The exclusionary rule is a judge‐made rule that evidence obtained by the government in violation of a defendant's constitutional rights can't be used against him or her. By filing a motion to suppress before the trial asking the judge to rule the evidence as inadmissible, a defendant may prevent the prosecution from using illegally obtained evidence. The exclusionary rule usually applies to suppression of physical evidence (for example, a murder weapon, stolen property, or illegal drugs) that the police seize in violation of a defendant's Fourth Amendment right not to be subjected to unreasonable search and seizure.
Weeks v. U.S. (1914)
The exclusionary rule was invented in Weeks v. U.S. Weeks is premised on the idea that when the police exceed their constitutional authority in conducting a search, then that search must be null and void. At the time of Weeks, the Bill of Rights was considered to apply only to the federal government.
Mapp v. Ohio (1961)
In Mapp, the liberal Warren Court extended the Weeks exclusionary rule to state courts. The Warren Court held that the exclusionary rule is part of a citizen's Fourth Amendment right and that the rule was needed because the states had not devised any effective remedies to the problem of arbitrary searches by police. Some police administrators and politicians denounced Mapp for handcuffing the police.
Erosion of Mapp
Lack of support for the rule among conservative U.S. Supreme Court justices who succeeded the liberal members of the Warren Court to the bench has limited the rule's impact. In a series of cases, the Court held that illegally obtained evidence could be used as the basis for grand jury questions, by the Internal Revenue Service in a civil tax proceeding, and in deportation hearings. In U.S. v. Leon (1984), the Court carved out the good faith exception: if the police make an honest mistake in conducting a search—that is, if the police act on the basis of a search warrant which a court later declares invalid—the seized evidence is still admissible.