How a Polygraph Machine Works
People lie to each other all the time. Sometimes these will be just small lies to protect a friend from a painful truth, and other times it may be a complete whopper of a lie to hide a terrible misdeed from the police or the courts.
Pretty much anyone who has breathed on this earth will have lied at some point in their life, and some cynics may say that people like politicians do it for a living. It has been a long held belief though that human reactions change when telling a lie through increased nervousness and changes in blood pressure.
Get Prices for Polygraph Tests
For example, in West Africa people suspected of committing a crime passed a bird’s egg to one another, with the theory being that if someone broke the egg it showed they were nervous and therefore guilty. This was a somewhat primitive method, and improved ways of detecting a liar were developed during the 20th century in the form of the polygraph.
The modern polygraph machine was invented by University of California medical student, John Larson in 1921 and has been used in police interrogations since 1924. The name polygraph comes from the way the machine records several different body responses simultaneously as an individual is questioned.
The polygraph works through a set of four to six sensors being attached to the interviewee’s body, so that changes in breathing, blood pressure, pulse and perspiration are measured. The signals from the sensors are instantly recorded by pens on a single strip of graph paper.
The test will usually start with the operator asking a set of control questions, which will have indisputable answers requiring a true or false answer to establish the pattern of how the person being interrogated will respond. This is then often followed by a “stim” test, where the interviewer asks the subject to deliberately lie so that the polygraph examiner can confirm that he was able to detect the lie.
Compare Quotes on Lie Detection Tests
Once the pattern has been established the actual interview or interrogation will begin, with the actual questions interspersed with filler questions that are largely irrelevant to the inquiry and “probable lie” control questions. This examination can last about two hours and both during and after the test, the polygraph examiner will look at the graphs to see whether there are any significant changes for any of the questions.
The examination is passed if the physiological responses during the probable lie control questions are larger than those during the actual questions. Should it appear that the suspect has failed the test it is likely he or she will be subjected to further examination in a post test interview in the hope of elciting an admission of guilt.
Generally such changes would occur as a result of a rise in heart rate, higher blood pressure or increased perspiration and provide the evidence that the person under interrogation is lying. Trained polygraph examiners can determine whether someone is lying with an extremely high level of accuracy although it is not one hundred foolproof as people can react differently when lying.