What is a lie detector? What are the procedures of lie detection?

Lie detectors are also called polygraphs because they graphically record several bodily reactions simultaneously which measure the bodily arousal of the individual. Typically a lie detector measures changes in blood pressure, heart-rate, breathing-rate and depth, and the Galvanic Skin Response (GSR) which indicates variations in the electrical conductivity of the skin.

The individual being tested is first asked a series of neutral (control) questions to establish the baseline. Simple questions are followed by specific questions that are designed to evoke responses from a guilty knowledge supposedly indicating the individual’s involvement in the crime being investigated. The lie detector or the polygraph records the changes in neurophysiological activities that occur while the suspected individual answers these questions.

Though the polygraph makes several objective recordings, the interpretation of these records relies heavily on the subjective judgement by the examiner. It is also probable that several unrelated factors like fear, pain or anxiety being felt by the individual during the test may affect her/his level of arousal. It is possible for the individual to lie with it. The validity of polygraph results in doubtful; however these are still used by law-enforcing agencies for lie detection.