The Polygraph: Does It Work?

The polygraph: does it work?

The polygraph, colloquially known as the “lie detector”, is an instrument that was invented in the 20th century. The term is derived from “poly” which means “many” and graph which refers to graphics and writing. So you could also say that a polygraph is a device that can produce many graphs at the same time. But what do these graphs indicate? They record the physiological reactions that a respondent experiences.

The polygraph was developed based on the idea that emotions can be expressed in physical reactions. For example, when we experience fear, that fear leads to sweating, increased heart and respiratory rates. The polygraph is mainly used on people who are suspected of having committed a crime. In the United States, lawyers use it to determine if a defendant is telling the truth while giving evidence.

A brief history of the polygraph

Since the 1920s, people tried to develop a device that could tell whether someone was cheating or not. Eventually the polygraph was developed, with William Marston being responsible for this project. He created a curve based on numerous blood pressure measurements and drew conclusions from this as to when a subject was lying or telling the truth. However, scientists did not accept the readings from his polygraph as evidence. In fact, the scientific community did not approve it as an acceptable practice.

Later, John August Larson, a psychiatrist and police officer from California, USA, built a modified version of his predecessor Marston’s lie detector. Larson wanted to modernize the police security forces and reduce the brutality sometimes used during testimony to extort a confession. In addition to blood pressure, Larson also measured the rhythm of breathing. He did this to increase the accuracy of the results. Therefore, in 1924, the police began using the polygraph in their investigations.

But Larson’s device continued to evolve. Leonarde Keeler improved the polygraph. He added a new variable, which is electrodermal activity. Electrodermal activity is the extent to which our skin can conduct electricity. Scientists believed that doubts, including lies, increased the electrodermal activity of our skin. This physiological response is related to anxiety and fear.

One hand is used for polygraphic examination.

How does a polygraph work?

There are two different tests that are commonly used and performed with a polygraph. The tests work differently , but both are based on questions designed to induce certain emotional changes in the suspect. The idea is that the suspect’s feelings show up in his or her physiological reactions.

Control Question Test (CQT)

This is the most common test done. There are three different types of questions that the interviewer asks: the irrelevant and the relevant questions as well as the control questions.

  • Irrelevant questions. These questions are not intended to provide important information. They are general questions and they do not relate to the case the interviewer is investigating. The interviewee is not expected to show any kind of excitement or reaction in answering these questions.
  • Relevant questions. These questions relate to the actual investigation. They are specific questions about the event that is being investigated. The guilty person is more aroused (both emotionally and physiologically) than an innocent person.
  • Control questions. These are ambiguous questions that are very imprecise. The questions should not be answered negatively without doubting the answer. Most of the time, these questions relate to non-case-related facts, but they can relate to things that the person did some time ago that might bear a resemblance to the event that happened now. For example, if the crime was murder, the interviewer asks the interviewee if he has hurt anyone in his life. At this question, a guilty and innocent person would probably show the same level of excitement because no one can seriously and safely deny them.

Factual knowledge test

This is a test to check the perpetrator’s knowledge. It refers to facts that only the perpetrator can know. The interviewer asks several questions that have several possible answers, only one of which is the correct answer.

It goes without saying that the perpetrator knows which option is the right one. He will be more excited when the interviewer reads the correct answer. However, the innocent who does not know the case should show the same level of arousal in each of the possibilities. This is because he doesn’t know what is right because he wasn’t there for the act.

Data curves of a polygraph

The boundaries of the polygraph

Despite the fact that the polygraph has been used for decades, we cannot ignore the fact that there are certain limitations related to the reliability of the device. In 2003 the National Research Council produced a report on the polygraph. He analyzed the psychological events on which the result of the polygraph is based. The Council also looked at the procedures that are being used. These are the main conclusions of the researchers:

  • Precision of the polygraph. The physiological reactions measured by the device do not only indicate reactions to deceptions. That is, there are a variety of physiological responses that are similar to attempting deception. This severely limits the accuracy of the results of a polygraph.
  • Theoretical foundations. The theoretical foundations on which the polygraph test is based are very weak. The terms fear and arousal are not precisely defined. For this reason, polygraph measurements are not entirely reliable as they cannot be easily transferred to other groups in society. In short, the data of a polygraph cannot be applied to any person unless he has been examined and the apparatus has been calibrated for him.
  • Accuracy of Evidence. The research results of studies do not agree with test results. However, this lack of correspondence can cause serious problems because the polygraph has a high margin of error when evaluating innocent people.

People use the polygraph when there is no clear evidence to incriminate a suspect. Nevertheless, the results of the polygraph are in no way meaningful.

There are also tricks that allow you to lie in a polygraph test. Some people have learned to control their physiological responses. They are able to manipulate the machine in such a way that the results are in their favor.

Is the use of a polygraph effective?

While we only mentioned a few problems that can arise with a polygraph test, the report identifies more. From this we can deduce that the polygraph does not give reliable results. This is very worrying when we think about the many areas in which polygraphs are used.

In fact, the polygraph has several flaws. This should set off all alarm bells. This is because using a method that does not guarantee accurate results greatly increases the likelihood that someone will be convicted of innocence.

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