Note: This is an educational course, not legal advice.
The Fourth Amendment applies only to "searches and seizures." So the first question in any case is whether the government's action was a search at all. If it was not, the amendment does not apply, no matter how intrusive it feels. You met the basics in Civics: rights of the accused; the amendment binds states through incorporation.
Why it works
Katz changed the question from property to privacy. But some things a person exposes to the public or hands to others are not protected under that test: garbage left at the curb (California v. Greenwood, 1988), open fields, and, under the third-party doctrine, records held by banks (United States v. Miller, 1976) and numbers dialed through the phone company (Smith v. Maryland (1979)). Modern technology has pressed on these lines. Kyllo v. United States (2001) held that aiming a thermal imager at a home was a search. United States v. Jones (2012) held that attaching a GPS tracker to a car was a search under a revived trespass test: physically occupying private property to gather information. Then Carpenter v. United States (2018) held, 5 to 4, that getting a person's historical cell-site location records (here, months of data) is a search that generally requires a warrant; the Court declined to extend the third-party doctrine to such detailed records. The dissents argued that the records belonged to the phone company and that Congress was better placed to set rules.
| Test | Source | Question |
|---|---|---|
| Reasonable expectation of privacy | Katz (1967), Harlan concurrence | Did the person expect privacy, and is that expectation reasonable? |
| Trespass (property) | Jones (2012); also Florida v. Jardines (2013) | Did the government physically intrude on a person, house, papers or effects to gather information? |
| Third-party doctrine | Smith v. Maryland (1979) | Did the person voluntarily convey the information to a third party? (Not extended to cell-site records in Carpenter) |
Key idea
A search occurs when government violates a reasonable expectation of privacy (Katz) or physically intrudes on property to gather information (Jones). The third-party doctrine limits protection for shared records (Smith), but Carpenter (2018) required a warrant for historical cell-site location records.
Worked example: A drug dog at the door
Police bring a drug-sniffing dog onto a home's front porch, without a warrant, to sniff at the door. A search?
- The porch is part of the home's curtilage, a protected area.
- The officers physically entered it to gather information, beyond the implied invitation to knock and leave.
- Florida v. Jardines (2013) held this a search under the trespass test.
Answer: Yes, under the property-based test.
Worked example: Bank records
Police get a suspect's bank statements from the bank without a warrant. Under the third-party doctrine?
- The suspect gave the information to the bank.
- United States v. Miller (1976) held no reasonable expectation of privacy in such records.
- Carpenter limited the doctrine for cell-site location data but did not overrule Miller or Smith.
Answer: Not a search under current doctrine (federal statutes may still require process).
Common mistake
Thinking anything a person shares with a third party is never protected. That was the third-party doctrine (Smith v. Maryland), but Carpenter declined to extend it to cell-site location records, which reveal a detailed record of a person's movements.