Definition
An investigative technique that measures physiological responses (blood pressure, pulse, respiration, skin conductivity) while the subject answers questions — used to detect deception, but inadmissible as evidence in Indian courts and cannot be administered without consent.
The polygraph test measures involuntary physiological responses on the premise that lying causes measurable physiological changes. In India, the Supreme Court in Selvi v. State of Karnataka (2010) 7 SCC 263 definitively held that polygraph tests, narco-analysis, and brain electrical oscillation signature (BEOS/brain mapping) tests cannot be administered compulsorily — compelling a person to take these tests violates Article 20(3) (right against self-incrimination) and Article 21 (right to mental privacy). Even with consent, the results are not admissible as evidence in court — but any facts discovered as a consequence of information obtained during these tests may be admissible (similar to the discovery provision under Section 25 BSA).
Statutory Definition
No statutory authorisation for polygraph tests — their use is governed by Selvi v. State of Karnataka (2010) 7 SCC 263 (Supreme Court). The NHRC Guidelines on Administration of Polygraph Test (2000) provide procedural requirements including written informed consent, presence of lawyer, and the right to refuse. Article 20(3) Constitution: 'No person accused of any offence shall be compelled to be a witness against himself' — this is the primary constitutional bar against compelled polygraph tests.
Etymology & Origin
From Greek 'polys' (many) + 'graphein' (to write). A polygraph 'writes many things' simultaneously — it records multiple physiological responses on a chart while the subject answers questions. The 'lie detector' label is a misnomer — the test detects physiological arousal, not lies per se.
Full Legal Analysis
Polygraph Test: The Lie Detector That Courts Don’t Trust
The polygraph test is the criminal investigator’s dream and the courts’ nightmare. Investigators value it as an interrogation tool — it may help identify leads, narrow suspects, or induce confessions. Courts distrust it — the science is contested, the results depend on the examiner’s interpretation, and the constitutional rights of the accused stand against compelling it. The Supreme Court’s 2010 judgment in Selvi has firmly placed India in the anti-compelled-polygraph camp.
Selvi v. State of Karnataka (2010): The Definitive Ruling
In Selvi v. State of Karnataka (2010) 7 SCC 263, a Constitution Bench (5 judges) of the Supreme Court held: (a) Compelling an accused or witness to submit to a polygraph test violates Article 20(3) (self-incrimination) and Article 21 (personal liberty and mental privacy). (b) Even with the subject's consent, the results are not admissible as direct evidence in court. (c) Any information obtained during such tests is not admissible — but facts discovered as a direct consequence of the information (e.g., a weapon discovered at a location revealed during the test) are admissible under Section 25 BSA (the discovery provision). (d) Tests can only be conducted with explicit written informed consent, with the presence of the subject's lawyer, and with the right to withdraw consent at any time.
Scientific Reliability Concerns
The polygraph's scientific reliability is contested: (a) Physiological arousal is caused by anxiety, not lying specifically — an innocent but anxious person may fail the test. (b) Trained individuals (pathological liars, psychopaths, those with low autonomic reactivity) can defeat the test. (c) The examiner's interpretation of the physiological readings is highly subjective. (d) Meta-analyses have found accuracy rates between 61-89% — better than chance but far below the certainty required for court evidence. India's Supreme Court's conclusion that polygraph results are not admissible reflects these scientific limitations.
“A polygraph test measures anxiety and arousal — it does not detect lies. An innocent person who is frightened by the test environment may 'fail'; a practiced deceiver may 'pass.' Courts require evidence that is reliable and independently verifiable. The polygraph provides neither.” — Selvi v. State of Karnataka
This Term in Indian Statutes
Constitution of India, 1950
"No person accused of any offence shall be compelled to be a witness against himself."
Polygraph test cannot be compelled — Article 20(3) bars forced self-incrimination; Selvi case (2010) extended this to polygraph, narco-analysis and brain mapping
