Explained | What is a lie-detector test? How accurate is it?

The ongoing investigation of the bone-chilling Shradha Walkar murder has put the spotlight on the use of the specialised examination as an advanced tool for interrogation.

Narco test, polygraph test (lie-detector test), and brain mapping are some of the psychological tools that are used during the interrogation process.

While a lot of details of the Mehrauli murder case are coming out, the police have got permission to conduct a polygraph test on the accused Aftab Poonawala in order to fast forward the investigation.

Also Read | Mehrauli Murder: Aftab to undergo polygraph test

So what is a polygraph test?

The polygraph test, which is also known as the lie detector test, is a non-invasive technique in which no medicine is used. The test is conducted to determine whether the accused is telling the truth or lie.

How is the polygraph test conducted?

The subject is attached to a machine and questions are asked to him/her about the matter related to any case or incident. The variation in the graph when the subject answers the questions are mapped to draw a conclusion.

The person getting interrogated has to answer the questions with yes or no.

As per the American Psychological Association (APA), in a polygraph test a person's heart rate/blood pressure, respiration, and skin conductivity are measured using various instruments.

Also Read | Explained: What is a narco test?

To measure the changes a regular blood pressure cuff is attached to the person’s arm, electrodes are attached to his/her fingers and a pneumograph tube is wrapped around the chest and abdomen area, as per a research paper.

The research paper was published in jlsrjournal.in titled 'Polygraphy test and its legal admissibility in India' by Thota Raghavendra.

The paper also mentioned that to record the body movements a special chair is used.

How does the polygraph test determine if the accused is guilty?

Polygraph tests work on the principle of psychosomatic interaction.

The test measures minor physical changes in the person’s body to determine if they are telling the truth or not. If the person is lying then he will fear getting caught and the 'mental disturbances' caused by it will lead to psychological changes.

How accurate are polygraph tests?

The debate around the accuracy of polygraphy tests has been going on for a long time. Some claim that liars who are aware of how this test works can work their way around it and pass the test.

Researchers suggest that polygraphs are accurate between 80 per cent to 90 per cent of the time, as per a report by The Conversation. Meanwhile, humans can spot lie around 55 per cent o the time. Polygraph tests are not 100 per cent accurate but can be considerably more reliable than an average person's judgement.

Some believe that after failing the polygraph there is a great chance that the accused will confess to the crime.

What are the legal requirements to conduct a polygraph test?

Such tests can be done only if the accused undergoing it has given consent. The consent must be recorded before a Judicial Magistrate.

The police and lawyer need to provide the person with knowledge of the test and its legal implications.