Lie Detector: Is It Effective?

The polygraph, commonly called lie detector, was created in the twentieth century. According to the etymology, the name is a compound of  poles , that is many, and grafos , that is related to spelling and writing. The polygraph is therefore a tool capable of generating multiple graphs simultaneously. But what do they measure? They determine the physiological response of the person.

The polygraph was born from the idea that our emotions are reflected in our physiological reactions. For example, if we are afraid, this emotion results in sweating, increased heart rate or shortness of breath. This tool is mainly used when interrogating people suspected of having committed a crime to determine if they are telling the truth or if they are lying.

A brief history of the lie detector

Beginning in the 1920s, the idea of ​​creating a machine capable of detecting lies began to spread. The polygraph or lie detector was born by the psychologist and inventor William Marston. He created a deception curve based on several blood pressure measurements. However, the results of the machine are not accepted as evidence, because the procedure in question is not approved by the scientific community.

Later, Berckley’s psychiatrist Larson built a modified version of the lie detector. Its purpose was to modernize law enforcement interrogation strategies, avoiding the brutality they often resorted to to obtain testimony. In addition to blood pressure, he introduced another value to increase the accuracy of the results, the rate of breathing. In 1924, the polygraph began to be used in police investigations.

This version of the polygraph underwent a later modification by Keeler . The new variable introduced, in addition to blood pressure and respiratory rate, was electrodermal or skin conductivity, which is how the skin is able to conduct electricity. Doubt and lying appear to increase the conductivity of the skin. This physiological reaction is related to fear or anxiety.

How does the lie detector work?

Typically, two different tests are used, both of which are based on the formulation of questions with the aim of creating certain emotional alterations in the suspect who will also manifest them physiologically.

CQT ( Control Question Test )

Literally, it is the control question test. Three different questions arise: irrelevant, relevant and controlling.

Irrelevant

These are questions that offer no important information. These are general questions and are not the relationship with the case under investigation. The person being questioned is not expected to exhibit any kind of arousal ( arousal , arousal ) in answering.

Relevant

These questions have a link with the investigation. These are specific questions about the events that occurred. A negative answer is expected (an affirmative answer would mean an admission of guilt) and that the offender will experience greater arousal (emotional and physiological).

Check

These are extremely ambiguous questions. They are very imprecise and are worded in such a way that it is impossible to answer in the negative without doubting your answer. Generally, they refer to facts very distant in time.

They are unrelated to the case under investigation, but may relate to the suspect’s past actions that may bear some resemblance to what happened. For example, if the crime is murder, the suspect is asked if he has ever harmed anyone. Following these questions, both guilty and innocent are expected to exhibit the same arousal .

The goal, however, is for the innocent to show greater activation to control questions. Being ambiguous questions, they will be afraid of giving the wrong answer. To the relevant ones they respond with a minor activation, precisely because they are innocent and unrelated to the fact in question. The guilty ones, on the other hand, show greater activation in the relevant questions, in fact the consequences related to them are greater than in the control questions.

GKT ( Guilty Knowledge Test )

Literally, the test of guilty knowledge. It refers to what the culprit knows about the case. Several multiple choice questions are asked, but only one is correct.

The culprit knows the correct answer to give, so he will have greater activation when he gives that answer. The innocent, on the other hand, who does not know the case, should have the same degree of activation to each answer, not knowing which is the correct one since he does not know the facts. In this sense, the correct answer will be totally recognizable by the guilty, but just as likely as the others by the innocent.

Limits of the lie detector

While the lie detector has been used extensively over the years, it cannot be denied that there are certain limits that would reduce its reliability. In 2003, the  National Research Council published a report on the polygraph, analyzing the psychological basis on which the instrument or the procedures followed is based. The most important conclusions are:

• Polygraph accuracy : Physiological responses recorded with the lie detector do not just reflect the lie. In other words, different psychological processes can be physiologically externalized on a par with deception, so this would greatly limit the precision that one wants to attribute to the instrument.

• Theoretical basis : the theoretical basis of the polygraph is very weak. The terms of fear,  arousal and other emotional conditions are not well defined.

The measured values ​​are not entirely reliable when it comes to generalizing the results to other populations and groups other than those who have them. Ultimately, it is not possible to generalize the data.

• Realism of the evidence : Research in the laboratory does not fit the reality of the evidence. In this case, determining whether a person is lying or not has very important consequences. However, the lack of realism in research can lead to serious problems in reality as the error rate in evaluating innocent people is very high.

• The polygraph is used when there is not strong enough evidence to indict a suspect, so the results of the tool cannot be contested in any way.

• There are countermeasures to disprove the polygraph. It is possible to learn to control the physiological measurements and responses, so the test subject can give the answers he wants and not the ones he aims to obtain with the lie detector.

Is the lie detector effective?

While we have only reported some of the limitations of this tool, there are many more. It follows from this that the polygraph is actually not a completely reliable machine, which is worrying if we think about the field in which this tool is used.

This should put us on alert, as using a method that does not ensure exact results on deception or lying can significantly increase the likelihood of actually convicting innocent people.

Bibliographical references:

National Research Council, (2003), The Polygraph and lie detection , Washington, DC: The National Academies Press.