Fourth Amendment Anti-Theory

Matthew Tokson* June, 2026

The Digital Fourth Amendment: Privacy and Policing in Our Online World. By Orin Kerr. New York: Oxford University Press. 2025. Pp. 6, 203. $30.

Introduction

The day after Thanksgiving, Randal Reid was pulled over on a Georgia highway and arrested for stealing purses in Louisiana. But he had never been to Louisiana. It turned out that Reid had been falsely identified as a purse thief by facial recognition technology, which erroneously determined that Reid was the “heavyset [B]lack male” depicted in video surveillance footage with the stolen purses.¹ Detective Andrew Bartholomew, a financial crimes detective in Louisiana, applied for a warrant for Reid’s arrest. He intentionally omitted any mention of facial recognition technology, instead claiming that he had identified Reid as the man in the footage based on “a credible source.”² A judge signed off on the warrant. Months later, a police officer in Georgia apparently ran Reid’s license through a warrant database despite the fact that Reid was driving lawfully and not acting suspicious in any way. The officer arrested Reid and took him to the DeKalb County Jail, where he was locked up for six days before police realized they had the wrong man.³

There are many anecdotes in Professor Orin Kerr’s⁴ important new book, The Digital Fourth Amendment: Privacy and Policing in Our Online World, but none of them are anything like Randal Reid’s story. Rather, virtually every anecdote and hypothetical in the book involves police successfully investigating a criminal via analog or digital means (pp. 1, 36–39, 58, 73, 114, 133, 162, 167–68, 187). This is hardly Kerr’s fault—most of these stories are drawn from litigated Fourth Amendment cases, which, by their nature, typically involve motions to suppress evidence filed by those who have committed crimes. But it is nonetheless emblematic of the strengths and weaknesses of the book. Kerr illuminates the major Fourth Amendment cases of the last century in an engaging, story-first way. He clarifies the issues that today’s judges are likely to face and explains them in a manner that readers with no prior knowledge of Fourth Amendment law can understand. His individual arguments are thoughtful and often compelling, and the breadth and depth of his knowledge of Fourth Amendment law is unparalleled. The Digital Fourth Amendment is an excellent overview of Fourth Amendment law in the modern world, written by America’s leading Fourth Amendment scholar.

At the same time, Kerr largely ignores the broader context of Fourth Amendment questions, focusing on particular cases and issues while sidelining themes of privacy, discrimination, and abuse. He rejects all but the most general forms of Fourth Amendment theory, leaving the normative basis for his proposals vague and blurry. He overlooks uses of surveillance technology that occur outside of criminal investigations or that police tend to hide from judicial scrutiny.⁵ And although Kerr’s principled approach to Fourth Amendment questions is evident throughout, he tends to approach these questions from the government’s perspective, asking how investigators might best pursue criminals without overreaching, rather than how civilians might be affected by government agents with unchecked surveillance powers.⁶

A central purpose of The Digital Fourth Amendment is to set out a big-picture account of Fourth Amendment law, connecting Kerr’s forty or so prior articles on individual Fourth Amendment issues into a larger synthesis.⁷ It is an exciting prospect, because, although I do not always agree with them, these articles constitute some of the best Fourth Amendment scholarship of the past several decades; some of them are among the best Fourth Amendment articles ever written.⁸

Yet there is no grand theory here to guide us through the many questions posed by modern digital surveillance. Rather than adopting some concept of the Fourth Amendment’s core value or values and using that concept to answer difficult questions posed by novel technologies, Kerr’s approach to the digital Fourth Amendment turns largely on analogies to the nondigital Fourth Amendment.⁹ Whenever possible, he would apply older rules to new settings, transposing the predigital Fourth Amendment into the digital world. However, he also recognizes that the novel characteristics of digital data and digital investigations can problematize analogies to the physical world and throw off the traditional balance between police efficacy and citizen privacy (p. 40). Kerr contends, drawing on his famous article from 2011,¹⁰ that when a new technology is transformative enough to dramatically disrupt the balance between police and criminal suspects, courts should engage in “equilibrium-adjustment” (p. 42) to try to restore the prior balance. Specifically, “[w]hen new technology and social practice dramatically expand government power, courts can adopt a more restrictive rule to restore the lost limits on government power. And when new technology and social practice dramatically curtail power, courts can loosen the legal regulation to help restore it” (p. 42).

