Polygraph

A polygraph, popularly but inaccurately called a "lie detector", records various physiological phenomena on paper, similar to an ECG: breathing, blood pressure, pulse, and skin conductivity. The premise is that when asked a series of questions by the examiner, the examinee will involuntarily reveal, through the above markers, whether or not he is being deceptive.

The U.S. is far and away the biggest user of the polygraph; many European countries find this ludicrous.

The polygraph's lack of accuracy has been well established through research. Because of its unreliability, polygraph results are not admissible as evidence in a court of law, though it is often used by probation and parole agencies (including by psychologists during court-ordered sex offender treatment), or to provide guidance during police investigations. The polygraph does not measure deception, it measures stress. A criminal who is nonchalant and untroubled about his crimes can pass a polygraph; an innocent man who is anything but calm can fail it.

A key function of the polygraph examination is to get the person under examination to reveal information to the examiner. If the examination is failed, the examiner will tell the examinee this and ask him to explain the failure. A naive examinee may conclude he has been "caught" and that he might as well confess. The correct response is "I was telling the truth; the machine is mistaken." If permitted, the presence of the examinee's attorney, who will always - and quite correctly - tell anyone under suspicion to "keep his mouth shut", can be helpful. A large proportion of those in prison are there not because of what the police discovered, but because of what was confessed or admitted.