A polygraph is an instrument that simultaneously records changes in physiological processes such as heartbeat, blood pressure, respiration and electrical resistance (galvanic skin response or GSR). The polygraph is used as a lie detector by police departments, the FBI, the CIA, federal and state governments, and numerous private agencies. The underlying theory of the polygraph is that when people lie they also get measurably nervous about lying. The heartbeat increases, blood pressure goes up, breathing rhythms change, perspiration increases, etc. A baseline for these physiological characteristics is established by asking the subject questions whose answers the investigator knows. Deviation from the baseline for truthfulness is taken as sign of lying.
There are three basic approaches to the polygraph test:
The Control Question Test (CQT). This test compares the physiological response to relevant questions about the crime with the response to questions relating to possible prior misdeeds. "This test is often used to determine whether certain criminal suspects should be prosecuted or classified as uninvolved in the crime" (American Psychological Association).
The Directed Lie Test (DLT). This test tries to detect lying by comparing physiological responses when the subject is told to deliberately lie to responses when they tell the truth.
The Guilty Knowledge Test (GKT). This test compares physiological responses to multiple-choice type questions about the crime, one choice of which contains information only the crime investigators and the criminal would know about. Psychologists do not think either the CQT or the DLT is scientifically sound, but a majority surveyed by the American Psychological Association think that the Guilty Knowledge Test is based on sound scientific theory and consider it "a promising forensic tool." However, they "would not advocate its admissibility [in court] in the absence of additional research with real-life criminal cases." One major problem with this test is that it has no controls. Also, unless the investigators have several pieces of insider information to use in their questioning, they run the risk of making a hasty conclusion based on just one or two "deviant" responses. There may be many reasons why a subject would select the "insider" choice to a question. Furthermore, not responding differently to the "insider" choices for several questions should not be taken as proof the subject is innocent. He or she may be a sociopath, a psychopath, or simply a good liar.
Is there any evidence that the polygraph is really able to detect lies? The machine measures changes in blood pressure, breath rate, and respiration rate. When a person lies it is assumed that these physiological changes occur in such a way that a trained expert can detect whether the person is lying. Is there a scientific formula or law which establishes a regular correlation between such physiological changes and lying? No. Is there any scientific evidence that polygraph experts can detect lies using their machine at a significantly better rate than non-experts using other methods? No. There are no machines and no experts that can detect with a high degree of accuracy when people, selected randomly, are lying and when they are telling the truth.
Some people, such as Senator Orrin Hatch, don't trust the polygraph machine, even if used by an expert like Paul Minor who trained FBI agents in their use. Anita Hill passed a polygraph test administered by Minor who declared she was telling the truth about Clarence Thomas. Hatch declared that someone with a delusional disorder could pass the test if the liar really thought she was telling the truth. Hatch may be right, but the ability of sociopaths and the deluded to pass a polygraph test is not the reason such machines cannot accurately detect lies with accuracy any greater than other methods of lie detection.
The reason the polygraph is not a lie detector is that...
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