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How a polygraph works.


2016-11-22
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People are used to lying. One could even say, that lying has become a natural part of each human beings life. Some lies appear to be enormous, able to cause serious consequences while others tend to be small with little influence on the future. There is also a type of lie, in which misinformation is given in order to stop something bad from happening, one could describe it as a “positive lie” which sounds absolutely absurd but is actually pretty real. Without doubt lying is our lifelong companion and there is absolutely no sense in denying so. This is why researchers, scientists and inventors finally came up with the polygraph commonly known as the “lie detector” . This device will not directly determine if a given statement is a lie or not but thanks to the ability of monitoring certain processes of the human body, it is capable of revealing changes in the body’s operation while the subject is lying. This gives an examiner the possibility to decide whether a given statement is real or not. But how does a polygraph actually work? The lie detector is basically a medical combination of devices capable of measuring different parameters of the human body. These consist of: blood pressure, heart rate, respiratory rate and skin electrical resistance.

While a given person is being questioned the polygraph measures the above factors and produces a result which is an analysis of the bodies reactions during the interviewer. This is then compared with the same parameters measured in normal non-interrogation conditions which leads to deciding whether someone is lying or not. Some may associate a polygraph with a device equipped with needles which would endlessly scribble single lines on an ever- ending scroll of paper. And indeed this is how a polygraph would look like some time ago. But through the last 20 years polygraph testing has undergone enormous improvement which has lead to the famous needles and paper scrolls to be replaced by digital computers. Together with the evolution of computer technology the accuracy of polygraph testing has also undergone great growth. Today’s technologies allow us to perform very precise examinations without the need of using miles of paper. In addition, available software can now also offer a very good analysis of results making it even easier for an examiner to make a verdict.

 All of the needed parameters are tracked by different means. Below are the methods used nowadays :

  • Respiratory rate – after the subject has been seated on the examination chair, two rubber tubes filled with air are placed on the chest and abdomen. As a result of the expansion of the chest and abdominal muscles, the air placed in the tubes is forced to be displaced. The pressure caused by the air being moved is monitored and transformed into information possible to be presented on the analysis sheet. If the subject’s respiratory rate changes, it is quickly discovered and presented in the examination results.
  • Blood pressure and heart rate – to monitor the subjects blood pressure a special cuff is placed on the upper part of the persons arm. When blood flows through the veins it makes a sound which is registered and analyzed by the polygraph connected to the cuff. As the blood flow changes so does the sound caused by it. Thanks to this it is possible to detect changes in the person’s blood pressure. Placing the cuff also allows examiners to measure the heart rate.
  • Skin electrical resistance – special pads placed in the subject’s fingertips are responsible for measuring the amount of sweat produced by the body. As it has been proven, stress which is associated with lying, causes a rise in sweat activity of the person’s internal system. Therefore if the pads detect such a situation, the reaction is sent to the computer which immediately places the measured changes on the result sheet.

After the exam is ended an analysis sheet is produced. It presents a precise analysis of the changes which took place during the questioning. On the basis of these results examiners are able to draw their conclusions.




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What is a polygraph lie detector test?

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history of lie detector invention and evolution

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Can a polygraph test be cheated?

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Ancient methods of lie detection

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