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Polygraph tests do lie

14 January 2005

The British Psychological Society is warning against relying on polygraph tests to detect guilt because the technique is inaccurate.

A report published today by the Society shows that the rate of incorrect decisions made on the results of polygraph tests is just too high to ignore. The report comes as the Government unveils its latest criminal justice legislation for England and Wales, including polygraph tests to monitor sex offenders' compliance with conditions of their licence on release from prison - though the results would not be admissible in any further prosecution.

A study of one particular polygraph technique used in criminal investigations 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 truth-tellers may also show similar changes when taking polygraph tests. This may be because the questions are particularly upsetting, for example focusing on a murdered relative, or because the truth-tellers have a fear their honest answers will not be believed.

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.

The report also cautions against the use of polygraph tests for employment and security screening and issues relating to human rights are also discussed.

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.”

The full report can be downloaded for free at