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The Science of Lie Detecting

Updated on December 1, 2016
The graph illustrates the classic lie response: decrease in depth of breathing (top line), sharp increase in sweating (middle line), and a delayed rise in blood pressure (bottom line).
The graph illustrates the classic lie response: decrease in depth of breathing (top line), sharp increase in sweating (middle line), and a delayed rise in blood pressure (bottom line).

The polygraph, or also known as the lie detector, is a device used primarily by police officers, in attempts to determine the truth of a person's answers to questions. The lie detector operates on the principle that when most persons lie, they undergo certain bodily changes: their blood pressure increases, they perspire more profusely, hold their breath, or otherwise show physiological responses. The polygraph can measure and record these responses very accurately. However, only trained and skillful operators can then interpret the record and determine, in some cases, which of the questions they asked were answered untruthfully.

How A Polygraph Works

The lie detector is actually a group of separate instruments. One measures the blood pressure, another the perspiration rate, and others the heartbeat, breathing rate, and body movements of the subject. The instruments are attached to the subject's body and connected to electrical recording pens. The subject is then asked a series of questions. The questions used require yes-or-no answers, and therefore give the subject no chance to be vague or misleading in answering. Some simple, straightforward questions are used to determine his responses when not under tension.

After the question session is completed, the graph traced out by the various instruments is studied. If the graph shows that the subject's physiological response to some questions differs sharply from his response to others, it may indicate that he has been lying. However, drugs and certain feelings, such as pain, fear, anger, and confusion, can produce bodily changes similar to the changes when a person lies.

The accuracy of any interpretation of the record obtained by a lie detector depends on the skill of the operator. If he does not use carefully formed questions, if the conditions under which the test is given are not proper or if the operator does not interpret the tracings accurately, the results are of no value. Most courts will not accept the results of a he detector test as evidence of guilt.


The Italian criminologist Cesare Lombroso first suggested using variations in blood pressure to determine guilt in 1895. Other measurements were suggested by the Swiss psychiatrist Carl Jung, Professor Hugo Miinsterberg of Harvard University, Vittorio Benussi of Italy, and the American lawyer William Moulton Marston.

In 1921, John A. Larson, an American psychologist, developed the first polygraph, a machine that recorded blood pressure and breathingchanges. The complete polygraph was assembled by the American criminologist Leonardo Keeler in 1926, only five years later.


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