abstract. For decades, Fourth Amendment protections have turned on “reasonable expectations of privacy.” But a new era may be dawning. There is growing interest among judges and scholars in turning away from privacy toward property or positive law as the touchstone for Fourth Amendment protections. Yet many questions remain about how that approach should work, such as where judges should look for positive law and precisely what role positive law should play in Fourth Amendment analysis.

This Article answers those questions, and in so doing lays forth a new, comprehensive theory of the Fourth Amendment. We argue that courts should interpret the Fourth Amendment’s protections by looking to “general law”—the common law under the control of no particular sovereign. Courts looking to general law would draw on ancient property concepts such as trespass, license, and bailments in determining the scope of protections. But they would also draw on custom, social practices, and modern legal developments to identify and flesh out common-law rules unknown at the Founding.

The general-law approach has numerous advantages over competitor theories. It makes better sense of the Fourth Amendment’s text and has deeper roots in its history. It is surprisingly easy to reconcile with a great deal of Fourth Amendment doctrine, while also suggesting important refinements in various areas. And it gives courts the flexibility to protect Fourth Amendment values in a changing world while also structuring and guiding the judicial task more than an untethered inquiry into privacy expectations. Private law, then, holds the key to understanding the Fourth Amendment’s limits on public power.

authors. Associate Professor of Law, Washington University in St. Louis; Treiman Professor of Law, Washington University in St. Louis. For helpful conversations and comments, we are grateful to William Baude, Jeffrey Bellin, Maureen E. Brady, Samuel L. Bray, Christian R. Burset, Andrew Guthrie Ferguson, Chad Flanders, Jack Landman Goldsmith, Orin S. Kerr, Michael Mannheimer, Richard M. Re, Neil M. Richards, David Alan Sklansky, James Y. Stern, and Matthew Tokson; participants in workshops at Washington University School of Law and at the Southwest Criminal Law Conference; and the editors of the Yale Law Journal. Samantha Kobor and Allison Walter provided excellent research assistance.

Introduction

The modern era of Fourth Amendment jurisprudence began in 1967 with Katz v. United States.1 That case, and especially Justice Harlan’s concurrence,2 heralded a new approach in which the Amendment’s protections turned on “reasonable expectations of privacy.”3 In the decades since, the Supreme Court has used this approach to build a grand edifice of Fourth Amendment doctrine. But now, just over half a century later, the Katz era could be nearing its end. Recent cases have revealed interest among some originalist Justices in restoring a supposed pre-Katz regime under which Fourth Amendment protections turn on concepts of property and trespass rather than amorphous notions of privacy.4 Aided by scholarly efforts,5 and perhaps by recent changes in the Court’s membership, some kind of “positive law” approach might be poised to flourish.

Yet, Justices drawn to a positive-law approach must still resolve fundamental questions about what exactly that approach would entail. The leading scholarly proponents of a positive-law approach, William Baude and James Y. Stern, argue that in determining questions of the Fourth Amendment’s scope, courts should ask whether “the government actor [has] done something that would be tortious, criminal, or otherwise a violation of some legal duty” under positive law if performed by a private person.6 But the Justices who are receptive to positive-law arguments have not yet endorsed Baude and Stern’s theory, which we might call the “pure” positive-law model. Notably, in his dissent in Carpenter v. United States,7 Justice Gorsuch stressed his uncertainty about several matters:

[I]f a house, paper, or effect is yours, you have a Fourth Amendment interest in its protection. But what kind of legal interest is sufficient to make something yours? And what source of law determines that? Current positive law? The common law at 1791, extended by analogy to modern times? Both?8

To Justice Gorsuch’s last question, this Article offers a different answer: neither. The Fourth Amendment should not be read as freezing specific common-law rules from the Founding Era in constitutional amber.9 Nor should it be understood as making Fourth Amendment protections wholly dependent on today’s positive law—that is, on whether the relevant jurisdiction in which a search or seizure occurs prohibits the conduct at issue for private parties. Instead, courts should interpret the Fourth Amendment by turning to general law. The general law, in Caleb Nelson’s words, is a set of “rules that are not under the control of any single jurisdiction, but instead reflect principles or practices common to many different jurisdictions.”10 In other words, this approach would ask courts to resolve Fourth Amendment questions not by looking to the common law of 1791, but instead by using the tools of the common law to determine the general law of the country today.

What would this approach look like in practice? A court would begin by verifying whether the government conduct at issue was a “search” or a “seizure.” Under some approaches, this threshold question is complex. For example, under Katz, government conduct is only a “search” if a court concludes that it violates someone’s reasonable expectations of privacy. Similarly, under Baude and Stern’s model, government only “searches” or “seizes” when it violates the positive law. Under the general-law approach, by contrast, “search” and “seizure” are read in a broader and more commonsense way.11 The general-law reading accords with the plain meaning of those words themselves and clarifies how the Amendment’s clauses interact.

