tests are inaccurate, psychologists warn
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
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.
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.
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.
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.
the report warns that those telling the truth may also show similar
changes when taking polygraph tests.
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.
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.
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."
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."
A review of the current scientific status
and fields of application of Polygraphic Deception Detection (pdf)
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