Polygraph tests are inaccurate, psychologists warn

January 17, 2005
by Angela Hussain

Polygraph tests to detect guilt and which are set to be used on sex offenders are inaccurate, psychologists have warned.

A report by the British Psychological Society (BPS) concludes the accuracy of the lie-detecting technique is "not high" and the rate of incorrect decisions is "too significant to ignore"

The report could be a blow to the government which has plans for convicted paedophiles to face mandatory lie detector tests in parts of the UK. The plans were outlined in the Management of Offenders and Sentencing Bill, published last week.

The Home Office has said 148 polygraph tests have been carried out on sex offenders since a pilot scheme began in September 2003.

However, the BPS's report also raises ethical issues relating to the use of polygraph tests when managing, supervising and treating criminals, including sex offenders.

The report, entitled "A review of the current scientific status and fields of application of Polygraphic Deception Detection" was produced by a BPS working party. It researched into the most popular polygraph tests and assessed their use in real life situations.

As an example of the technique's dubious accuracy the report highlights a field study of one polygraph technique used in criminal investigations which found that 10 to 17 per cent of guilty suspects 'cheated' the test and were classified as innocent, while between 11 and 47 per cent of innocent suspects were classified as guilty.

Polygraph tests are based on the premise that liars will experience changes in bodily activity, such as sweating or changes in heart rate, blood pressure and respiration, because they fear getting caught.

But the report warns that those telling the truth may also show similar changes when taking polygraph tests.

Guilty people can also beat polygraphs by suppressing their physiological reactions with the help of mental countermeasures such as meditation or physical ones such as drugs. More commonly they increase their arousal on control questions by inflicting physical or mental pain on themselves or producing muscle tension. This reduces the differentiation in bodily activity.

Professor Ray Bull, of the University of Leicester and chair of the working party, said: "The polygraph is one of a number of procedures that could be used in attempts to detect deception and integrity but, like all procedures, it has inherent weaknesses.

"Error rates in polygraph deception detection can be high, so the belief that people who 'pass' a polygraph test are, therefore, cleared of suspicion is a false belief."

He added: "Polygraph procedures should not be ascribed a special status. We must not deceive ourselves into thinking there will ever be an error-free way of detecting deception."

Read for yourself:
A review of the current scientific status and fields of application of Polygraphic Deception Detection (pdf)

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