Liar Liar

By Matthew Purchase

Every good spy film I’ve ever watched has involved some combination of interrogation and lie-detector tests to assist the hero in unravelling the villain’s plot for global domination. Similarly, companies and organisations in South Africa are increasingly turning to the same measures in a forensic context to protect themselves against fraud and theft.

‘Lie detectors’ in one form or another are as old as civilisation itself. For example, in ancient China suspects were required to hold a handful of rice in their mouths whilst the allegations against him were presented. On the premise that guilty people would have dry mouths because of their emotional anxiety, any person with ‘dry rice’ at the end of the prosecutor’s presentation would be considered guilty. Modern lie detectors still rely on the measurement of involuntary psycho-physiological changes in the body (sweaty palms, quickened pulse and changes in respiration and blood pressure) though thankfully science has made considerable advances since the ‘dry rice’ test. However, the use of lie detectors and the validity of evidence obtained remain controversial even today.

The polygraph test is the most popular and most widely used lie detector test used today. A polygraph is a device that records physiological changes to respiration, blood pressure, pulse, and skin current or galvanic skin response. Because a polygraph test is a complex process and the results of a polygraph test need to be interpreted by the person administering the test, it is important to ensure that the person administering the test is fully qualified from an accredited polygraph school and is a member of the Polygraph Association of South Africa (

Some companies place great reliance on polygraph tests and make testing part of their pre-employment selection, some also include clauses in their employment agreements requiring employees to submit to polygraph testing on demand. There is currently no legislation regulating or prohibiting the use of the polygraph in South Africa and it is legal to request employees to voluntarily subject themselves to polygraph tests whether or not there is a term in their employment agreements providing for polygraph testing. Some legal commentators have however pointed out that testing can be used in an intimidatory or coercive fashion, even likening the test to a ‘psychological rubber hose’. Accordingly, great care must be taken to ensure that polygraph testing is done fairly and that the procedures adopted do not infringe individual’s rights - particularly their rights in terms of the Labour Relations Act and the Constitution. Potential abuse of polygraph testing has been explicitly recognised in America, where polygraph tests are regulated in terms of the Employee Polygraph Protection Act.

The accuracy, reliability and admissibility of polygraph evidence has been treated inconsistently in our courts and remain hotly debated issues, despite the fact that hundreds of studies have been performed to confirm the accuracy and scientific basis for polygraph testing. The polygraph institute of South Africa claims testing to be 85% to 98% reliable in investigations concerning a specific issue, while critics of the test have stated that polygraph evidence is opinion evidence of little value and “…is in reality little different from a police officer giving evidence that that during an interview the accused shuffled, stammered and sweated profusely”.

Our courts (including cases from the Commission for Conciliation, Mediation and Arbitration) have sometimes discarded polygraph evidence as inadmissible, but have frequently allowed that evidence as a form of corroborative evidence. Polygraph results can never provide irrefutable probative evidence in court but can sometimes be used to corroborate other admissible evidence. The acceptance of polygraph evidence and the weight afforded to that evidence will depend largely on the credibility and qualifications of the person administering the test and the procedures that were adopted.

In appropriate cases, polygraph testing can be a very useful tool in forensic investigations. A polygraph test should never be implemented in isolation and can of course never replace a proper investigation, but instead may help to focus that investigation and provide additional information. It may also help to limit the number of implicated persons in large investigations involving many persons. Participation in a polygraph test is voluntary and no person can be compelled to take the test. Persons guilty of dishonest or fraudulent conduct may be more inclined to co-operate in the investigation and admit to their role in the dishonest or fraudulent conduct when they are confronted with the results of their polygraph test.

We suggest that polygraph testing, in a forensic context, only be undertaken as part of a larger forensic investigation and that the selection of the person administering the test, the procedures adopted before, during and after the testing and the subsequent use of the information obtained from the polygraph test be overseen by an attorney or other suitably qualified forensic investigator.