The Polygraph Technique, Part I: Theory

Written By: Reid
Aug 01, 2001
The polygraph instrument, erroneously called a 'lie detector', is nothing more than a monitoring device to record different physiological systems. The first polygraph was developed in 1908 by a British Doctor to monitor a mother's physiology during labor contractions. In the 1930's this technology was applied to a technique used to question criminal suspects.

The polygraph instrument monitors three physiological systems. It records changes in respiratory rate and volume, heart rate and blood pressure as well as sweat gland activity. The reason these systems are monitored is that they each reflect arousal of the autonomic (involuntary) nervous system (ANS). The ANS contains two types of nerves. The first type, parasympathetic nerves, conserve energy and keep the systems in a relatively resting state. The second type of nerve, sympathetic nerves, prepare the body for action in what is called a fight or flight response. During intense emotional states such as surprise, fear, anger and pain, sympathetic nerves are activated. From polygraph charts a trained examiner can identify whether or not a particular question stimulated sympathetic arousal.

As can be seen, the polygraph instrument itself plays a minor role in detecting deception. Rather, it is the questioning technique that is critical. The simplest technique relies on subject recognition to stimulate autonomic arousal. Consider a burglary in which $500 in cash was stolen from a cookie jar. A suspect who is innocent of this burglary would have no idea where the $500 was stolen from. However, the guilty burglar would certainly remember that unusual hiding place. A suspect could be given a polygraph examination where the central question asked is, "Was the stolen $500 hidden in a _________?" In the blank the examiner could read the words (1) shoe box, (2) desk drawer, (3) night stand, (4) cookie jar, (5) jewelry box, (6) pocket book, (7) stereo cabinet. If this series of questions was asked on three different tests and during each test a subject exhibited autonomic arousal to the item, "cookie jar" this result would clearly support the finding of guilty knowledge, and therefore imply involvement in the burglary. This simple recognition test is called a Peak of Tension test because often the deceptive subject's autonomic arousal gradually builds to the point of recognition whereupon the body exhibits relief, thus forming a literal peak on the polygraph records.

Oftentimes however, through police questioning or media coverage, details of a crime become known to innocent suspects and the above mentioned Peak of Tension test is not a viable design. In this circumstance the polygraph technique is used to identify whether or not a subject is telling the truth when he answers "no" to a crime question such as, "Did you steal $500 from a cookie jar in a home on Main Street?"Common sense might indicate that a lying subject would exhibit autonomic arousal to this question and a truthful subject would not. However, this is rarely the case. A truthful subject may exhibit arousal to this question as a result of recognizing the importance of the question. Furthermore, a deceptive subject may not exhibit arousal to this question because of fatigue, ingestion of drugs or a variety of underlying psychological factors. To overcome these difficulties, in the 1940's John E. Reid developed the Control Question polygraph technique.

Whereas the Peak of Tension polygraph technique relied on recognition, the Control Question technique relies on a more involved psychological process called attention. In any high-motivation situation a person focuses his thoughts, perceptions and actions toward a particular goal. Attention is the process of excluding competing stimuli in favor of a particular goal.

To illustrate, consider that a polygraph instrument was attached to two parents attending a high school band concert who also happened to be close friends. One of the parent's son plays oboe while the other's son plays trumpet. During a particular piece of music there is an oboe solo as well as a trumpet solo and both parents are aware of each other's apprehension over the upcoming solos. The process of recognition would predict that both parents would exhibit similar autonomic arousal during both the oboe and trumpet solos. However, because these are competing stimuli, attention takes over. The polygraph charts of the parent of the oboe player would show much more arousal during the oboe solo compared to the trumpet solo. The polygraph charts of the parent of the trumpet player would show opposite results. Consequently, even though recognition occurred in both parents during the playing of each boys’ solo, their respective attention was direction toward one solo or the other. The fascinating feature of attention as a psychological process is that it excludes competing stimuli. It results in not only heightened arousal to a targeted stimulus, but also inhibits arousal to competing stimuli.

Returning to the earlier burglary example, consider that it is known that the subject taking the examination has a previous juvenile conviction for stealing a car from a neighbor's driveway. During an interview that preceded the polygraph examination the subject explained that he pled guilty to this charge even though he did not steal the car. Investigative information, as well as the subject's behavior, leave no doubt that he did steal the car. Based on this information, the examiner may establish the following set of polygraph questions where questions (1), (2) and (4) are relevant questions and questions (3) and (5) are control questions.

(1) "On July 15th, 2001 did you break into a home on Main Street?"

(2) "On July 15th, 2001 did you steal money from a home on Main Street?"

(3) "Did you ever steal anything from a person you knew?"

(4) "Did you steal $500 from a cookie jar in a home on Main Street?"

(5) "Did you ever take someone else's property without their permission?"

Assume that we are examining the person guilty of the burglary. He is lying to all five of the above questions and yet, because the purpose of the examination is to establish his truthfulness in the burglary, through the process of attention he will exclude the questions concerning general thefts even though he is lying to them (the polygraph is not a lie detector). His attention will be focused to the relevant questions concerning the July 15th burglary.

Assume now that this subject is innocent of the burglary. He is telling the truth to relevant questions (1) (2) and (4) but lying to control questions (3) and (5). Even though the burglary questions are important to him (through recognition) the process of attention will exclude those questions in favor of the much more threatening questions concerning general theft. John Reid described this phenomenon in the terms of emotionally weighted questions. He theorized that a subject will exhibit the most significant arousal to questions which hold the greatest emotional weight as a result of threatening a particular goal during a polygraph examination. The goal of an innocent subject is to be reported as telling the truth during the examination. Thus, the control questions are the most threatening. The goal of a deceptive subject is to not be detected as lying to the issue under investigation. Thus the relevant questions are the most threatening.

The control question polygraph technique compensates for many of the inherent problems associated with a recognition test. In short, for a subject to be reported as telling the truth during a polygraph examination he must demonstrate the ability to exhibit sympathetic arousal through his responses to control questions. A deceptive suspect who has ingested drugs or for some psychological reason does not exhibit autonomic arousal during deception will not be mistakenly reported as telling the truth because those factors will also inhibit responses to control questions. An innocent subject, who may exhibit false arousal to crime questions because of recognition, is provided stronger competing stimuli through control questions.

The next web tip will present information on the proper use of the polygraph technique during a criminal investigation.

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