Why (some) Americans believe in the lie detector while others believe in the guilty knowledge test.

The accuracy of polygraphic lie detection in real life applications is very little better than chance. Yet, at least in the United States, many agencies and the polygraphers themselves have great faith in the technique. The reasons why polygraph examiners, and their clients, genuinely believe in the myth of the polygraph are explained and illustrated. A more plausible method of polygraphic interrogation, the Guilty Knowledge Test (GKT), is described and it is shown how the GKT, but not the lie test, might have resolved doubts about the case of Demjanjuk, the alleged "Ivan the Terrible."