given. The examiner asks 10 or 11 questions, only three of four of which are relevant to the issue or crime being investigated. The other questions are control questions. A control question is a very general question, such as "Have you ever stolen anything in your life?" -- a type of question that is so broad that almost no one can honestly respond with a "no." If the person answers "no," the examiner can get an idea of the reaction that the examinee demonstrates when being deceptive.

  • Post-test - The examiner analyzes the data of physiological responses and makes a determination regarding whether the person has been deceptive. If there are significant fluctuations that show up in the results, this may signal that the subject has been deceptive, especially if the person displayed similar responses to a question that was asked repeatedly.

There are times when a polygraph examiner misinterprets a person's reaction to a particular question. The human factor of a polygraph exam and the subjective nature of the test are two reasons why polygraph exam results are seldom admissible in court. Here are the two ways that a response can be misinterpreted:

  • False positive - The response of a truthful person is determined to be deceptive.
  • False negative - The response of a deceptive person is determined to be truthful.
"If we look at laboratory-based studies, false-positive errors occur somewhat more often than false-negative errors," Horvath said.

Critics of polygraph exams say that even more false-positive errors occur in real-world scenarios, which biases the system against the truthful person. These errors are likely to occur if the examiner has not prepared the examinee properly or if the examiner misreads the data following an exam.­