What is a polygraph test, which police wants Aaftab Poonawala to undergo?

How are polygraph tests administered, and how do they help in an investigation? We explain.

How Polygraph tests work

A polygraph test is based on the assumption that physiological responses (heartbeat, changes in breathing, sweating, etc.) triggered when a person is lying are different from what they would be otherwise. Instruments like cardio-cuffs or sensitive electrodes are attached to the person, and variables such as blood pressure, pulse, blood flow, etc., are measured as questions are put to them. A numerical value is assigned to each response to conclude whether the person is telling the truth, is deceiving, or is uncertain.

A test such as this is said to have been first done in the 19th century by the Italian criminologist Cesare Lombroso, who used a machine to measure changes in the blood pressure of criminal suspects during interrogation. Similar devices were subsequently created by the American psychologist William Marstron in 1914, and by the California police officer John Larson in 1921.

Are such tests accurate?

Neither polygraph tests nor narco tests have been proven scientifically to have a 100% success rate, and remain contentious in the medical field as well.

However, recently, investigating agencies have sought to employ these tests in investigation, and they are sometimes seen as being a “softer alternative” to torture or ‘third degree’ to extract the truth from suspects.

So are the results of these tests admissible as evidence?

The results of the tests cannot be considered to be “confessions”. However, any information or material subsequently discovered with the help of such a voluntarily-taken test can be admitted as evidence, the Supreme Court said, in ‘Selvi & Ors vs State of Karnataka & Anr’ (2010).

Thus, if an accused reveals the location of a murder weapon in the course of the test, and police later find the weapon at that location, the statement of the accused will not be evidence, but the weapon will be.

The Bench took into consideration international norms on human rights, the right to a fair trial, and the right against self-incrimination under Article 20(3) of the Constitution, as it is feared that a false confession could be obtained at times when a case needs to be solved quickly.

“We must recognise that a forcible intrusion into a person’s mental processes is also an affront to human dignity and liberty, often with grave and long-lasting consequences,” the court said, observing that the state’s plea that the use of such scientific techniques would reduce ‘third degree’ methods “is a circular line of reasoning since one form of improper behaviour is sought to be replaced by another”.

Can these tests be administered to anyone?

Some conditions need to be satisfied. The Supreme Court Bench comprising then Chief Justice of India K G Balakrishnan and Justices R V Raveendran and J M Panchal in the 2010 case ruled that no lie detector tests should be administered “except on the basis of consent of the accused”.

Those who volunteer must have access to a lawyer, and have the physical, emotional, and legal implications of the test explained to them by police and the lawyer, the Bench said.

It said that the ‘Guidelines for the Administration of Polygraph Test on an Accused’ published by the National Human Rights Commission in 2000, must be strictly followed. The subject’s consent should be recorded before a judicial magistrate, the court said.