How Does a Polygraph Test Work?

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Polygraph Description

  • Polygraphs, or lie detector tests, are actually instruments that trained professionals can read to determine whether an individual is telling the truth or lying. The polygraph has between 4 and 6 sensors attached to it while the free end is attached to the interviewed subject. The polygraph machine also has a strip of paper where it prints out what the sensors are detecting. This strip of paper is what the polygraph interpreter will read to get the results of the test.

Taking the Test

  • A suspect for interviewing is placed in a normal chair. Then the interpreter places the sensors on the individual. One sensor measures perspiration and another measures heart rate. A third sensor monitors blood pressure while the fourth detects a person's breathing rate. The interviewer than takes his or her seat and gets the machine ready. The interviewer than asks the suspect several general questions such as "Is your name John?" or "Are you 30 years old?" These are called the base questions. Base questions answers will give the interviewer a guide to show when the subject is telling the truth. All questions are asked in a yes or no manner. When finished with the base questions, the interviewer will ask the "bait" questions whose answers are pivotal in determining the truth. When this is finished, the interviewer will unhook the subject from the sensors and review the strip of paper from the machine.

Reading the Test

  • The interviewer will use the base questions as the truth. Whenever the interviewer sees an increase in perspiration, heart rate, blood pressure, or respiratory rate, they will know that the subject was lying on that particular question. The increase will show up on the graph as a larger line. He or she can then prepare a report based on the findings from reading the strip of paper from the interview. This report is often used by police to eliminate suspects or to confirm that the suspect they are considering is the right one. There is no formula to determining if the suspect is lying or telling the truth. It is solely based on the interpretation of the one administering the polygraph. That is why polygraphs are not admissible in a court of law.

  • Photo Credit JaeYong
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