The Truth About Lie Detectors (aka Polygraph Tests)

Most psychologists agree that there is little evidence that polygraph tests can accurately detect lies.

Findings

Lie detector tests have become a popular cultural icon - from crime dramas to comedies to advertisements - the picture of a polygraph pen wildly gyrating on a moving chart is readily recognized symbol. But, as psychologist Leonard Saxe, PhD, (1991) has argued, the idea that we can detect a person's veracity by monitoring psychophysiological changes is more myth than reality. Even the term "lie detector," used to refer to polygraph testing, is a misnomer. So-called "lie detection" involves inferring deception through analysis of physiological responses to a structured, but unstandardized, series of questions.

The instrument typically used to conduct polygraph tests consists of a physiological recorder that assesses three indicators of autonomic arousal: heart rate/blood pressure, respiration, and skin conductivity. Most examiners today use computerized recording systems. Rate and depth of respiration are measured by pneumographs wrapped around a subject's chest. Cardiovascular activity is assessed by a blood pressure cuff. Skin conductivity (called the galvanic skin or electrodermal response) is measured through electrodes attached to a subject's fingertips.

The recording instrument and questioning techniques are only used during a part of the polygraph examination. A typical examination includes a pretest phase during which the technique is explained and each test question reviewed. The pretest interview is designed to ensure that subjects understand the questions and to induce a subject's concern about being deceptive. Polygraph examinations often include a procedure called a "stimulation test," which is a demonstration of the instrument's accuracy in detecting deception.

Several questioning techniques are commonly used in polygraph tests. The most widely used test format for subjects in criminal incident investigations is the Control Question Test (CQT). The CQT compares responses to "relevant" questions (e.g., "Did you shoot your wife?"), with those of "control" questions. The control questions are designed to control for the effect of the generally threatening nature of relevant questions. Control questions concern misdeeds that are similar to those being investigated, but refer to the subject's past and are usually broad in scope; for example, "Have you ever betrayed anyone who trusted you?"

A person who is telling the truth is assumed to fear control questions more than relevant questions. This is because control questions are designed to arouse a subject's concern about their past truthfulness, while relevant questions ask about a crime they know they did not commit. A pattern of greater physiological response to relevant questions than to control questions leads to a diagnosis of "deception." Greater response to control questions leads to a judgment of nondeception. If no difference is found between relevant and control questions, the test result is considered "inconclusive."

An alternative polygraph procedure is called the Guilty Knowledge Test (GKT). A GKT involves developing a multiple-choice test with items concerning knowledge that only a guilty subject could have. A test of a theft suspect might, for example, involve questions such as "Was $500, $1,000, or $5,000 stolen?" If only a guilty suspect knows the correct answer, a larger physiological reaction to a correct choice would indicate deception. With a sufficient number of items, a psychometrically sound evaluation could be developed. GKTs are not widely employed, but there is great interest in doing so. One limitation of the GKT is that it can be used only when investigators have information that only a guilty subject would know. The interpretation of "no deception" is also a potential limitation, since it may indicate lack of knowledge rather than innocence.

The accuracy (i.e., validity) of polygraph testing has long been controversial. An underlying problem is theoretical: There is no evidence that any pattern of physiological reactions is unique to deception. An honest person may be nervous when answering truthfully and a dishonest person may be non-anxious. Also, there are few good studies that validate the ability of polygraph procedures to detect deception. As Dr. Saxe and Israeli psychologist Gershon Ben-Shahar (1999) note, "it may, in fact, be impossible to conduct a proper validity study." In real-world situations, it's very difficult to know what the truth is.

A particular problem is that polygraph research has not separated placebo-like effects (the subject's belief in the efficacy of the procedure) from the actual relationship between deception and their physiological responses. One reason that polygraph tests may appear to be accurate is that subjects who believe that the test works and that they can be detected may confess or will be very anxious when questioned. If this view is correct, the lie detector might be better called a fear detector.

Some confusion about polygraph test accuracy arises because they are used for different purposes, and for each context somewhat different theory and research is applicable. Thus, for example, virtually no research assesses the type of test and procedure used to screen individuals for jobs and security clearances. Most research has focused on specific incident testing. The cumulative research evidence suggests that CQTs detect deception better than chance, but with significant error rates, both of misclassifying innocent subjects (false positives) and failing to detect guilty individuals (false negatives).

Research on the processes involved in CQT polygraph examinations suggests that several examiner, examinee, and situational factors influence test validity, as may the technique used to score polygraph charts. There is little research on the effects of subjects' differences in such factors as education, intelligence, or level of autonomic arousal.

Evidence indicates that strategies used to "beat" polygraph examinations, so-called countermeasures, may be effective. Countermeasures include simple physical movements, psychological interventions (e.g., manipulating subjects' beliefs about the test), and the use of pharmacological agents that alter arousal patterns.

Despite the lack of good research validating polygraph tests, efforts are on-going to develop and assess new approaches. Some work involves use of additional autonomic physiologic indicators, such as cardiac output and skin temperature. Such measures, however, are more specific to deception than polygraph tests. Other researchers, such as Frank Andrew Kozel, MD, have examined functional brain imaging as a measure of deception. Dr. Kozel's research team found that for lying, compared with telling the truth, there is more activation in five brain regions (Kozel et al., 2004). However, the results do not currently support the use of fMRI to detect deception in real world individual cases.

Significance & Practical Application

Polygraph testing has generated considerable scientific and public controversy. Most psychologists and other scientists agree that there is little basis for the validity of polygraph tests. Courts, including the United States Supreme Court (cf. U.S. v. Scheffer, 1998 in which Dr.'s Saxe's research on polygraph fallibility was cited), have repeatedly rejected the use of polygraph evidence because of its inherent unreliability. Nevertheless, polygraph testing continues to be used in non-judicial settings, often to screen personnel, but sometimes to try to assess the veracity of suspects and witnesses, and to monitor criminal offenders on probation. Polygraph tests are also sometimes used by individuals seeking to convince others of their innocence and, in a narrow range of circumstances, by private agencies and corporations.

The development of currently used "lie detection" technologies has been based on ideas about physiological functioning but has, for the most part, been independent of systematic psychological research. Early theorists believed that deception required effort and, thus, could be assessed by monitoring physiological changes. But such propositions have not been proven and basic research remains limited on the nature of deceptiveness. Efforts to develop actual tests have always outpaced theory-based basic research. Without a better theoretical understanding of the mechanisms by which deception functions, however, development of a lie detection technology seems highly problematic.

For now, although the idea of a lie detector may be comforting, the most practical advice is to remain skeptical about any conclusion wrung from a polygraph.

Cited Research & Additional Sources

Kozel, F.A., Padgett, T.M. & George, M.S. (2004). A Replication Study of the Neural Correlates of Deception. Behavioral Neuroscience, Vol. 118, No. 4, pp. 852-856.

Lykken, D. (1998). A Tremor in the Blood: Uses and Abuses of the Lie Detector, 2d ed. New York: Perseus.

National Academy of Sciences (2002). The Polygraph and Lie Detection. Washington, DC: National Academy Press.

Saxe, L. (1991). Lying: Thoughts of an applied social psychologist. American Psychologist, Vol. 46, No. 4, pp. 409-415.

Saxe, L. & Ben-Shakhar, G. (1999). Admissibility of polygraph tests: The application of scientific standards post- Daubert. Psychology, Public Policy and the Law, Vol. 5, No. 1 pp. 203-223.


American Psychological Association, August 5, 2004