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Also known as a polygraph, the lie detector is widely used in crime detection and in security-sensitive areas of employment. It makes use of physiological responses to emotion by assuming that lying involves an emotional reaction which is characterized by physiological changes in the subject. The physiological variables measured by lie detectors are changes in relative blood pressure, heart rate, respiration movements and often sweating of the palms or a galvanic skin response. The first lie detector was invented in 1895 and since then many, more sophisticated machines have been designed.
The apparatus is attached to a subject who is then asked a series of questions to which he or she can answer simply yes or no. Some questions are neutral and are designed to establish a subject's characteristic responses to known truthful answers. The questions about the crime or other area of investigation are phrased so that the subject must either admit knowledge of the crime or lie.
Lying is thought to involve an emotional response because of the conflict between the strong amount of conditioning towards truthfulness instilled in most of us and the desire to escape detection. People without a so-called 'conscience' may still show an emotional response to lying if they believe in the infallibility of the machine.
According to some experts, the accuracy of the polygraph in practice is 80 per cent correct, 3 per cent wrong and 17 per cent not interpretable. The accuracy depends to a great extent on the skill of the examiner who must be thoroughly trained and experienced. Most courts do not as yet accept lie detector reports as evidence, although confessions elicited by means of a polygraph examination are often accepted.