The FBI’s listening device never went inside the phone booth. Agents taped it to the outside of the glass, recorded Charles Katz’s side of his calls without a warrant, and in 1967 the Supreme Court held that this still violated the Fourth Amendment. Katz v. United States, 389 U.S. 347, retired the old trespass rule and rebuilt search law around a new idea: because the Amendment “protects people, not places,” the requirement of a warrant travels with the surveillance, not the property.
The groundwork came a year earlier. In Warden v. Hayden the Court had already written that “The premise that property interests control the right of the government to search and seize has been discredited” and that the Fourth Amendment exists chiefly to protect privacy rather than property. Katz turned that logic into doctrine: no physical entry is required for a search.
What replaced trespass is the test every surveillance case now starts from. Justice Harlan, concurring, proposed a two-part inquiry: “first that a person have exhibited an actual (subjective) expectation of privacy and, second, that the expectation be one that society is prepared to recognize as ‘reasonable.’” The Court’s own gloss has aged just as well: “What a person knowingly exposes to the public, even in his own home or office, is not a subject of Fourth Amendment protection. But what he seeks to preserve as private, even in an area accessible to the public, may be constitutionally protected.”
Where The Line Actually Falls
The cases that followed drew it unevenly, and the pattern is strategic, not random. An official holding a shared office he neither owned nor leased had a protected expectation, the Court held in Mancusi v. DeForte, and an overnight guest in another person’s home was covered in Minnesota v. Olson. But a visitor who spent a few hours in an apartment bagging cocaine had no such claim in Minnesota v. Carter, and car passengers lost in Rakas v. Illinois for the glove compartment and the space under the seat. The closer the tie to a home, the stronger the claim; the more public the setting, the weaker.
Property did not vanish entirely. Soldal v. Cook County found a seizure when sheriff’s deputies helped haul off a mobile home during an eviction, regardless of any privacy interest, and United States v. Jacobsen let DEA agents reasonably examine a damaged package a private carrier had already opened. Privacy governs searches; possessory interests still govern seizures.
Technology Is The Pressure Point
The test’s hardest cases involve tools the Framers never imagined. Kyllo v. United States, decided in 2001, struck down the warrantless aim of a thermal imager at a home from a public street. The Court’s rule: new devices that can “shrink the realm of guaranteed privacy” amount to a search when they pull information about a home’s interior that no one could have gathered without physically entering, at least where the technology is not in general public use. Otherwise, the majority warned, the homeowner would be left at the mercy of advancing machines that could discern all human activity inside a house.
Carpenter v. United States carried the logic into the digital age in 2018, anchoring it in founding history. The Fourth Amendment “seeks to secure ‘the privacies of life’ against ‘arbitrary power,’” the Court wrote, quoting a nineteenth-century precedent, and “a central aim of the Framers was ‘to place obstacles in the way of a too permeating police surveillance.’”
What this signals for the current era is the same calculus in new hardware. The subjective half of Harlan’s test has largely withered — expectations, as a later opinion of his observed, largely mirror the laws and customs people live under — so everything turns on what society is prepared to call reasonable. That determination must draw on sources outside the Amendment itself: property concepts, or understandings society recognizes and permits. Which is to say, the line between surveillance and search will be drawn wherever the technology, and the public’s tolerance for it, happen to stand.
Sources
- Katz and the Adoption of the Reasonable Expectation of Privacy Test | U.S. Constitution Annotated | US Law | LII / Legal Information Institute
- Chatrie: Old Doctrine, New Surveillance — Secure Justice
