Any privacy advocate who has read the Supreme Court’s decision in Carpenter v. United States, which came down Friday, can see plenty in the decision to cheer about but also plenty to worry about. At a basic level, the case is about cell-site location information, which your telephone service may collect about the location of your mobile phone—and not so incidentally, about your location if, like most of us, you tend to keep your mobile phone close. As the Atlantic put it just a few years ago:

Right now, many Americans generate a detailed database of their whereabouts over time—as they move throughout the day from their workplace to their doctor, from their own house to their partner’s—that they do not see or control.

That location data can stretch back weeks, months, and even years. And until last week, government agencies could seek your cell-site location information without obtaining a warrant for it (that is, without proving to a judge that they have probable cause to believe that information will provide evidence related to a crime).

On a deeper level, the 5–4 decision in Carpenter has reinvigorated the question of whether our personal data, when entrusted to the companies that provide services to us, fall outside the Fourth Amendment’s protection (so that government agencies don’t need a warrant to obtain that data). Specifically, it has challenged what used to seem like a black-and-white rule that, when the government is seeking information you voluntarily give to third parties, the Fourth Amendment doesn’t require the government to seek a search warrant: Government agents can use less-demanding procedures instead. (This is commonly referred to as the third-party doctrine.)

Still, the 5–4 vote was narrow. Chief Justice Roberts wrote the majority opinion, which was joined by the court’s most civil liberties–oriented justices (Ginsburg, Breyer, Sotomayor, Kagan). The four remaining (and more law enforcement–friendly) associate justices (Kennedy, Thomas, Alito, Gorsuch) each wrote a separate opinion that challenged not merely the outcome in Carpenter, but also the Fourth Amendment framework that grew out of a 51-year-old court case called Katz v. United States. That’s disturbing to Fourth Amendment fans, because it seems entirely possible that the composition of the court will change significantly in the next few years, especially if President Trump is re-elected in 2020.

Katz gave us the basis for the entire statutory framework for lawful wiretapping by the government (you have to get a court order, generally, to wiretap), and also for more modern communications issues like email privacy. (I’ve written about Katz and email privacy issues here.) So it’s troubling that among the conservative justices there is some sentiment to roll back Katz either partially or completely, and to limit the Fourth Amendment’s scope to some version of property rights or the law of trespass.

The Katz case also gave us, a half-century ago, the idea that to determine whether the Fourth Amendment applies outside one’s home or possessions (like your car or your backpack), courts must consider whether you have “a reasonable expectation of privacy.” (If you do, then the Fourth generally applies.) It’s a phrase that has haunted Fourth Amendment jurisprudence all these decades, partly because no one is entirely sure what it means in different fact situations raised by different cases, but also because it wasn’t actually part of the original majority opinion. In 1967 the eight-justice court (Justice Thurgood Marshall recused himself because he’d been involved with the case as solicitor general under President Lyndon Johnson) voted 7–1 in Katz, concluding that Charles Katz’s Fourth Amendment rights had been violated when the FBI planted electronic listening devices in the public phone booths he used place illegal bets in violation of federal law.

At the time this was a novel theory—the Fourth Amendment requires that “persons, houses, papers, and effects” be “secure” against “unreasonable searches and seizures.” The amendment also specifies a process for the issuance of search-and-seizure warrants that’s still with us today. But Katz himself wasn’t personally searched or arrested, and neither were his house, papers, and effects: Only his conversations had been recorded. Katz didn’t own the phone booths—he didn’t have a property interest that was violated. But the Supreme Court reasoned five decades ago that a public telephone booth, once you closed its door, gave you the same right to expect privacy that you’d have in, say, a hotel room you rented. And a line of pre-Katz cases had been expanding on the idea that maybe recording your conversations through increasingly advanced wiretap and recording technologies was some kind of search after all. Ultimately, the majority opinion put the issue this way:

[T]he Fourth Amendment protects people, not places. 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.

Of the seven justices who voted that year in favor of recognizing that Katz had Fourth Amendment rights implicated by the bugging of his phone booth, four (including the author, Justice Potter Stewart) simply backed the majority opinion outright, but three filed separate concurring opinions. It was Justice John Marshall Harlan’s concurring opinion that first surfaced the idea that a “reasonable expectation of privacy” ought to be the standard of whether the Fourth Amendment applies. That “reasonable expectation” test he proposed would have two parts—first, did the person in question have a subjective expectation of privacy regarding the matters subject to search or seizure, and second, was it an expectation that society would take to be “reasonable.”

Harlan’s proposal would become the tail that wagged the dog—in two subsequent cases, the Supreme Court narrowed the Fourth Amendment by applying the third-party doctrine. In United States v. Miller (1976), the court concluded that bank records created no “expectation of privacy,” and in Smith v. Maryland (1979), it reached the same conclusion regarding phone-call records. In those two cases, Harlan’s notion of “reasonable expectation of privacy” moved front and center to Fourth Amendment jurisprudence.

The “reasonable expectation of privacy” test has been less than entirely successful in putting the brakes on the expanding scope of government searches. The “subjective” part of the “reasonable expectation” test has more or less withered way, and as for the “objective” part—what society is willing to recognize as reasonable—well, who among us who’s aware of the flood of digital data we generate about ourselves every day really believes a reasonable citizen should expect not to be trackable digitally and searchable digitally?

