Polygraph

Earlier societies utilized elaborate methods of lie detection which mainly involved torture; for instance, the Middle Ages used boiling water to detect liars as it was believed honest men would withstand it better than liars. Early devices for lie detection include an 1895 invention of Cesare Lombroso used to measure changes in blood pressure for police cases, a 1904 device by Vittorio Benussi used to measure breathing and an abandoned project by American William Moulton Marston which used blood pressure to examine German prisoners of war (POWs).Marston’s machine indicated a strong positive correlation between systolic blood pressure and lying.

    A polygraph, popularly referred to as a lie detector test, is a device or procedure that measures and records several physiological indicators such as blood pressure, pulse, respiration, and skin conductivity while a person is asked and answers a series of questions.

In some countries, polygraphs are used as an interrogation tool with criminal suspects or candidates for sensitive public or private sector employment. US law enforcement and federal government agencies such as the FBI, NSA and the CIA and many police departments such as the LAPD and the Virginia State Police use polygraph examinations to interrogate suspects and screen new employees. Within the US federal government, a polygraph examination is also referred to as a psychophysiological detection of deception (PDD) examination.

How it works

The examiner typically begins polygraph test sessions with a pre-test interview to gain some preliminary information which will later be used to develop diagnostic questions. Then the tester will explain how the polygraph is supposed to work, emphasizing that it can detect lies and that it is important to answer truthfully. Then a “stim test” is often conducted: the subject is asked to deliberately lie and then the tester reports that he was able to detect this lie. Guilty subjects are likely to become more anxious when they are reminded of the test’s validity. However, there are risks of innocent subjects being equally or more anxious than the guilty. Then the actual test starts. Some of the questions asked are “irrelevant” (“Is your name Fred?”), others are “diagnostic” questions, and the remainder is the “relevant questions” that the tester is really interested in. The different types of questions alternate. The test is passed if the physiological responses to the diagnostic questions are larger than those during the relevant questions.

Does it work?

Polygraphy is widely criticized. Despite claims of 90% validity by polygraph advocates, the National Research Council has found no evidence of effectiveness. The utility among sex offenders is also poor, with insufficient evidence to support accuracy or improved outcomes in this population.

There are no specific physiological reactions associated with lying, making it difficult to identify factors that separate liars from truth tellers. Polygraph examiners also prefer to use their own individual scoring method, as opposed to computerized techniques, as they may more easily defend their own evaluations.

Polygraphs measure arousal, which can be affected by anxiety, anxiety disorders such as posttraumatic stress disorder (PTSD), nervousness, fear, confusion, hypoglycemia, psychosis, depression, substance-induced states (nicotine, stimulants), substance withdrawal state (alcohol withdrawal) or other emotions; polygraphs do not measure “lies”. A polygraph cannot differentiate anxiety caused by dishonesty and anxiety caused by something else.

The reality is that multiple factors – including nervousness in a high-stakes situation – can affect the readings detected by a polygraph machine, and give an impression that the subject is lying. For that reason, polygraphs are not generally admissible in any criminal case, even though police interrogators will sometimes trick a suspect into submitting to one. Polygraphs may be admissible in civil cases, depending on the state, and some states allow polygraph tests to be used in criminal cases if everyone agrees to it.