Practice 1
The point is that, apart from many of the polygraphers being unqualified, the tests themselves are by no means free from error, primarily because they discount human imagination and ingenuity. Think of all those perfectly innocent people, with nothing to be afraid of, who blush and stammer when a customs officer asks them if they have anything to declare. Fear, and a consequently heightened electrical response, may not be enough to establish guilt. It depends on whether the subject is afraid of being found out or afraid of being wrongfully convicted. On the other hand, the person who is really guilty and whose past experience has prepared him for such tests can distort the results by anticipating the crucial questions or deliberately giving exaggerated responses to neutral ones!
The success rate of up to 90% claimed for lie detectors is misleadingly attractive. If we refer such a figure to a company with 500 employees, twenty of whom are thieves, the lie detector could catch 18 of them but in doing so would place 32 innocent employees under suspicion.