The Supreme Court judgment in Selvi
v. State of Karnataka (2010) dealt with the admissibility of expert opinion evidence, specifically regarding scientific tests like polygraph examinations. The Court established principles applicable under what is now Section 39-45 of the Bharatiya Sakshya Adhiniyam. Which principle did the Court establish regarding such scientific evidence?

Selvi v. State of Karnataka established critical principles regarding scientific evidence under Section 39-45 of BSA (and previously IEA). The Court held that while expert opinions on matters of science are admissible, the results of intrusive scientific tests (polygraph, narco-analysis, brain-mapping) require explicit informed consent and cannot violate the right against self-incrimination. More importantly, results of such tests are admissible as corroborative evidence to corroborate other direct evidence, but cannot form the primary or sole basis of conviction. This principle balances Section 39 (admissibility of expert opinion) with Constitutional protections and fair trial rights. The judgment clarified that "opinion evidence" differs from conclusive proof—expert opinions inform the court but do not displace the burden of proof standards under Section 104 of BSA, and guilt must still be proved beyond reasonable doubt through the lens of Section 2(j) of BSA ("proved").