The Challenge of the Concealed Knowledge Polygraph Test

This paper introduces the concealed knowledge polygraph test (CKT) and suggests that it might be useful for the criminal justice system. The main point is that the CKT is a standard and objective psychological test that can to some extent protect the innocent from false allegation. A review of 15 mock crime studies of the CKT supports this contention. It revealed average detection rates of 80.6% for guilty examinees and 95.9% for the innocent. Furthermore, in 11 of these studies no false positive identifications (i.e. failures to exonerate innocent suspects) were observed. Two field studies revealed similar results, with false-positive error rates that are not larger than expected by chance. However, in the field studies, the detection rates for guilty suspects were substantially lower than those obtained in the 15 experimental studies. Suggestions for enhancing the detection rate of guilty suspects in real life settings are provided.Finally, the effects of several possible countermeasures, such as the use of drugs, biofeedback training and behavioural countermeasure maneouvres, are discussed. It seems that drugs and biofeedback are not effective in the CKT. However, at least the electrodermal measure is vulnerable to the effects of behavioural countermeasures. Possible ways to deal with the problem are discussed.