How a Lie Detector (Polygraph) Works

A lie detector, or polygraph, measures physiological responses like heart rate, blood pressure, respiration, and skin conductivity to detect deception based on stress-induced changes.

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Context: The Mumbai Police’s Economic Offences Wing (EOW) conducted a polygraph test on Hitesh Mehta, the prime accused and former General Manager of New India Cooperative Bank.

Working Principle of a Lie Detector/Polygraph

A polygraph records changes in bodily functions, which are believed to occur due to the emotional response to lying.
The key parameters measured include: Blood pressure, Heartbeat (pulse rate), Respiration (breathing rate), Perspiration (sweat levels, also known as electrodermal response or psychogalvanic skin reflex), & Electrical impulses in the body.
The modern polygraph was invented in 1921 by John Larson, a medical student at the University of California, in collaboration with a police officer.

Equipment Used in a Polygraph Test

Pneumograph Tube – Wrapped around the chest to measure breathing rate.
Blood Pressure-Pulse Cuff – Strapped to the arm to track pulse and blood pressure changes.
Electrodes – Pick up electrical impulses from different body parts.
Moving Graph Paper – Records changes in physiological functions when the subject answers questions.

How the Polygraph Detects Lies

The examiner asks the subject a series of questions to the subject.
The polygraph records baseline physiological responses when neutral questions are asked.
When a subject lies, one or more body functions deviate from normal, leading to changes in the recorded graph.
The examiner analyzes these deviations to infer whether the person is lying.

Accuracy and Reliability of Polygraphs

Although widely used in interrogations, polygraphs are not scientifically proven to detect lies accurately.
Many researchers argue that physiological responses can be influenced by factors other than lying, such as stress, anxiety, or nervousness.
Polygraphs are not accepted as conclusive evidence in courtrooms due to their unreliability.
Some individuals can train themselves to manipulate their physiological responses, making it easy to deceive the polygraph.

Legal Admissibility of Polygraph Tests in India

The Supreme Court of India Selvi & Ors vs State of Karnataka & Anr (2010) ruled that polygraph tests cannot be forced on any individual.
The court emphasized that such tests violate human rights, including the right against self-incrimination under Article 20(3) of the Constitution.
Key legal points from the ruling:
- Results of polygraph tests are NOT considered confessions.
- Statements made during the test cannot be used as direct evidence.
- However, evidence discovered as a result of these tests can be admissible.
- Example: If the accused reveals the location of a murder weapon during the test, the weapon itself can be used as evidence, but the statement revealing the location cannot.