Lie Detection

A procedure (or machine) designed to distinguish truth-tellers from liars.

Conducting lie detection exam.

Conducting lie detection exam.

The commonly used lie detector test relies on an instrument called the polygraph to measure and record the subject's physiological response to specific questions. In a conventional polygraph examination, galvanic skin response (GSR), blood pressure, and heart rate are monitored. Sweating causes a brief drop in the electrical resistance of the skin. This resistance (the GSR) can be measured by means of electrodes attached to the hand. An arm band is used to measure blood pressure and pulse rate. Thus the polygraph does not measure lying directly, it measures the physiological responses that are assumed to accompany lying.

Because emotional states are often accompanied by physiological arousal, researchers theorized that physiological measurements could be used to detect what a person is thinking or feeling. The assumption that physiological cues may reveal an individual's guilty reaction to certain statements or questions forms the basic rationale behind the polygraph test. It is assumed that an individual will have increased autonomic arousal in response to certain key questions if they have specific knowledge that is associated with a feeling of guilt. An innocent person, on the other hand, will not experience such autonomic arousal to the same questsion. Nevertheless, there is a real danger that innocent people could be misidentified as liars, simply because of high anxiety triggered by a potentially incriminating question (e.g., “Did you steal the car?” ). Alternatively, accomplished liars may be able to lie without flinching.


Control Question Technique (CQT)—
A polygraph questioning technique for psychophysiological evaluation of subjects’ responses.
Galvanic skin response—
A term describing electroconductance of the skin or electrodermal response; a way to measure the active and passive electrical properties of the skin.
The technique and practice of using a polygraph, especially for the purpose of deception detection.

Besides its use in forensics and criminal investigations, the polygraph has been used in pre-employment screening and employee evaluations during the course of employment. However, this use has been largely prohibited by the Employee Polygraph Protection Act (EPPA) of 1988. The U.S. government remains the biggest user of the polygraph for job applicants, including 70,000 people subjected to the test in 2013, mainly for security clearance purposes. The U.S. military uses a hand-help version of the polygraph known as The Preliminary Credibility Assessment Screening system (PCASS). Unlike the polygraph, it relies less on physiological responses and more on an integrated algorithm to identify deception.

Polygraphy does not enjoy general acceptance from the scientific community, which includes findings of the National Academy of Sciences showing that polygraph research is unreliable. The American Psychological Association also suggested that “because of the nature of deception, there is no good way to validate the [polygraph] test for making judgments about criminal behavior.” Although some psychologists maintain that the polygraph test is both reliable and valid, this is the minority view. Many researchers consider the polygraph test “pseudoscience” and dispute its usefulness, primarily because no physiological pattern of activity is a foolproof reflection of deceit. To complicate matters, a commonly used polygraph technique known as Control Question Technique (CQT) is typically conducted in the manner of interrogation, which may elicit similar nervous responses from truthful individuals or liars alike. Psychologist William Iaconohas said that CQT is based on “naive, implausible assumptions” and that its error rate cannot be assessed adequately. Since physiological arousal can be caused by anxiety or anxiety disorders such as fear, nervousness, depression, or a range of other emotions, the polygraph test is not able to differentiate between those sources of anxiety and anxiety caused by dishonesty.



Iacono, W., and D. Lykken. “The scientific status of research on polygraph techniques: The case against polygraph tests.” In Modern scientific evidence: The law and science of expert testimony, edited by D. Faigman, et al. St. Paul, MN: West, 1997.


Iacono, W.G. “Forensic lie detection: Procedures without scientific basis.” Journal of Forensic Psychology Practice. 1 (Mar 2001): 75–86.

Lewis, J.A., and M. Cuppari. “The polygraph: The truth lies within.” Journal of Psychiatry and Law. 37 (2009): 85-92.


American Psychological Association. Monitor on Psychology: The polygraph in doubt. July 2004. . (accessed August 2015).