Assuming that the government conduct qualified as a search or seizure, a court would then inquire whether it intruded on someone’s “person[], house[], papers, or effects.”12 In many cases, this inquiry is easy: when police barge into someone’s home or physically restrain someone, there is little doubt that the Amendment’s protections apply. But other questions are harder: can an overnight guest be said to be in her own “house” if the home is searched while she is staying there? Is a digital copy of an email that resides on a cloud-storage company’s servers the “papers” of a suspect?13 Is a homeowner’s trash left at the curb that person’s “effects”?14 Courts would answer these questions by looking to well-established general-law property concepts.

If the search or seizure did intrude on one of the Fourth Amendment’s protected categories, a court would then determine whether it was “unreasonable” by asking whether it would violate the general law. In this analysis, courts would no longer make untethered and speculative inquiries into “reasonable expectations of privacy,” as the Katz test requires (though it confusingly does so at the threshold step of determining whether a “search” occurred). But neither would courts ask whether common-law jurists in 1791 would have seen the government conduct as unlawful. Instead, they would use the tools of the common law, particularly the private law, to aid in determining how the general law would resolve the question today. To be sure, musty property-law concepts like licenses, bailments, and abandonment can help to resolve many hard Fourth Amendment questions. But in determining what searches were “unreasonable,” courts would not be limited to the specific common-law rules, or even to the broader common-law categories, known when the Fourth Amendment was ratified. Instead, judges would look to how common-law rules have evolved since the Founding. And, in so doing, judges would also contribute to that continued evolution.

Unlike the pure positive-law model, the general-law approach would not treat any one jurisdiction’s law governing private parties as controlling. Instead, in trying to identify the country’s general law, courts would look to the laws and practices of different jurisdictions as relevant data points—persuasive precedent—not as dispositive authorities. That is, the inquiry would be distinct from the way that federal courts approach questions of state common law governed by Erie Railroad Co. v. Tompkins, under which a particular state’s law dictates the rule of decision for a federal court.15 This is because questions of the Fourth Amendment’s scope and protections are best understood as matters of general law rather than “local common law.”16

In fleshing out the general law, courts would apply the common-law method to new scenarios, just as early twentieth-century courts did in recognizing new privacy-based torts for the modern commercial era. If a court identified a right recognized by the general law that the government might have infringed, it would ask what protections the general law would extend to that right. For example, if an overnight guest is a licensee under general law,17 are such licensees protected against nonconsensual intrusions?18 If a cloud-storage company is best understood as a bailee of a person’s data,19 then what protections should the law of bailment provide the bailor to guarantee her security over her belongings? And under the ad coelum doctrine,20 should a homeowner have the right to exclude others from flying a drone forty feet above her backyard?21

Sometimes existing common-law case law will reveal a well-developed consensus about whether a particular right exists and what protections it deserves. At other times, especially in situations involving emerging technologies, a court will have much less to draw on. But a court in such a position is no worse off than any common-law court (or, for that matter, a court applying the Katz framework) confronting a novel factual scenario. Just as common-law courts have applied the common-law framework to new fact patterns for centuries, so would courts interpreting the Fourth Amendment using a general-law approach.

If a court concluded that a search or seizure did violate a claimant’s rights under the general law, the search will be presumptively unlawful if government actors did not obtain a warrant in advance. Current doctrine speaks of a warrant “requirement” and its “exceptions.”22 But the better way to read the Fourth Amendment’s text—one more consistent with its historical background—is that a proper warrant immunizes otherwise unlawful conduct. In most contexts where current doctrine sees an “exception” to the supposed warrant requirement, the better understanding is that the government conduct simply does not violate the general law in the first place—and thus is not “unreasonable”—for reasons rooted in traditional general-law principles.

The general-law approach has many advantages over its competitors. Some are pragmatic: it is more straightforward to apply and produces more attractive results. Moreover, unlike the dominant Katz approach, it also gives courts a firmer foundation upon which to build doctrine than judges’ own intuitions about privacy expectations.

But it is also superior to previously recognized positive-law-based approaches. Unlike Baude and Stern’s pure positive-law model, the general-law approach leads to more uniform rules, avoids results that seem arbitrary or strange, and neither permits nor encourages legislatures to eradicate protections by rewriting rules that govern private parties.23 It also enables courts to answer questions that the pure positive-law model struggles to resolve. As civil-procedure scholars have observed, the rise of mandatory arbitration is rendering swaths of substantive law invisible, even meaningless.24 A court exercising its judgment over general law could protect Fourth Amendment values even when the positive law of a particular jurisdiction provided no explicit basis for doing so.

