The exclusionary rule is a rule of evidence which prevents introducing information at trial that directly or indirectly stems from illegal police investigation. The U.S. Supreme Court developed the rule to deter police from violating the Fourth Amendment right to be free from unreasonable searches and seizures.
By the early 1980s a majority of justices had stated a desire either to abolish the rule or to sharply curtail its operation, and numerous opinions had rejected all doctrinal bases save that of deterrence. The most severe curtailment of the rule came in 1984 with adoption of a ''good faith'' exception. In United States v. Leon, the Court created an exception for evidence obtained as a result of officers' objective, good-faith reliance on a warrant, later found to be defective, issued by a detached and neutral magistrate.