Fourth Amendment to the United States Constitution

The Fourth Amendment (Amendment IV) to the United States Constitution prohibits unreasonable searches and seizures and requires any search warrant to be judicially sanctioned and supported by probable cause. It is part of the Bill of Rights. The Fourth amendment was adopted in response to the abuse of the writ of assistance, a type of general search warrant issued by the British government. It was a major source of tension in pre-Revolutionary America. The Fourth Amendment was introduced in Congress in 1789 by James Madison, along with the other amendments in the Bill of Rights. They were proposed in response to Anti-Federalist objections to the new Constitution.

Congress sent 12 amendments to the states in August of 1789. Of these, 10 were approved by the states. The last state, Virginia ratified the amendments (including the fourth amendment) on December 15, 1791. On March 1, 1792, Secretary of State Thomas Jefferson announced the adoption of the amendment.

The Bill of Rights did initially apply to the states. Also, federal criminal investigations were less common in the first century of the nation's history. For these reasons there is little case law for the Fourth Amendment before the 20th century. The amendment was held to apply to the states in Mapp v. Ohio (1961).

Under the Fourth Amendment, search and seizure (including arrest) should be limited in scope to specific information supplied to the issuing court. This is usually by a law enforcement officer who has sworn by it. Fourth Amendment case law deals with three central questions. What government activities constitute "search" and "seizure"? What constitutes probable cause for these actions? How should violations of Fourth Amendment rights be addressed? Early court decisions limited the amendment's scope to a law enforcement officer's physical intrusion onto private property. But with Katz v. United States (1967), the Supreme Court held that its protections, such as the warrant requirement, extend to the privacy of individuals as well as physical locations. Law enforcement officers need a warrant for most search and seizure activities. But the Court has defined a series of exceptions for consent searches, motor vehicle searches, evidence in plain view, exigent circumstances, border searches, and other situations.

The exclusionary rule is one way the amendment is enforced. Established in Weeks v. United States (1914), this rule holds that evidence obtained through a Fourth Amendment violation is generally inadmissible at criminal trials. Evidence discovered as a later result of an illegal search may also be inadmissible as "fruit of the poisonous tree," unless it inevitably would have been discovered by legal means.