Home » The truth about Lie Detectors: Science behind It!

The truth about Lie Detectors: Science behind It!

by admin
The truth about Lie Detectors: Science behind It!

Why is there only a lie detector and not any truth detector?


The first Lie Detector or Polygraph was created in 1921. An apparatus to simultaneously measure continuous changes in blood pressure, heart rate and respiration rate was devised by a California-based policeman and physiologist John A. Larson

When an individual experiences anxiety, fear, or emotional excitement, his or her rate of respiration, blood pressure, and galvanic skin resistance increases and this is how a LIE is being detected. A polygraph instrument records the noticeable changes as the individual is questioned by a trained examiner. The examiner interprets the recordings and renders an opinion to the truthfulness of the person examined.


How does a Lie Detector/Polygraph work?

You have heard about lie detectors at the times of police investigations, and sometimes a person who is applying for a job will have to undergo a polygraph test (certain government jobs with the FBI or CIA require polygraph tests). The goal of a lie detector is to check if the person is telling the truth or lying when answering certain questions.

When a person takes a polygraph test, he is being attached to four to six sensors. A polygraph is a machine in which the multiple signals from the sensors are recorded on a single strip of moving paper (“graph”). 


What do the sensors usually record?

  • The person’s breathing rate
  • The person’s pulse
  • The person’s blood pressure
  • The person’s perspiration
  • A polygraph will also sometimes record things like arm and leg movement


When the polygraph test begins, the questioner asks three or four simple and easy questions to establish the normalcy of the person’s signals. Then the actual questions are asked. Throughout questioning, all of the person’s signals are recorded on the graph.

A polygraph examiner can look at the graphs both during and after the test and can see whether the principal signs changed significantly on any of the questions. In general, a noteworthy change (such as a faster heart rate, higher blood pressure, palpitation) indicates that the person is lying.

When a well-trained examiner uses a polygraph, he or she can accurately detect lying. However, a polygraph test is not always perfect and can be fooled because the examiner’s interpretation is subjective and different people react differently to lying.


How do Lie Detectors operate?

When a person is undergoing a polygraph test, the examiner begins by asking two types of trivial questions; questions which the person is expected to answer truthfully and questions which the person is expected to answer with a lie. This way, when the examiner of the test asks more relevant questions, later on, the subject’s physiological reactions can be compared with the reactions to the trivial questions to determine whether or not the subject is telling the truth.

If the trivial questions do not accurately show how the person is reacting when lying, it becomes difficult for the administrator to decide whether or not the person is lying when answering relevant questions. So, while the polygraph might be effective at measuring physiological factors that do not necessarily mean it will always be able to differentiate between a person lying and a person telling the truth.

The polygraph measures physiological factors that are associated not just with lying but also with being nervous—one might experience when being interrogated. Overall, it is important to consider the chance for minute errors when examining the results of a polygraph test, but it is possible to catch a person when he/she is lying. 


Significance & Practical Application

Polygraph testing has considerably created a scientific and public controversy. Most psychologists and other scientists have agreed that there is little basis for the validity of polygraph tests.

Polygraph tests are also sometimes used for seeking the convenience of the individual’s innocence from others and also by private agencies and corporations.

There is no evidence that any pattern of physiological reactions is unique to deception. An honest person might feel nervous when answering truthfully and a dishonest person may be non-anxious. Also, few good studies validate the ability of polygraph procedures to detect deception.

Early theorists believed that deception required effort and could be simply assessed by monitoring physiological changes. But such propositions have not been proven in years and basic research remains limited on the nature of deceptiveness. Efforts to develop actual tests have always surpassed theory-based basic research. The development of a lie detection technology seems highly problematic without a better theoretical understanding of the mechanisms by which deception functions.

Also published on Medium.

You may also like