Polygraph, any of various devices designed to register a person's bodily responses to being questioned. Popularly known as a lie detector, the polygraph has been used chiefly in criminal investigations. Because no machine can unerringly recognize when a person is lying, the polygraph results are used in conjunction with other evidence, observations, and information. Ordinary nervousness, individual physical or mental abnormalities, discomfort, excessive pretest interrogation, or indifference to a question affect test accuracy. A polygraph is actually several instruments combined to simultaneously record changes in blood pressure, pulse, and respiration.

The electrical conductivity of the skin's surface can also be measured; increased sweat-gland activity reduces the skin's ability to carry electrical current. The Reid polygraph, devised in 1945 by American criminologist John Edward Reid, also records muscular movement.

Physiological changes are transmitted, through a small panel unit, into synchronized readings on moving graph paper; these parallel graphs are then correlated and interpreted to determine whether the subject is lying.