AP® United States Government and Politics · Unit 3 · Topic 3.6
The Fourth Amendment and Digital Surveillance
The Fourth Amendment bars unreasonable searches, and courts decide what counts as a search by asking whether a person had a reasonable expectation of privacy. Digital data, especially location data, is stretching that old test.
The Fourth Amendment protects against unreasonable searches and seizures, and normally the government needs a warrant, based on probable cause, before searching. But courts first have to decide whether something counts as a search at all. The test comes from Katz v. United States (1967): a search happens when the government violates a reasonable expectation of privacy, meaning both that the person expected privacy and that society sees that expectation as legitimate. Content of a call or letter gets strong protection, because you clearly expect what you say to stay private. Metadata, like which numbers you called and when, got treated differently for decades under the third-party doctrine, the idea that once you hand information to a company like a phone carrier, you've given up your privacy claim to it.
Metadata sounds harmless, just numbers and times, but stacked together it can reveal who your doctor is, when you visit a lawyer, who you call at 2 a.m. That gap between what metadata seems to show and what it actually exposes is the center of modern Fourth Amendment fights. The sharpest version of the problem is cell phone location tracking, where towers log your phone's position continuously, building a complete map of your movements over weeks or months.
In Carpenter v. United States (2018), the Supreme Court ruled that police need a warrant to get this kind of long-term cell-site location data, even though a company technically holds it. The Court reasoned that comprehensive location tracking reveals so much about a person's private life that handing data to a third party doesn't erase the privacy interest in it. This case connects Unit 3's bigger theme: old doctrines built for mail and telephones keep getting retested against technology that collects more, remembers longer, and reveals more than anyone imagined when the rule was written.
Key terms
- Reasonable expectation of privacy
- The Katz test asking whether a person expected privacy and whether society accepts that expectation as legitimate.
- Metadata
- Data about communication, like numbers called and timing, rather than the content of the communication itself.
- Third-party doctrine
- The idea that sharing information with a company or person can reduce or eliminate a person's privacy claim to it.
- Carpenter v. United States
- A 2018 case ruling that police need a warrant for long-term cell-site location tracking data.
- Probable cause
- The evidentiary standard required to justify a warrant or a search.
Watch it explained
- How it worksWalk me through what metadata reveals about a person without anybody reading a message.
- How it worksHow does a reasonable expectation of privacy decide whether something counts as a search?
- Going furtherWhat happens to Fourth Amendment doctrine when a phone tracks your location all day?
- Common mistakeWhy isn't handing data to a company the same as giving up all privacy in it?
Check yourself
End of topic.