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What Data Are Measured in Polygraph Tests? For centuries, humans have searched for accurate means of detecting lies. In prehistoric Hindu and Chinese societies, authorities “detected” lies by having the suspect chew a grain of rice and spew it out. A dry grain of rice would be associated with the dry mouth of a liar. In India, if rice stuck to the mouth, it would prove guilt. Even if these methods were archaic and non-scientific, they nevertheless underscored the basic supposition humans make in lie detection: lying can be detected through physiological signs. Every time a person lies or is asked a delicate question, his heart may start to race, increasing the body’s blood pressure. The test subject may as well as hold his breath, take in a deep breath, or perspire. Such physiological irregularities are spotted by the polygraph and read by the polygraph examiner. 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. Changes in blood pressure affect the air pressure in the cuff. Such changes are recorded by the polygraph machine and then displayed on a computer monitor, together with respiratory and perspiratory data.
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Respiration The subject’s respiration pattern is recorded by two pneumograph devices which capture thoracic movement or volume change when a person is breathing. One pneumograph tube is fastened around the abdomen and the other around the chest. Like the arm cuffs used to detect cardiovascular changes in a subject, the pneumograph tubes are also filled with air and connected to the machine. The polygraph machine records every change in the tubing air pressure as the subject inhales and exhales. Perspiration The measurement of sweat, scientifically called the measurement of galvanic skin resistance, is made possible by attaching a two-piece galvanometer to two of the subject’s fingertips. 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, when a person perspires, the water and salt from his sweat lowers his skin’s resistance, allowing a greater amount of electric current to travel on the skin surface. Hence, the amount of electric current the galvanometer records, is a reflection of how much sweat that was produced in fingertips of the subject. Although not completely accurate, polygraph tests are often used by as an instructive tool by government authorities and especially law enforcement agencies. With the advancements in technology, humans will be able to better connect the psychological state of lying with physiological manifestations.