Lie Detector Testing

Polygraph testing is popularly referred to as lie detector testing, but this is not strictly accurate, as it is not possible to detect lies with a machine, but rather the test determines the body’s physiological reaction to a number of questions. In fact, the U.S. Federal Government refer to such tests as “Psychophysiological Detection of Deception” (PDD).

The History of the Polygraph Machine


A medical student, John Larson, from the University of California invented the forerunner of the modern polygraph in 1921.  It superseded a more primitive version that was developed in the early 1900s.  The later version was more accurate than its predecessor and, like the modern version, it recorded physiological responses, including blood pressure, and pulse and respiratory rates. In the 1990s digital equipment was introduced and those machines are now the most popular.


The term “polygraph” derives from the Greek words “poly” meaning “many” and “graph” meaning “writing.” So a literal translation would be “many writings.” In the case of a polygraph machine, the name refers to the simultaneous recording, originally by pens, of the physiological changes in the subject as a series of questions is asked. The machines using pens to trace lines on paper, called analog machines, are now considered less accurate than the modern technology that uses digital equipment to show the results on a computer screen.


How the Lie Detector Works

The information being recorded during a polygraph examination comes from three main signals:

Respiratory: To monitor the breathing rate, a rubber tube is placed on the upper chest and another on the abdominal region.

Cardiovascular: a blood pressure cuff records changes in heart rate, pulse, and pressure.

Skin condition: sweat gland activity is recorded via small electrodes placed on the fingers.

The responses from these instruments are compared with the timing of the questions being asked, to correlate the answers with the changes recorded.

The Polygraph Test Procedure

The test procedure begins with an interview where the examiner will introduce himself and explain the testing process. He will also cover the questions that are going to be asked, usually between 3 and 5 questions related to the subject matter plus some additional ones concerning general information.

With the initial stage completed the actual test will take about 20 to 30 minutes, after which the examiner will review the charts collected and form an opinion as to the veracity of the examinee. These results are usually described as NDI (no deception indicated), DI (deception indicated) or INC (inconclusive).

Is the Lie Detector Test Accurate?

It is generally accepted, and backed by scientific studies, that polygraph tests are highly accurate.  However, errors can occur – no testing procedure or apparatus is infallible. Errors usually fall into two categories: false positives and false negatives. A false positive reports that deception has been recognized when the examinee was being truthful, and the opposite situation is when deception occurred, but the subject was identified as being truthful. In order to minimize the likelihood of errors, examiners are highly trained in both techniques and procedures.


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