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. Sometimes other factors are monitored; for example, using a wired pad which if sat on reveals if the examinee is contracting his muscles. 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 is routinely used to determine if offenders comply with all the conditions of probation, parole, and supervised release, or have thoughts in the course of sex offender treatment deemed inappropriate by the treatment provider. (All boylovers have such thoughts.) Civil commitment centers also use polygraphs to make sure -- as sure as the device will allow -- that an offender has admitted all his crimes, including those for which he was never charged (and the authorities might not know about). This is a requirement for progressing past the early stage of relapse prevention treatment. However, release from civil commitment, like any other prison, is in no way guaranteed upon completion of any treatment program.
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, such as a psychopath, who is nonchalant and untroubled about his offenses 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. Before the exam, the examinee will be asked if there is anything he wants to confess that would cause him to fail the examination; then the examination proceeds. If the examination is failed, the examiner will tell the examinee this and ask him to explain the failure. A naïve 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.
Meijer et al write, "In Europe, there is an increased interest in using the polygraph ('lie detector') as a tool in the treatment and risk assessment of convicted sex offenders. This interest originated from optimistic reports by American clinicians who argued that polygraph testing in the treatment of sex offenders is akin to urine analysis in the treatment of drug addiction. . . . Our review shows that the available evidence for the claims about the clinical potential of polygraph tests is weak, if not absent."
- Ewout H. Meijer, Bruno Verschuere, Harald L.G.J. Merckelbach, Geert Crombez (October–November 2008). "Sex offender management using the polygraph: A critical review". International Journal of Law and Psychiatry 31 (5): 423–429. doi:10.1016/j.ijlp.2008.08.007.