Fourth Amendment

The Fourth Amendment is part of the U.S. Bill of Rights, ratified in 1791. Its text bars "unreasonable searches and seizures" and provides that "no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized." It was a direct response to British use of general warrants and writs of assistance in the American colonies, which had allowed officers broad, suspicionless searches of homes and businesses.

In modern practice, the amendment governs how federal, state, and local officials may investigate crime, conduct surveillance, and detain individuals. Two questions typically drive Fourth Amendment analysis: (1) did a search or seizure occur, and (2) was it reasonable? Since Katz v. United States (1967), a search occurs when the government intrudes on a "reasonable expectation of privacy." Terry v. Ohio (1968) established that brief investigative stops and frisks require only reasonable suspicion, a lower standard than probable cause. Mapp v. Ohio (1961) applied the exclusionary rule to the states, meaning evidence obtained in violation of the amendment generally cannot be used at trial.

Key contemporary battlegrounds include:

• Digital privacy — Riley v. California (2014) required a warrant to search a cell phone seized during arrest, and Carpenter v. United States (2018) required a warrant for historical cell-site location data.
• Border and airport searches, where courts have recognized broad exceptions.
• Use of drones, license-plate readers, and third-party data, where doctrine is still developing.

For international relations researchers, the Fourth Amendment is also relevant to debates over foreign-intelligence surveillance under FISA, NSA bulk-collection programs disclosed in 2013, and how U.S. privacy standards compare with the EU's Charter of Fundamental Rights and the European Convention on Human Rights.