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Polygraph tests must be supported by expert evidence

By Jacques van Wyk and Andre van Heerden

19 Aug 2014

A party seeking to rely on the results of a polygraph test in disciplinary proceedings must adduce expert evidence for the purposes of establishing such evidence's conceptual cogency and accurate application. What is key is that evidence be led to establish the cogency of the concept of a polygraph, as well as evidence to establish the technical integrity of the process.

Polygraph tests must be supported by expert evidence

In the case of DHL Supply Chain (Pty) Ltd and National Bargaining Council for the Road and Freight Industry, R Dube and Masinga [2014] ZALAC, the Labour Appeal Court examined the validity of an employer's reliance on the results of a polygraph test to charge two employees with theft in a disciplinary enquiry and subsequently dismiss them. Both the Arbitrator and the Labour Court hearing the matter on review found that the employer's reliance was not justified in the absence of evidence corroborating the results of the test.

The Labour Appeal Court agreed with this finding and acknowledged that courts have previously admitted such evidence but held that this alone cannot justify its admission in all subsequent proceedings. The court held that polygraph evidence may only be relied upon if the party relying on it adduces expert evidence to prove its reliability as well as the accuracy of its use and application in a given case.

Absence of expert evidence

The inference drawn from the failure of a polygraph test is only useful in determining probabilities. The Labour Appeal Court held that in the absence of expert evidence to explain what such inferences are, either generically or in the bounds of the specific instance itself, and also to justify the explanation of what that is, there is nothing usable at all that might contribute towards probabilities.

The employer in this case had not adduced such evidence; the court therefore held that they had not proven that the employees were guilty of theft and thus ordered that the employees be reinstated.

Employers should be aware that whilst they are permitted to rely on the results of a polygraph test in disciplinary proceedings, the inferences drawn from the results should be corroborated with other evidence. Further, the employers should have expert evidence beyond the mere opinion of the polygraph operator to prove that the results are reliable.

Importantly, because employers will be required to prove the accuracy of its application in a given case, the decision to resort to polygraph testing should be carefully considered, taking into account the nature of the case as well as other available evidence.