Polygraph

Introduction to Polygraph

Polygraph, any of various scientific recording 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, although it is also used in employment and security screening practices. Because no machine can unerringly recognize when a person is lying, the polygraph results are used in conjunction with other evidence, observations, and information. Emotional stress reflected by this test, for instance, need not be due to lying. On the other hand, a subject may be a pathological liar and therefore show no measurable bodily responses when giving false answers. Ordinary nervousness, individual physical or mental abnormalities, discomfort, excessive pretest interrogation, or indifference to a question also affect test accuracy. The polygraph can, however, provide a basis for an evaluation of whether or not the subject’s answers are truthful. This test has also been helpful in exonerating innocent persons accused of crimes.

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 the American criminologist John Edward Reid, also records muscular movement.

Apparatus worn by the seated subject includes a so-called pneumograph tube around the chest, an ordinary blood-pressure cuff, and electrodes on the fingers and surfaces of the hand. The actual 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.

Proper conditions and procedure are essential in polygraph testing. The room used should be plain, quiet, comfortable, and private; two-way mirrors and microphones are sometimes used as legal precautions. The examiner’s role is also important. He must be consistently objective and should be thoroughly trained in scientific interrogation to reduce the inherent human error.

Polygraph results are generally considered inadmissible as legal evidence in U.S. courts, except where mutual agreement is given by the opposing parties in a case, and the use of the device in private employment procedures was severely restricted by federal law in 1988. The chief objections to the polygraph are that its use is unconstitutional, that it constitutes an invasion of privacy, and that it is still too inconclusive scientifically to be considered valid as evidence. In several countries the use of the polygraph is prohibited by their governments on the basis of violation of free will.” (1)

Resources

Notes and References

• Information about Polygraph in the Encarta Online Encyclopedia

Guide to Polygraph