The other leading positive-law approach reads the Fourth Amendment as freezing 1791 common-law rules in place.25 But this approach suffers from its own defects. As David A. Sklansky has persuasively argued, 1791 rules were “hazier and less comprehensive” than originalists often claim—and even where those rules do provide clear guidance, it is often “guidance we should hesitate to follow.”26

The general-law approach avoids these pitfalls. It recognizes that the common law evolves and that divining common-law rules is no scientific inquiry. Trying to map novel factual situations onto 1791 common-law rules is often an indeterminate inquiry that may simply conceal hard value judgments. The general-law approach would create space for judges to acknowledge those judgments when the general law provides no definitive answers. It also gives judges the flexibility to recognize when old rules no longer make sense in light of modern conditions. But it does so while providing enough structure and constraint to prevent the inquiry from becoming a discretionary free-for-all.27

These pragmatic benefits are not the general-law approach’s only selling point. Compared with other approaches, it is also easier to square the general-law approach with the originalist methodology endorsed by Supreme Court Justices attracted to a positive-law model. Given that the Founding Generation understood the common law as an evolving, almost-organic entity,28 there is little reason to believe that the Fourth Amendment’s ratifiers would have understood it as simply locking the state of the common law in place for all time. There is also little evidence that they would have understood the Amendment as making protections subservient to any particular jurisdiction’s positive law. Indeed, the proponents of a pure positive-law model appear to be accidentally infusing the Fourth Amendment with decidedly modern legal values. More specifically, that approach seems to take for granted the idea underlying Erie that “the” common law does not exist—that common law is nothing more than what the relevant jurisdiction says it is. Instead, the most plausible conclusion consistent with originalism is that courts should determine what counts as an “unreasonable” search and seizure using the traditional methods of the common law. And that means an inquiry into general law, not unbending deference to any one sovereign.

Finally, the general-law approach is surprisingly easy to reconcile with a great deal of extant Fourth Amendment case law, and indeed provides a better explanation of some recent cases than alternative theories. In the recent cases in which the Court seemingly turned toward positive law, the Justices did not search for answers in 1791 common law or in the positive law of a particular jurisdiction. Instead, the Court applied traditional common-law concepts, such as trespass and license, to new facts. The best way to understand what the Court did in those cases is that it made an inquiry into general law.

But the general-law approach finds support in precedent going far beyond the recent positive-law turn. The general law has the tools to resolve questions about customs, practices, and expectations of privacy. Indeed, common-law courts have been grappling with such questions for over a century, first recognizing several new common-law causes of action protecting privacy rights at the turn of the twentieth century. Because a general-law approach could look to this rich body of common-law decisions, a good deal of the Court’s Katz jurisprudence can be justified under the general-law approach. That is, even if Katz conceptualized the relevant inquiry imprecisely—as one into social expectations, rather than into general law as demonstrated by customs and practices—the results that approach has produced may be largely, though not perfectly, consistent with a general-law approach. This means that the Court could put Fourth Amendment jurisprudence on a firmer conceptual foundation without tearing down much of the edifice of existing doctrine.

We make the case for the general-law approach as follows. In Part I, we set the stage by describing the current state of Fourth Amendment jurisprudence. We explain how Katz arose and how doctrine applying Katz developed. We then explore the recent rise of a positive-law approach and lay out the unresolved questions for those drawn to that model.

Part II lays out our general-law approach. We begin by discussing recent scholarship on the general law and how general law differs from the positivist, sovereign-focused conception of common law embraced in Erie and its progeny. Although Erie claimed that “there is no federal general common law,”29 scholars have increasingly argued that this claim is incorrect both as a descriptive matter and as a normative claim about the role of common law in the constitutional system. We then apply these insights to the Fourth Amendment. We contend that “unreasonable” in the Fourth Amendment is best understood as meaning inconsistent with the general law. Rather than freezing in place what the common law would have treated as unreasonable in 1791, however, the Fourth Amendment is best understood as requiring courts to engage in the general-law mode of analysis, developing rules to govern new factual situations while building on old common-law concepts and tools developed over the centuries. Next, we describe how a general-law method would work in practice and what questions it would require courts to ask: how courts would identify rights triggering Fourth Amendment protections and how they would look to determine whether government conduct impinges on those rights. Finally, we lay out the normative case for the general-law approach, explaining its superiority over other approaches on textual, historical, doctrinal, and pragmatic grounds.

Part III shows the general-law approach’s great promise by applying it to a wide range of factual scenarios. As we show, the general-law approach provides compelling answers to many perplexing Fourth Amendment questions. In many cases, those answers are consistent with existing doctrine. But elsewhere, the general-law approach suggests a different path. Our analysis proceeds by looking to several concepts recognized in the private law and showing how they can map onto Fourth Amendment fact patterns in a useful and illuminating way. For example, we consider how bailment doctrine and rules about abandonment could inform Fourth Amendment questions about digital privacy; how trespass doctrine can help inform questions about drones; and how privacy torts recognized in the early twentieth century might shed light on twenty-first century questions about digital surveillance. We also reconcile case law governing seizures of the person as well as the Fourth Amendment’s warrant “requirement” with our theory. And we briefly consider how our theory might approach the problem of racialized policing.

We close by noting several additional ways the general-law approach might be generative. It could inform the debate on the proper remedies for Fourth Amendment violations. It could provide solutions to hard questions in other areas of constitutional law. And it could be beneficial to the private law, as it would enable federal courts to develop the general law in ways that state courts could find helpful in other contexts.