What is a polygraph and how does it work?

What is a polygraph and how does it work?
Posted on 20-02-2022

We are so obsessed with the truth that we even have an instrument that allows us to detect the lies that a person tells, and it is the polygraph. Which you have undoubtedly heard at some point because it is widely used in police movies to confront a person while they are being interrogated.

In the following article, we will explain what a polygraph is and how it works, so we invite you to continue reading.


what is a polygraph

polygraph is a piece of equipment that measures and records the body's responses to some stimuli, to certain questions or words, and thus be able to observe the anomalies that occur in the responses; and that relates to lies. With it, you measure heart rate, blood pressure, respiratory rate, skin behavior, and of course nerve stimuli.



The polygraph was not designed as a lie detector, as everyone thinks, it is used to measure the difference between the current flow transmitted by the index fingers of the hands, among other parameters. Only this person can perceive responses that the body gives physiologically, according to what was asked, and the results are interpreted by other people.

In the US, the police and intelligence services use the polygraph as a lie detector, while in Europe it is one of the stages to be selected as personnel in public or private security. However, not everything is good, since there are entities that reject its use since those who interpret the data are people subject to interpretations; not precise at all.

But the most bizarre thing is that they even suspend a television program after a guest who underwent a polygraph committed suicide when it was known that he was unfaithful, so extreme care and responsibility must be taken when using this type of device.


The polygraph consists of different parts and works by measuring the respiratory rate, heart rate, blood pressure, pulse, and electrical conductivity of the skin. This information is recorded on graphs that appear as the test progresses.

What are the parts of a polygraph?

The parts of the polygraph are the following:

  • Pneumograph: with this the frequency, quality is measured and when breathing is suppressed, it is placed around the Taurus, capturing its movements.
  • Galvanometer: it is to measures electrical signals and the difference in perspiration, by means of sensors placed on the fingers of the hands.
  • Cardiograph: this is in charge of measuring the heartbeat, heart rate, and blood pressure.


The inventor of the polygraph was John Augustus Larson, who was studying biology and a thesis on fingerprint typing. They became interested in forensic science, in 1920 he completed his doctorate in physiology, He was a police collaborator in Berkley - California, and dedicated himself to the ingenuity of this device that mediates arterial, pulse, and respiration.

This yielded bodily results, and for this reason, the polygraph was used for the first time, in 1921, in a trial, and to make it more accepted the press nicknamed it the lie detector.


When the person is in front of the polygraph, a series of questions are asked, but the environment must be as comfortable as possible, as well as private, only the expert and the person to be interrogated. There are 3 types of questions which are as follows:

  • The irrelevant ones: have little importance or they give quite obvious answers, they are more useful to relax the one who is questioned.
  • The control ones: they allow to compare the reaction with the relevant questions related to the investigation that is carried out.
  • Those of importance: they are direct, concise, and easy to understand.


Deceiving the polygraph is possible, although the one who is really deceived is the expert, who is the person who managed to interpret the results that the polygraph throws before the situation. There are external elements that manage to influence the results too much as if nothing can be taken an anxiolytic, which will allow the answers given to be given very calmly before the polygraph.

It must be clarified that psychopaths in their answers give very different results from other people, for this reason, we will mention some tips that will allow the polygraph exam :

  • Normal breathing is maintained, for about 15 to 30 breaths per minute while the relevant ones are made.
  • Alter breathing when the control questions are asked, which will cause confusion in the evaluator when comparing the answers obtained from the control questions with those of relevance.
  • Answer the control questions strangely and with it the answers obtained well will not be normal to be compared,
  • Stress the mind by answering control questions, making the results of the normal state the same as in cases of truly real stress.


The polygraph is used in the following tests :

  • Pre-employment: an individual who applies for a job and has a large number of questions, in two to two and a half hours.
  • Routine: with them, the performance of the employer is measured and it is determined if there is no misconduct, and they are focused on the exercise of the profession and can last about two hours.
  • Specific: with them, a person is implicated in a specific act and they are used as exculpatory evidence, limited to small questions about the past, they last from two and a half to three hours.


polygraph tests

The former president of the Polygraphy Association of Guatemala, Karina Soto de Rodas, assures that it is a very reliable test according to the evaluation: in pre-employment, they have 82%; the routine ones 86%, and in the specific ones it goes up to 95%. All this is conducted by certified and honest polygraphy professionals; exempt from presenting these tests to people who suffer from:

  • epileptic seizures.
  • Have a cardiac pacemaker.
  • People with more than 24 hours without sleep.
  • People with emotional problems of great magnitude in previous time.


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