How Does a Lie Detector (Polygraph) Work?

In police investigations, you hear about lie detectors all the time, and sometimes a person applying for a job will have to undergo a polygraph test (for example, some government jobs with the FBI or CIA require polygraph testing). A lie detector’s goal is to see if when answering certain questions, the person tells the truth or lies.

A polygraph, popularly referred to as a lie detector test, is a device or procedure that measures and records several physiological indicators such as blood pressure, pulse, breathing and conductivity of the skin while a person is being asked and answers a number of questions

The belief that underpins the polygraph’s use is that deceptive response will produce physiological responses that can be distinguished from those associated with non-deceptive responses. However, there are no specific physiological reactions associated with lying, making it difficult to identify factors separating liars from tellers of truth.

In contrast to computerized techniques, polygraph examiners also prefer to use their own individual scoring method as they can more easily defend their own evaluations.

Four to six sensors are attached to him when a person takes a polygraph test. A polygraph is a machine in which a single strip of moving paper (“graph”) records the multiple (“poly”) signals from the sensors. Usually, the sensors record:

  1. The breathing rate of the person
  2. Blood pressure of the person
  3. The breathing rate of the person
  4. The breathing rate of the person.

A polygraph will sometimes record things like arm and leg movement as well.

The questioner asks three or four simple questions when the polygraph test begins to establish the norms for the signals of the person. Then the polygraph will ask the real questions being tested. All of the signals of the person are recorded on the moving paper throughout the questioning.

A polygraph examiner can look at the graphs during and after the test and see if the vital signs on any of the questions have changed significantly. A significant change (such as a faster heart rate, higher blood pressure, increased suddenness) generally indicates that the person lies.

Using a polygraph, a well-trained examiner can detect lying with high precision. Since the interpretation of the examiner is subjective, however, and because different people react differently to lying, a polygraph test is not perfect and can be fooled.

HowStuffWorks Related Articles

  1. How Lie Detectors Work
  2. How DNA Evidence Works
  3. How Wiretap Works

Rules and regulations:

  1. Polygraph Examiners Advisory Board
  2. Department of Justice Criminal Resource Manual: Polygraphs – Introduction to Trial
  3. US v. Scheffer Brief Analysis
  4. How to Set the Polygraph

The test of the control question, also known as the likely lie test, was designed to overcome or mitigate the problems with the relevant, irrelevant test method. While the relevant questions in the probable lie test are used to obtain a reaction from liars, the physiological reactions that “distinguish” liars can also occur in innocent people who fear a false detection. Thus, although there may be a physiological reaction, the reasoning behind the reaction may be different. Further examination of the likely lie test has shown that it is biased towards innocent subjects. Those who are unable to think of a lie related to the relevant question will automatically fail the test.

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