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What Data Are Measured in Polygraph Tests?

For hundreds of years, humans have pursued reliable methods of detecting lies. In early Hindu and Chinese cultures, authorities “detected” lies by instructing the subject to chew a grain of rice and spew it out. A dry grain of rice would be considered a sign of the dry mouth of a liar. In India, if rice stuck to the mouth, it would be a sign of guilt. Though these methods were ancient and non-scientific, they however stressed the elementary theory humans make in lie detection: lying may be detected by observing physiological signs.

Whenever a person lies or is asked a critical question, his heart may begin to race, increasing his blood pressure. As well, the test subject may also hold his breath, inhale a big one, or perspire. Such physiological irregularities are spotted by the polygraph and read by the polygraph examiner. It is the judgment of the examiner to equate the significant data changes with dishonesty.

Cardiovascular Activity
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An encircling, air-filled cuff placed around the upper arm records blood pressure and heart rate. When blood pressure changes, so does the air pressure in the cuff. The polygraph machine records such changes and displays them on a computer screen, side by side with respiratory and perspiratory data.
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Respiration

The respiration pattern of a subject is recorded by two pneumograph devices capturing thoracic volume change or movement as a person breathes. One pneumograph tube is placed around the chest and the other around the abdomen. Like the arm cuffs that detect cardiovascular changes in a subject, the pneumograph tubes are also air-filled and connected to the machine. The polygraph machine records every change in the tubing air pressure as the subject inhales and exhales.

Perspiration

Sweat measurement, scientifically referred to as the measurement of galvanic skin resistance, is conducted by attaching a two-piece galvanometer to two fingertips of the subject. Through the galvanometer, a small electric current is sent into the skin from one fingerplate, and the amount of current that got through on the other fingerplate will be recorded. Dry skin conducts electricity poorly. However, during perspiration, water and salt from the sweat drives down skin resistance, allowing a bigger amount of electric current to flow on the surface of the skin. Thus, how much electric current is recorded by the galvanometer depends on how much sweat the subject’s fingertips produced.

While not a hundred percent accurate, polygraph tests are commonly used as an instructive tool by law enforcement agencies and many government authorities. Through rapid technological advancements, humans will soon to strengthen the correlation between the psychological state of lying and its physiological indications.