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Polygraph tests do lie

By Professor Ray Bull, from insidetime issue September 2007

After extensive research, Professor Ray Bull concludes that even in the most favourable circumstances, polygraph lie-detection accuracy is not high


Polygraph tests do lieThroughout history it has often been assumed that lying can be detected by examining changes in bodily activity but we are actually deceiving ourselves if we believe there will ever be an error-free way of detecting deception. Polygraph tests in particular should not be ascribed special status.

I make this conclusion as the chair of a working party convened by the British Psychological Society to examine research into the most popular polygraph tests and assess their use in real life situations. The working party’s report, which was published in January and entitled “A review of the current scientific status and fields of application of Polygraphic Deception Detection”, concluded that the accuracy of polygraphs is not high and that the rate of incorrect decisions is too significant to ignore.

Of course polygraph tests are not currently used in criminal investigations in the UK, but they are in many other countries including Belgium, Canada, Israel, Japan, Turkey, Singapore, South Korea, Mexico, Pakistan, the Philippines, Taiwan, Thailand and the USA. In a number of countries the courts have been apprehensive about admitting testimony concerning the ‘outcomes’ of polygraphic lie detection and the BPS report should be of assistance in this regard.

Polygraph tests work by measuring changes in bodily activity such as heart rate, blood pressure, respiration, and palmar sweating. Three out of the four most popular lie detection procedures assume that while answering so-called ‘relevant’ questions, liars will be more aroused than while answering so-called ‘control’ questions.

Yet this premise is somewhat naďve, as truth tellers may also be more aroused when answering the relevant questions, particularly when these relevant questions are emotion evoking, for example an innocent man, questioned about murdering his beloved wife, might experience strong feelings about her. An innocent examinee can also become more aroused due to fear, which may occur, for example, when the person is afraid that his or her honest answers will not be believed.

Moreover, a suspect may admit having guilty knowledge but nevertheless deny guilt. This happens when the suspect admits being present but denies the specific alleged acts, for example in an alleged sexual assault where the suspect admits the sexual acts but claims that they were consensual.

In our report we examined the available evidence to establish whether the polygraph is a useful procedure for the UK. Scientific laboratory studies, which generally show somewhat favourable results for polygraph testing, are strongly attacked by polygraph opponents. Amongst other things, they argue that the ‘guilty’ participants, who are asked to commit a mock crime, have little incentive to try to beat the polygraph test and that innocent participants are unlikely to be concerned about the relevant questions.

Field studies illustrate the accuracy of polygraphs in the ‘real world’ but their quality is subject to debate. One of the main problems is establishing with certainty whether the suspect is actually innocent or guilty. Confessions are widely accepted as ways to establish the ground truth, however a guilty suspect who passes a polygraph test is unlikely to confess, and with no confession the incorrect polygraph decision will not be noted.

Most field studies have been carried out using the Control Question Test (CQT) technique, which compares responses to specific questions about the crime (relevant questions) with responses to control questions, which are expected to arouse anxiety but to a lesser extent than the relevant questions. Overall field studies show the CQT polygraph technique catches guilty suspects in 83 per cent to 89 per cent of cases. But innocent suspects do less well, with between 11 per cent and 47 per cent being classified as guilty.

The two field studies conducted using another polygraph technique, the Guilty Knowledge Test, revealed very good results regarding the classification of innocent suspects (94 per cent and 98 per cent of innocent suspects were correctly classified) but rather poor results regarding the classification of guilty suspects. In these two tests only 76 per cent and 42 per cent of guilty suspects were ‘caught’ and correctly classified.

This is probably because some guilty suspects may be able to ‘cheat’ polygraphs by suppressing their physiological reactions with the help of countermeasures. Mental countermeasures include meditation, training in hypnosis to produce ‘amnesia’ for the offence, and biofeedback training. Guilty people can also use physical countermeasures such as using drugs prior to the examination to dampen physiological responses or increasing their arousal on control questions by inflicting physical or mental pain on themselves or producing muscle tension. This reduces the differentiation in bodily activity.

Proponents of the polygraph test argue that it is highly improbable that countermeasures can succeed because properly trained examiners would notice that the examinee is trying to fool them. However, several studies, some conducted by polygraph supporters, have shown that the use of countermeasures can be very effective in defeating polygraph tests, and that they sometimes remain unnoticed by polygraph examiners.

One of the most famous countermeasures test was conducted by Floyd ‘Buzz’ Fay, a man who was falsely convicted of murder in the USA on the basis of a failed polygraph examination. He took it on himself to become a polygraph expert during his two-and-half years of wrongful imprisonment. He coached 27 inmates, who all freely confessed to him that they were guilty, in how to beat the control question polygraph test. After only 20 minutes of instruction, 23 of the 27 inmates were successful in defeating the polygraph examination.

After studying such evidence as this we concluded that even in the most favourable circumstances polygraphic lie detection accuracy is not high, so an over-reliance on an imperfect procedure may lead to undue relaxation concerning the developing of: other methods of identifying or screening wrongdoers; and other ways of ensuring security and preventing crime. The belief that people who ‘pass’ a polygraph test are, therefore, cleared of suspicion is a false belief.

* Professor Bull is from the University of Leicester and Chair of the British Psychological Society’s working party on Polygraph Deception Detection. * Reproduced by kind permission of Barrister magazine

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This article appears under the following categories...
Unsafe Prosecutions

Summary of headlines for September 2007
High Court Judge orders immediate release
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Current page: Polygraph tests do lie
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The fear factor
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