Reasonable Expectation of Privacy

Edition label: 2026-10

Definition: The two-prong Fourth Amendment test from Justice Harlan’s Katz concurrence: a person enjoys constitutional protection where (1) they exhibit an actual subjective expectation of privacy, and (2) the expectation is one that society is prepared to recognize as reasonable.¹

Overview

Articulated in Harlan’s concurring opinion in Katz v. United States (1967), the two-prong reasonable-expectation inquiry added a privacy-based framework to Fourth Amendment doctrine.² It did not displace the property/trespass test: United States v. Jones (2012) confirms that a physical trespass to obtain information remains an independent route to finding a search.³

The test’s structural ambiguity — what counts as a “reasonable” expectation, and whose perspective on “society” decides — has been a central interpretive question in later surveillance cases. Smith v. Maryland (1979) applied the third-party doctrine, first set out for bank records in United States v. Miller (1976), to dialed telephone numbers: information voluntarily disclosed to a third party (e.g., the phone company) carries no reasonable expectation of privacy,⁴ opening the constitutional space the post-9/11 Section 215 bulk-telephony program exploited.⁵ United States v. Jones (2012) confirmed that trespass remains an independent route,⁶ and Carpenter v. United States (2018) partially limited the third-party doctrine for cell-site location information.⁷

Analytical significance

The reasonable-expectation test matters to both judicial review and public oversight of classified surveillance. The Privacy and Civil Liberties Oversight Board (PCLOB) Section 215 report addresses the government’s and the Foreign Intelligence Surveillance Act (FISA) court’s view that under Smith v. Maryland there is no reasonable expectation of privacy in dialed numbers; the Wyden-Udall correspondence addressed the secret legal interpretation of Section 215 itself.⁸ The third-party doctrine — set out in United States v. Miller (1976) and applied to dialed numbers in Smith v. Maryland (1979) — is the doctrinal hinge.⁹

Related Entities

• John Marshall Harlan II — author of the operative concurrence

Related Concepts

• fourth-amendment-doctrine
• trespass-doctrine — the doctrine Katz displaced as the exclusive test; Jones (2012) confirmed that trespass remains an independent route¹⁰
• surveillance-doctrine
• foreign-intelligence-surveillance

Source References

• Supreme Court of the United States. Katz v. United States, 389 U.S. 347 (1967). Decided December 18, 1967. — origin of the two-prong test¹¹
• Supreme Court of the United States. United States v. United States District Court for the Eastern District of Michigan (Keith), 407 U.S. 297 (1972). Decided June 19, 1972. — applied the Katz framework to domestic-security wiretaps¹²
• Administration White Paper. Bulk Collection of Telephony Metadata Under Section 215 of the USA PATRIOT Act. August 9, 2013.
• Carpenter v. United States, 585 U.S. 296 (2018) (slip opinion, No. 16-402).
• Privacy and Civil Liberties Oversight Board. Report on the Surveillance Program Operated Pursuant to Section 702 of the Foreign Intelligence Surveillance Act. Washington, DC, September 28, 2023.
• Privacy and Civil Liberties Oversight Board. Report on the Telephone Records Program Conducted under Section 215 of the USA PATRIOT Act and on the Operations of the Foreign Intelligence Surveillance Court. Washington, DC, January 23, 2014.
• Smith v. Maryland, 442 U.S. 735 (1979).
• United States v. Jones, 565 U.S. 400 (2012).
• Wyden, Ron, and Mark Udall. Letters to Eric Holder, Attorney General of the United States. U.S. Senate, September 21, 2011, and March 15, 2012.

Open Questions

• Question: How did the post-Snowden litigation (Klayman, American Civil Liberties Union (ACLU) v. Clapper, the post-Carpenter cases) treat the third-party doctrine of Miller and Smith in relation to bulk-collection? Given PCLOB’s account of the broad third-party doctrine¹³ and Chair Franklin’s description of Carpenter’s cell-site-location holding,¹⁴ how do those principles apply to bulk-telephony metadata?