In Selvi v. State of Karnataka (2010), a three-judge Supreme Court bench held that polygraph, narcoanalysis, and brain mapping tests cannot be administered without the subject's explicit consent. Forced administration violates Article 20(3) — protection against self-incrimination — and Article 21 — right to life and personal liberty. Voluntary results are admissible as corroborative evidence only. The National Human Rights Commission had earlier issued guidelines in 2000 restricting these tests.

Polygraph measures physiological responses (blood pressure, respiration) to detect deception. Narcoanalysis involves injecting sodium pentothal ('truth serum') to induce a semi-conscious state where subjects are less inhibited. Brain mapping (BEAP — Brain Electrical Activation Profile) maps neural responses to crime-related stimuli to detect concealed knowledge. All three were widely used by Indian investigative agencies before the Supreme Court's intervention.

The Selvi judgment is constitutionally pivotal for three reasons. First, it expands Article 20(3) beyond formal legal proceedings to cover any compelled self-incrimination — closing the loophole that agencies used to conduct tests outside courtrooms. Second, it roots bodily and mental autonomy within Article 21, establishing that the right to privacy includes the right not to have one's mind involuntarily probed. Third, it distinguishes between testimonial acts (protected) and physical evidence like fingerprints or blood samples (not protected), drawing a line that directly informs subsequent privacy jurisprudence including Justice Puttaswamy (2017).

For CLAT PG candidates, this case is an anchor judgment: examiners extract passage-based hypotheticals where a character is subjected to narcoanalysis without consent and ask whether a constitutional violation has occurred, which article is breached, and whether the evidence gathered would be admissible. The consent-admissibility distinction is the most-tested fault line.