Effective Policing

• Author: William G. Iacono
• DOI: 10.1177/0093854808321529
• Published date: 01 October 2008
• Date: 01 October 2008
• Subject Matter: Articles

EFFECTIVE POLICING

Understanding How Polygraph Tests

Work and Are Used

WILLIAM G. IACONO

University of Minnesota

Forensic applications of polygraph techniques rely primarily on the control or comparison question test (CQT). The author

describes the CQT and its theoretical basis, and how it is used and evaluated by the polygraph professionals, and by scien-

tists at arms length from the polygraph community. Because the CQT (a) has a weak theoretical foundation, making it

unlikely that it can be as accurate as polygraph proponents claim, (b) is biased against the innocent, and (c) may be subject

to countermeasures used by the guilty to appear truthful, CQT results cannot constitute evidence of either deception or truth-

fulness. In the absence of insight into brain mechanisms that underlie deception, it may be difficult to develop a valid lie

detector. However, methods are available for detecting guilty knowledge, information that only the perpetrator of a crime and

the police possess, which are ripe for further development as forensic applications.

Keywords: polygraph; control question technique; comparison question test; police interrogation; guilty knowledge test

To the dismay of a fearful public and the police, many crimes with a plausible suspect

go unprosecuted due to a lack of hard evidence. It is possible that no amount of detec-

tive work will turn up the leads necessary to clear or indict the suspect. However, the evi-

dence needed to resolve the case is present in the mind of the perpetrator. If only we could

accurately access this information, more crimes would be solved, the innocent would rarely

be falsely imprisoned, and the guilty liar would always be convicted. For more than half a

century, the polygraph profession has claimed that we can resolve seemingly unsolvable

cases by tapping into the physiological basis of a suspect’s veracity. In this article, I will

examine how well we can detect lying, using conventional police polygraph methods, and

how law enforcement agencies can best use deception detection procedures as investigative

aids. My focus will be the so-called control or comparison question test (CQT), a proce-

dure that is widely used in forensic settings.

PSYCHOPHYSIOLOGICAL BASIS OF THE CQT

It is generally recognized that there is no physiological response that is uniquely associ-

ated with lying. Moreover, the brain mechanisms involved in lying are unknown, rendering

difficult the development of techniques that can distinguish liars from truth tellers. Hence,

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CRIMINAL JUSTICE AND BEHAVIOR, Vol. 35 No. 10, October 2008 1295-1308

DOI: 10.1177/0093854808321529

© 2008 International Association for Correctional and Forensic Psychology

AUTHOR’S NOTE: Please address all correspondence to William G. Iacono, Department of Psychology,

University of Minnesota, 75 East River Road, Minneapolis, MN 55455; e-mail: wiacono@tfs.psych.umn.edu.