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What Data Are Measured in Polygraph Tests? For hundreds of years, humans have pursued reliable methods of detecting lies. In very old Hindu and Chinese communities, authorities “detected” lies by making the subject to chew a grain of rice and spit 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. Each time a person lies or is asked a critical question, his heart can begin to race, elevating the body’s blood pressure. Also, the test subject may also hold his breath, inhale a large one, or perspire. These physiological irregularities are caught by the polygraph for the polygraph examiner’s interpretation. It is the discretion of the examiner to associate the sudden 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 there are changes in blood pressure, the air pressure in the cuff changes as well. 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 Two pneumograph devices, which capture changes in volume or movements in the thoracic cavity, record the subject’s respiration pattern while he breathes. One pneumograph tube is fastened around the abdomen and the other around the chest. As with the arm cuffs used for detecting cardiovascular changes in a subject, the pneumograph tubes are air-filled while 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. The galvanometer functions by sending a small electric current into the skin from one fingerplate and recording the amount of current that was able to reach the other fingerplate. Dry skin is a bad conductor of electricity. 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. Although not completely accurate, polygraph tests are often used by as an instructive tool by government authorities and especially law enforcement agencies. Through rapid technological advancements, humans will soon to strengthen the correlation between the psychological state of lying and its physiological indications.