The third party doctrine is a key Fourth Amendment principle established by the U.S. Supreme Court in United States v. Miller, 425 U.S. 435 (1976). In this case, the Court held that the defendant did not have a Fourth Amendment expectation of privacy in bank records obtained by the government with subpoenas. The Court reasoned that there is no reasonable expectation of privacy in the banking documents because they were “not confidential communications but negotiable instruments to be used in commercial transactions” and the other “documents obtained contain only information conveyed to the banks and exposed to their employees in the ordinary course of business.” In holding that the Fourth Amendment did not protect these documents, the Court relied on the principle behind the third party doctrine; that the Fourth Amendment does not prohibit the government from obtaining information that is voluntarily revealed to a third party.
Three years later, the Supreme Court revisited the third party doctrine in Smith v. Maryland, 442 U.S. 735 (1979). At issue in this case was whether the installation and use of a pen register (device used to record phone numbers from outgoing calls) at the defendant’s home constituted a Fourth Amendment search. Applying Miller, the Court held that there was no reasonable expectation of privacy in the phone numbers that someone calls. The Court reasoned that a customer of a telephone company knows that the phone numbers of outgoing calls are collected by the phone company for billing purposes. Thus, the customer “voluntarily conveyed” the information to a third party and assumed the risk that the third party would reveal the information to law enforcement.
The Supreme Court addressed the third party doctrine's applicability to the collection of historical cell-site location information (CSLI) in Carpenter v. United States, 585 U.S. 296 (2018). In this case, the Court refused to extend the third party doctrine to CSLI for two reasons:
- First, the Court rejected an automatic application of the third party doctrine.
- In both Miller and Smith, the Court relied not only on voluntary disclosure but also on the nature of the information at issue. In Miller, the checks were “not confidential communications” and the other documents were exposed to bank employees during the ordinary course of business. In Smith, the Court considered the limitations of pen registers and the narrow scope of some information in call logs. By contrast, CSLI can provide a “detailed chronicle of a person’s physical presence compiled every day, every moment, over several years.” Thus, the privacy interests at stake in Carpenter greatly exceeded those in Miller and Smith.
- Second, the third party doctrine’s “voluntary disclosure” rationale did not apply to CSLI.
- The majority rejected the view that users voluntarily share CSLI. As the Court explained, modern life creates a constant need and expectation to carry a cell phone. A user does not need to take any affirmative steps in order to transmit CSLI beyond turning the phone on, as “any activity on the phone generates CSLI, including incoming calls, texts, or e-mails and countless other data connections that a phone automatically makes.” Because users cannot avoid CSLI collection without disconnecting from the cellular network, the defendant in Carpenter did not voluntarily share that information with the carrier and thus did not assume the risk that law enforcement would obtain it.
In 2026, the U.S. Supreme Court decided in Chatrie v. United States, 609 U.S., that the police did in conduct a Fourth Amendment search with the execution of a geofence warrant, in violation of the Fourth Amendment. In this case, the government argued for the application of the third party doctrine and the petitioner argued against its application. The Court ultimately decided that the reasonableness standard applied when law enforcement sought this data from Google because individuals do have a reasonable expectation of privacy with their location data.
[Last reviewed in August of 2026 by the Wex Definitions Team]