Throughout 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.
Of course polygraph tests are currently used in criminal investigations in the US. However, in a criminal case, when someone’s liberty is at stake, a polygraph is not admissible in Wisconsin. Further Wisconsin employers are not supposed to use them on employees. In a number of countries the courts have been apprehensive about admitting testimony concerning the ‘outcomes’ of polygraphic lie detection.
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 naïve, 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. In other words, your heart rate goes up, you sweat and breath heavier for reasons other than lying. Polygraphs therefore do not weed out false positives. That decision is instead left to the subjective opinion of the person giving the polygraph.
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.
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.
In light of present scientific evidence the Department of Justice continues to agree with the conclusion of the Committee on Governmental Operations of the House of Representatives, which held after extensive hearings in 1965:
There is no “lie detector.” The polygraph machine is not a “lie detector,” nor does the operator who interprets the graphs detect “lies.” The machine records physical responses which may or may not be connected with an emotional reaction–and that reaction may or may not be related to guilt or innocence. Many, many physical and psychological factors make it possible for an individual to “beat” the polygraph without detection by the machine or its operator.
H.R.Rep. No. 198, 89th Cong., 1st Sess. 13 (1965). Following further hearings and study, the same conclusions were reached in 1976. The Use of Polygraphs and Similar Devices by Federal Agencies: Hearings on H.R. 795 Before the House Comm. on Government Operations, 94 Cong., 2d Sess. (1976). And in 1988, as a result of continuing doubts about the usefulness and accuracy of polygraphs as a means of detecting deceit, Congress restricted the use of polygraphs in employment decisions. 29 U.S.C. §§ 2001 et seq.