It should be pointed out that the polygraph, often referred to as "lie detector" is not in fact such a device. The instrument is designed to record under proper stimuli emotional responses in the form of physiological variations which may indicate and accompany deception. The FBI feels that the polygraph technique is not sufficiently precise to permit absolute judgements of deception or truth without qualifications. The polygraph technique has a number of limitations, one of which relates to the mental fitness and condition of the examinee to be tested.

During the proceedings at Dallas, Texas, on July 18, 1964, Dr. William R. Beavers, a psychiatrist, testified that he would generally describe Jack Ruby as a "psychotic depressive." In view of the serious question raised as to Ruby's mental condition, no significance should be placed on the polygraph examination and it should be considered nonconclusive as the charts cannot be relied upon.134