In his influential dissent in Olmstead v. United States, Supreme Court Justice Brandeis wrote:

[E]very unjustifiable intrusion by the Government upon the privacy of the individual, whatever the means employed, must be deemed a violation of the Fourth Amendment.

Brandeis was the first in a decades-long line of justices who rejected the idea that whether a “search” has occurred depends on whether there was a physical trespass onto something protected by the Fourth Amendment (our “persons, houses, papers, and effects”).

The trend culminated in what is now known as the “reasonable expectation of privacy” test from Katz v. United States. As of Katz, a Fourth Amendment search takes place whenever the government violates an actual expectation of privacy that society is prepared to recognize as reasonable, whether or not there is a physical trespass.

While the doctrine at first seemed to offer us more privacy protection than we had under the “trespass doctrine,” things quickly changed. Twelve years after Katz, the Supreme Court decided Smith v. Maryland, a case involving the warrantless installation of a pen register — a device that collects telephone “metadata,” such as phone numbers dialed, and perhaps time and length of phone calls. The Court held that installation of a pen register is not a search, because the data being collected by the device is data that the customer has shared with the phone company, a “third party.” Once one has shared data with a third party for any purpose, the Court held, one no longer has a “reasonable expectation of privacy” in it.

And so the “reasonable expectation of privacy” test, once believed to be privacy’s greatest ally, quickly became its nemesis. Legal protection of information we share with a third party is now given only by the grace of statute. Today, more and more of what we do involves sharing information via third parties, either online or via cellular network. In this context the third-party doctrine, combined with policies of liberal data sharing among federal agencies, has transported us into a virtual Benthamite panopticon.

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