Explained: Aaftab Poonawala Undergoes Polygraph Test, What's Polygraph Test And How It Works?

Polygraph test is often called lie detector test. It tracks a person's physiological responses when questions are asked to judge whether a person is lying.

Aaftab Poonawala

Shraddha Walkar murder accused Aaftab Poonawala on Monday underwent the remaining sessions of polygraph test. 

Aaftab is accused of murdering his live-in partner Shraddha. After killing her, he allegedly cut her body in 35 parts, stored them in his fridge, and disposed them in forests of Delhi in batches. 

There have reportedly been inconsistencies in Aaftab's statements to police so far and hence the investigators have turned to polygraph test. The polygraph test is also called lie-detector test.

The purpose of a polygraph test is to see whether a person is lying in their responses. 

Here we explain the development of polygraph tests, how it works, its accuracy, and its admissibility in court.

How polygraph test was developed

The polygraph test, commonly called the lie-detector test, has been used in investigations since 1924, according to The Britannica Encyclopaedia

The idea behind the test is that a change in person's physiological response while answering questions indicates whether the person is lying. The physiological responses monitored are:

  • Breathing rate
  • Pulse rate
  • Blood pressure
  • Sweating
  • Limb movement

These responses are monitored by electrodes and BP-measuring and pulse-measuring devices fitted on to the person. 

Traditionally, a pen would keep plotting the responses on a graph. Hence the name polygraph as 'poly' means multiple. The name literally means something that puts multiple things on a graph. These days, such data is recorded digitally. 

How are polygraph tests conducted?

Initially, the person undergoing the test is asked few simple question to set a baseline of sorts for the person's responses. Once the baseline is set, the real questioning starts.

Sudden spikes in physiological responses on certain questions indicate that the person might be lying. 

There are two main techniques for questioning in a polygraph test — Control Question Test (CQT) and Guilty Knowledge Test (GKT).

Control Question Test (CQT)

This technique divides the questions into two categories of 'relevant' and 'normal'.

'Relevant' questions are specific to the case but 'control' questions are broad in its scope and address past misdeeds, such as "Have you ever committed theft?".

"A person who is telling the truth is assumed to fear control questions more than relevant questions. This is because control questions are designed to arouse a subject's concern about their past truthfulness, while relevant questions ask about a crime they know they did not commit," says American Psychological Association (APA) while explaining CQT.

Guilty Knowledge Test (GKT)

This technique uses multiple-choice tests to seek answers to questions that only a guilty person would know.

"A test of a theft suspect might, for example, involve questions such as 'Was $500, $1,000, or $5,000 stolen?' If only a guilty suspect knows the correct answer, a larger physiological reaction to a correct choice would indicate deception," says APA.

This technique is not widely used.

How accurate are polygraph tests?

While physiological markers can indicate whether a person is lying, it's not a 'truth serum'. It can be inaccurate as well. 

The basic assumption of polygraph test is that a person's physiological responses, such as spike in sweating, breathing rate, of blood pressure, would indicate a person is lying. However, all of these responses are also seen when a person is nervous or stressed. Therefore, it's possible for an innocent person to have results that might indicate they are lying when they are not.


"An innocent person may fail the test out of pure nervousness. This is why these results are often dismissed in court...According to the National Research Council, there is evidence suggesting that lie detector results can be fabricated," notes explanatory website How Stuff Works.

The APA says, "Most psychologists agree that there is little evidence that polygraph tests can accurately detect lies."

The APA says that the popular term 'lie detector test' is also incorrect.

"So-called 'lie detection' involves 'inferring' deception through analysis of physiological responses to a structured, but unstandardized, series of questions," says APA.

While innocents can end up appearing to be guilty in polygraphs, the actual guilty persons can also cheat the test with proper training. This is applicable in high-stake fields like international espionage or top secret missions where agents are given training to evade interrogation and polygraphs. Techniques like biting your tongue to skew the graph readings and calming techniques like picturing calming imagery in your mind can help beat polygraphs.

Are polygraph tests admissible in court?

The courts have been cautious about the admitting polygraph test findings as evidence because of their unreliability and potential of fabrication.

The Supreme Court of India in 2010 in Smt. Selvi & Ors Vs State of Karnataka case said that methods like polygraph tests cannot be conducted on the accused without their consent.

Notably in an earlier judgement, Madras High Court in Dinesh Dalmia v. State said that the police must complete the investigation in a stipulated time to get the benefits of such tests. It also mentioned that when the accused is not cooperating with the investigating agency, these tests become the only resort.

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