How Does Polygraph (Lie Detector) Test Work?

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A polygraph machine records the body’s involuntary responses to an examiner’s questions in order to ascertain deceptive behavior.

The test measures physiological data from three or more systems of the human body-generally the respiratory, cardiovascular, and sweat gland systems-but not the voice.

A polygraph, popularly referred to as a lie detector, measures and records several physiological indices such as blood pressure, pulse, respiration, and skin conductivity while the subject is asked and answers a series of questions.

Rate and depth of respiration are measured by pneumographs wrapped around a subject's chest. Cardiovascular activity is assessed by a blood pressure cuff. Skin conductivity (called the galvanic skin or electrodermal response) is measured through electrodes attached to a subject's fingertips.

The accuracy (i.e., validity) of polygraph testing has long been controversial. An underlying problem is theoretical: There is no evidence that any pattern of physiological reactions is unique to deception. An honest person may be nervous when answering truthfully and a dishonest person may be non-anxious. Also, there are few good studies that validate the ability of polygraph procedures to detect deception.

2) Who invented the lie detector test?

Marston is credited as the creator of the systolic blood pressure test, which became one component of the modern polygraph invented by John Augustus Larson in Berkeley, California.

3) When was the first lie detector invented?

In 1902 an inadequate lie detector test was invented by a man named James McKenzie. Later on in the 18th century, 1921, a medical student named John Larson from the University of California invented the modern polygraph instrument, which was much more accurate in its results than the previous machine.

4) How do they do lie detector tests?

If you’re like most people, lying makes your heart race. It makes you pant. It drives up your blood pressure and makes you drip sweat.

A polygraph machine detects lies by looking for signs of these physiological changes.

5) How do you pass a lie detector test?

Tice says it’s also easy to beat a polygraph while telling a real lie by daydreaming to calm the nerves. “Think of a warm summer night… or drinking a beer, whatever calms you. You’re throwing them off,” he says. “The needle might nip a little [because you’re lying], but not off the charts.”

6) Do Lie detectors lie?

Calling it a “lie-detector” suggests that a polygraph machine can detect lies. Commonly used in criminal investigations, this device actually measures nervous excitement. It operates on the premise that if a person is telling the truth they will remain calm.

7) How long does it take to take a lie detector test?

Most polygraph examinations last between one and two hours, but the examinee is only attached to the polygraph instrument for 15 to 20 minutes. There are exceptions some of which include criminal testing or screening tests.

The typical polygraph examination involves three separate phases.

8) How much does it cost to take a lie detector test?

Typical costs: Lie detector tests conducted by certified

professionals typically cost $200-$2,000. The length of the test plays a factor in the price, with all-day tests on the high end of the range.

Typical two-hour, single-issue polygraph exams are generally $200-$800.