What is a Lie Detector Test?

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  • Written By: Charity Delich
  • Edited By: Bronwyn Harris
  • Last Modified Date: 04 April 2020
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A lie detector test, also called a polygraph test, is a type of examination designed to uncover untruthful or deceptive behavior by using an instrument to measure a person’s involuntary responses to an examiner’s questions. Most lie detector tests work by attaching multiple sensors to a person’s body. While the person is answering the examiner’s questions, the sensors record signals on a strip of moving paper. The signals measure changing physiological responses when the subject is asked certain questions. These changes are recorded on a graph, which is later analyzed by the examiner.

In general, a lie detector test records physiological data from several systems in a person’s body. Most tests record the person’s respiration, perspiration, and pulse rate. Some lie detector systems record blood pressure as well as arm and legal movement. Voice stress analysis is usually not part of a lie detector test.

Substantial changes in a person’s sensory responses typically indicate that the person is lying. For example, if the person’s heart rate increases when an examiner asks a certain question, the sensors would pick this up and record it on the graph. The examiner takes note of these variances when analyzing the graph. Examiners who have had a good deal of training and experience are generally able to pick up signs of lying with a great degree of precision.


Lie detector testing is frequently used by law enforcement officials during examinations of criminal suspects. Private employers occasionally also use lie detection systems when hiring prospective employees. For instance, some government agencies, like the Central Intelligence Agency (CIA) in the United States, require certain employees to undergo lie detector tests as a condition of employment.

A lie detector test can prove to be unreliable. Since the test is recording bodily responses, it can be manipulated to some extent. For example, a person may take sedatives to lessen nervous reactions before taking a test. A subject may even bite certain areas of his or her mouth in order to produce a steady physiological response to the questions and to prevent the machine from detecting changes in vital signs.

In some circumstances, a lie detector test can lead to erroneous results. For instance, a person who is prone to anxiety may be extremely nervous during testing and thus skew the results. The manner in which an examiner phrases questions can also lead to errors. Additionally, how an examiner interprets the results of a lie detector test is subjective and can result in some inaccuracies.


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