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A polygraph test–popularly known as a lie detector test–is a machine that measures a person’s physiological responses when they respond to questions. According to the American Psychological Association (APA), polygraph tests measure a person’s “heart rate/blood pressure, respiration, and skin conductivity.” The purpose of the test is usually to prove whether or not a person committed a crime. The test cannot actually test for honesty, however. Instead, it relies on the polygraph operator’s analysis of the tested person’s responses, which can be inaccurate. As such, polygraphs are usually not admissible as evidence in United States courts.

This article from the APA explains that the polygraph test begins by attaching devices such as pneumographs, blood pressure cuffs, and electrodes to a person’s body. The person is then told how the test works and asked some sample questions. Next, the tester will use a technique such as the Control Question Test (CQT)–which is more common–or the Guilty Knowledge test (GKT). 

Under the CQT, the person answers both control and relevant questions; control questions are broader and pertain to a person’s past while relevant questions pertain to the crime a person is under suspicion of committing. The underlying assumption is that a person innocent of suspected crime will be more agitated answering the control questions than the relevant questions. This is because the control questions actually pertain to their actual past behavior whereas the relevant questions relate to a crime they did not commit.

During the GKT, a person answers multiple-choice test of sorts that contains answers that only a person who committed the crime would know. The thought behind this test is that the correct answer out of a lineup would cause a greater reaction from a person who was guilty, whereas an innocent person’s response would not differ.

[Last updated in July of 2020 by the Wex Definitions Team]