Forensic Polygraph

Since the recording of history, human beings have believed that certain physiological changes occur in individuals when they are lying. In ancient China, individuals accused of being deceptive were required to speak with their mouths full of dry rice. The person was determined to be truthful if the rice could be easily spewed out. If the rice stuck to the inside of the mouth, the person was assumed to be lying. Similarly, Egyptians and other peoples used a red-hot poker to determine an individual's truthfulness: The searing end of the poker was placed on the tongue of the accused, and if it sizzled and did not stick to the tongue, the individual was set free. If, however, the poker stuck to the tongue, the accused was deemed guilty and often promptly executed.

This primitive “lie detector” process does have some physiological basis. A person in a highly emotional state, fearing detection, could have difficulty forming saliva, which could cause the rice to stick to the sides of the mouth, and the poker to stick to the tongue. Unfortunately, the innocent as well as the guilty can be emotionally aroused when facing such a process. People can get a dry mouth from being excited, nervous, or fearful, all while telling the truth.

Although the validity of these techniques for the purpose of lie detection can be seriously questioned, the current state of the art for the psychophysiological detection of deception is somewhat more promising. Current polygraph instruments (“polygraph” meaning literally, “many writings or recordings”) record tracings on paper or in computers of several different physiological measures. Most instruments used by licensed polygraph examiners have a minimum of three recording channels: the cardiosphygmograph, which measures relative changes in cardiovascular activity such as blood pressure and heart rate; the pneumograph, which measures changes in the thoracic area of the body (mostly due to respiration); and the galvanic skin response (GSR), which measures skin resistance or conductance. There are many other physiological measures that may be recorded, including cardiac activity monitors, plethysmographs (for measuring variations in the size of an organ or limb), and other instruments, to assess if the individual is attempting to employ countermeasures during the examination. Also, the use of computer programs for assessment of the physiological data has become commonplace.