Lie Detector, or Polygraph, a device used to help determine whether a person is lying. Lie detectors are used primarily by government security agencies and criminal investigators, and by business firms as part of internal investigations of theft. The typical lie detector consists of devices that continuously monitor and record on a chart the subject's blood pressure, pulse, breathing rate, rate of perspiration, and galvanic skin response (a measure of the skin's electrical conductivity). These physical processes and conditions are readily affected by psychological stress.

The examiner usually asks a series of questions, some irrelevant, others bearing on the subject's knowledge of the situation in which the subject's truthfulness might be in question. Large fluctuations in the conditions and processes being monitored may indicate a strong emotion, which, in turn, may indicate lying.

The examiner must be highly skilled in phrasing the questions and interpreting the results. With some subjects, lying does not produce large fluctuations, and with certain others, large fluctuations can occur even when a truthful answer is given. Tests may sometimes prove to be inconclusive. Under United States law, no one can be forced to take a lie-detector test, and results can be used as evidence in court only under certain conditions.

The forerunner of the modern lie detector was devised by William M. Marston in 1915. Important improvements were added by John Larson in 1921, Leonarde Keeler in 1926, W. G. Summers in 1936, and John Reid in 1945. By the late 1980's, lie detector tests were being used routinely by many businesses. Many people, however, viewed the testing as an invasion of privacy and objected to the testing because the results could be misused. The Employee Polygraph Protection Act of 1988 specifies the conditions under which lie detectors can be used in the United States. Among other restrictions, the Act prohibits private business from using lie detector tests to screen job applicants.