This isn’t to say the “reasonable expectation” test has lost all currency. As Mark Joseph Stern notes, Justice Sonia Sotomayor’s use of it in her concurrence in the 2012 decision United States v. Jones provided some key reasoning for Roberts’ opinion in Carpenter. Chief Justice John Roberts tries to leverage Sotomayor’s concurrence when he writes this (citing Jones): “A majority of this Court has already recognized that individuals have a reasonable expectation of privacy in the whole of their physical movements.”

The problem for the court’s majority in Carpenter is that the dissenting justices, all of whom can chapter-and-verse cite all the cases drawing on the Katzdecision’s “reasonable expectation of privacy,” and on the “third-party doctrine” of the Miller and Smith decisions. The dissenters are fully aware of the shifting sands under the first standard and the logical implications of the second. Justice Anthony Kennedy thinks third-party doctrine more or less entails that cell-site location information lacks strong Fourth Amendment protections (because the information is held by the phone company, and not by you, and it’s not your property, beside). Justice Clarence Thomas would abandon Katz altogether because, well, the language of the Fourth Amendment and the Constitution as a whole don’t recognize a privacy interest at all, much less a “reasonable expectation” of it. And Justice Samuel Alito argues that a lawful subpoena has plenty of restrictions on its scope already—there may be Fourth Amendment interests involved, but the subpoena used in Carpenter to obtain cell-site location information doesn’t require a showing of probable cause the way a search warrant does because “no search or seizure of Carpenter’s home or his property occurred in this case.”

Justice Neil Gorsuch’s dissent is published last, and in some ways it’s the best of the dissents. Gorsuch builds on Kennedy’s reading of the third-party doctrine and on Thomas’ skepticism about whether “reasonable expectation of privacy” has any constitutional roots at all. Gorsuch asks a number of questions about both doctrines that make clear he’s aware of the Fourth Amendment interests that maybe should be implicated by cloud services and other modern third-party services:

Can the government demand a copy of all your e-mails from Google or Microsoft without implicating your Fourth Amendment rights? Can it secure your DNA from 23andMe without a warrant or probable cause? Smith and Miller say yes it can—at least without running afoul of Katz. But that result strikes most lawyers and judges today—me included—as pretty unlikely.

So much for the third-party doctrine. And Gorsuch is equally critical of the “reasonable expectation” test, which he says has “yielded an often unpredictable—and sometimes unbelievable—jurisprudence”:

Smith and Miller are only two examples; there are many others. Take Florida v. Riley, 488 U. S. 445 (1989), which says that a police helicopter hovering 400 feet above a person’s property invades no reasonable expectation of privacy. Try that one out on your neighbors.

Gorsuch doesn’t mention drones and their potential to intrude on privacy, but it’s pretty clear he’s thinking about them.

More than the other dissenters—and to a certain degree, he’s more like the court’s majority in this case—Gorsuch is looking toward the future as well as the precedential past. Still, like the other dissenting justices, Gorsuch would like to tie the Fourth Amendment back to property interests, and he’d amend or replace third-party doctrine with some kind of property law (he invokes the law of bailments—your rights regarding your property when it’s in the hands of others) with new statutory privacy law, which Congress could theoretically develop. (As he must, Gorsuch assumes Congress may someday become more functional than it is just this moment.)

But all the justices seem to be overlooking yet another way to sort out “reasonable expectation of privacy” and the third-party doctrine—and that’s the path of going back to not to what Justice Harlan proposed in his concurrence about “reasonable expectations” but what the majority opinion in Katz v. United States actually said: The Fourth Amendment protects “people, not places.” And no matter how much some jurists want to underscore that “reasonable expectation of privacy” has no direct textual roots in the Constitution, no one can deny that the Fourth Amendment’s own text protects “[t]he right of the people to be secure in their persons, houses, papers, and effects,against unreasonable searches and seizures.”

Security “in their persons” is arguably the real heart of the Fourth Amendment interests raised by cases like Carpenter—not ownership of property. When your phone company gathers location information about your phone (and necessarily, about you, the subscriber who’s holding that phone), the issue isn’t whether they’re searching or seizing your house, papers, or effects. Instead, the issue is whether they’re searching (or in some sense, seizing) you. Roberts, who through role and temperament seems predisposed to be an incrementalist focused on precedent, took pains to insist that Carpenter is “narrow,” but he also concluded that cell-site location information is different in character from the list of dialed numbers in Smith or the banking records in Miller. “[W]hen Smith was decided in 1979,” he writes, “few could have imagined a society in which a phone goes wherever its owner goes, conveying to the wireless carrier not just dialed digits, but a detailed and comprehensive record of the person’s movements.” Third-party doctrine doesn’t apply because the issue isn’t whether the phone company has your documents—their records are not your papers or effects.

Instead, the issue is whether the phone company has, in some sense, searched you as a person, or is enabling the government to search you personally by searching everywhere you’ve been over weeks or months or years. That’s why I hope that all the justices, whether conservative or liberal, find common ground in seeing past third-party doctrine, or bailments, or the blurriness of “reasonable expectations” to remember that the Fourth Amendment protects not just property, and not just places like the home. Fundamentally, it’s actual individual human beings whose autonomy and liberty—and not just their property—is obviously, indisputably, and directly the focus of the Bill of Rights.

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