How Does a Lie Detector Work?
Sometimes it is hard to determine when a person is lying. But most people feel guilty when they lie. Then they cannot control inward reactions. There may be a change in blood pressure or breathing. A polygraph, popularly referred to as a lie detector is a device that detects these hidden reactions to questions.
When a person is given a lie detector test, various parts of the machine are attached to his body. A person who falsely answers a question will usually show a change in breathing and blood pressure. He usually will not show these changes when he answers questions truthfully.
In some countries polygraphs are used as an interrogation tool with criminal suspects or candidates for sensitive public or private sector employment. US law enforcement and federal government agencies such as the FBI and the CIA and many police departments such as the LAPD use polygraph examinations to interrogate suspects and screen new employees.
Within the US federal government, a polygraph examination is also referred to as a psychophysiological detection of deception (PDD) examination. The polygraph was invented in 1921 by John Augustus Larson, a medical student at the University of California at Berkeley and a police officer of the Berkeley Police Department in Berkeley, California.
Polygraph testing is designed to analyze the physiological reactions of subjects. However, research has indicated that there is no specific physiological reaction associated with lying and that the brain activity and mechanisms associated with lying are unknown, making it difficult to identify factors that separate liars from truth tellers. Polygraph examiners also prefer to use their own individual scoring method, as opposed to computerized techniques, as they may more easily defend their own evaluations.
The validity of polygraph testing is again called in to question with the relevant-irrelevant testing technique, designed to gauge reactions of subjects against crime questions and other non-crime related questions. Studies have indicated that this questioning technique is not ideal, as many innocent subjects exert a heightened physiological reaction to the crime relevant questions.
The control question test, also known as the probable lie test, was developed to combat the issues with the relevant-irrelevant testing method. Although the relevant questions in the probable lie test are used to obtain a reaction from liars, it can also gain a reaction from the innocent subject who is afraid of false detection.
The physiological reactions that “distinguish” liars may also occur in individuals who fear a false detection, or feel passionately that they did not commit the crime. Therefore, although a physiological reaction may be occurring, the reasoning behind the response may be different. Further examination of the probable lie test has indicated that it is biased against innocent subjects. Those who are unable to think of a lie related to the relevant question will automatically fail the test.