The polygraph, or lie detector, measures physiological responses when a person is asked a question. The test will record changes in blood pressure, pulse, sweating, breathing and body temperature, amongst other changes. When subjected to the polygraph a person is asked a number of control questions to see how they act when telling the truth, after which the subject is then asked a number of questions to which they will probably lie. Finally the subject will be asked the questions to which the tester wishes to know the answer. While the polygraph itself is often criticised for inaccuracy, there are a number of other related tests which are thought to be more accurate, such as the Guilty Knowledge Test and the Concealed Information Test. None, however, has been found to give accurate-enough answers to be used in the criminal courts in the UK.
Currently polygraph testing is not admissible because of concerns over its accuracy. It is thought that a polygraph test is between 60% and 95% accurate, but this is not known for certain and there have been concerns that if used, the polygraph could lead juries to make decisions based on its result, leading to convictions of innocent people and non-convictions of those able to beat the polygraph.
In the United States, polygraph evidence is used in courts in some states and is also used in conducting police investigations. In the UK, polygraph testing is increasingly used outside of the courts system, for example between warring couples and by employers. In addition to this, legislation passed last year allows the use of polygraph tests as part of probation conditions for certain criminals, though it specifically forbids its use in criminal courts. Polygraph testing is currently being used to determine whether sex offenders are ready to be released from prison and will continue to be used as a pilot in certain areas of England and Wales for two more years. Whether this will alter the opinion on court use remains to be seen. Lie detector results can be admissible in some tribunal and civil cases, but their introduction into criminal courts would require legislation.