The polygraph or so-called lie detector measures physiological responses to stress experienced by a subject during the course of an interrogation. The instrument monitors three physiological states: (a) cardio-vascular responses manifested by changes in blood pressure and pulse rate; (b) galvanic skin resistance that lowers as perspiration increases; and (c) breathing patterns that respond to changes in tension. Changes in any of these patterns can be detected as the subject experiences emotional reactions. The theory behind the polygraph assumes that people encounter measurable physiological changes in the act of deception. The heartbeat increases, blood pressure goes up, breathing rhythms change, and perspiration increases. All of these reactions are recorded on a moving chart for analysis by a trained polygraph technician.

The physiological connection with deception was assumed in the eighteenth century. English novelist Daniel Defoe suggested that "Guilt always carries fear around with it, there is a tremor...