Kerr’s combination of analogy and adjustment provides a framework of sorts for Fourth Amendment law, but it is a framework with little descriptive or normative bite. As a descriptive account of Fourth Amendment cases, the theory is virtually unfalsifiable. A court addressing a Fourth Amendment question involving new technology can either apply an old rule (using an analogy) or adopt a new rule (equilibrium adjusting)—either approach can be said to be consistent with Kerr’s account.¹¹ Kerr’s framework likewise has little purchase as a normative theory of the Fourth Amendment. It becomes clear over the course of the book that equilibrium adjustment is more of an “intuitive,” practical inquiry than a normative theory of law (p. 45). It is a search for a “ballpark level of limits on police power” within the “range of the reasonable, somewhere in the middle, that most will feel permits enough enforcement of the law without giving the government too much power” (p. 45). In practice, the equilibrium adjustment inquiry is often a totality-of-the-circumstances-style approach, where Kerr lists various facts about a surveillance issue and reaches an intuitive judgment about whether to adjust.¹²

Kerr’s approach amounts to an anti-theory of the Fourth Amendment. It addresses few of the values that scholars associate with Fourth Amendment law, including privacy, power, liberty, oppression, trust, or fidelity to original law.¹³ Fourth Amendment theories generally tie one or several of these values to some prescription for regulating government searches and seizures. They are driven by ideals claimed as foundational to Fourth Amendment law. By contrast, The Digital Fourth Amendment consistently elevates narrow, pragmatic considerations over any overarching theory of value. This is made even clearer in the parts of the book where Kerr indirectly (and occasionally overtly) rejects the idea of theory-driven Fourth Amendment law.¹⁴

While there are substantial benefits to a flexible and pragmatic approach to the Fourth Amendment, there are also significant downsides. Kerr’s anti-theoretical framework contributes to his tendency to advocate for a remarkably narrow, underprotective approach to online data, as demonstrated in the book’s third and final part. In prior parts, Kerr’s uses of analogies and interest balancing are relatively straightforward. Kerr, to his credit, follows the logic of his approach wherever it leads, at times advocating for an expansion of Fourth Amendment protections to more closely preserve the function of the old doctrines (pp. 87, 101–02).

Yet the further away new technologies get from traditional Fourth Amendment contexts, the less guidance and constraint the analogical approach provides. In those situations, Kerr’s focus on the physical location of internet data and his use of distant analogies to real-world settings leads him astray. His proposals tend to underprotect privacy and underestimate the harms caused by unchecked police surveillance. The result is a long series of frontier Fourth Amendment questions that are nearly all answered the same way: The Fourth Amendment should not apply to online data in most controversial cases.¹⁵

The way forward for Fourth Amendment law is not mining the past for increasingly inapposite analogues to the digital world. The task for Fourth Amendment scholars in this area is to help ease the transition away from property-based, physical-world conceptions of law—but to do so with the clarity, pragmatism, and persuasiveness that Kerr exemplifies.

Part I of this Review discusses Professor Kerr’s anti-theory of Fourth Amendment law and details his elevation of pragmatic considerations over broader principles throughout the book. Part II looks at the subtle yet substantial advantages of a largely atheoretical approach to Fourth Amendment law. It then examines one of the downsides of the anti-theoretical approach—namely, the lack of constraint and the elevation of discretion. In the absence of a clear normative theory, Kerr’s tendency to favor rough analogies to the physical world often leads him to undervalue digital privacy. Part III surveys some potential theoretical alternatives to Kerr’s proposed framework. The Review concludes by defending the importance of robust constitutional protections for personal data and discussing the Fourth Amendment’s potential for combating authoritarian drift.

I. Analogy and Adjustment

A. Analogy

At the center of Kerr’s anti-theoretical approach to digital Fourth Amendment law is a reliance on analogies to the physical world (p. 66). It is easy to grasp the idea that rules developed to govern police investigations in the physical world should be applied to the transformed, but roughly analogous, terrain of the digital world. But this notion is less a theory of the digital Fourth Amendment and more a useful tactic in the absence of a theory. Lacking an account of the central values of the Fourth Amendment, or the nature of digital surveillance, we might rely on analogies to the law of the physical world to reach some sort of conclusion. However, as Kerr details, analogy is often fraught with peril because of the many ways that digital information and evidence have changed modern police investigations (ch. 2). In such a transformed terrain, analogy is less a means of preserving the past and more of an erratic makeshift. Its vague prescriptions are sometimes conservative and sometimes transformative, depending on the specifics of how the technology in question has changed.

Kerr’s commitment to the physical Fourth Amendment is evident from the start of the book, where he interprets Katz v. United States, the foundational 1967 Supreme Court case establishing that a Fourth Amendment search occurs whenever the government violates a person’s reasonable expectation of privacy.¹⁶ The Katz Court concluded that the government searched Katz when they taped a microphone to the top of a public phone booth he was using and recorded his phone conversations.¹⁷ In doing so, the Court uncoupled the Fourth Amendment from property concepts, focusing instead on the privacy Katz was entitled to expect as he made his phone call. The Fourth Amendment, the Court declared, “protects people, not places.”¹⁸

The conventional interpretation of Katz considers it an expansion of the Fourth Amendment to cover intangible things like conversations, or any other form of data in which a person has a reasonable expectation of privacy.¹⁹ But Kerr reads Katz in an unusual, and remarkably narrow, manner. To him, Katz merely protected “phone booth conversations,” on the grounds that a phone booth is a place that the Constitution protects (pp. 15–16). Katz does not protect intangible data from government intrusion, he posits; rather, it establishes that both physical invasions and “equivalents to physical invasions” into certain protected places are covered by the Fourth Amendment—nothing more.²⁰

Setting aside the bizarre implications of this interpretation for the constitutionality of wiretapping and bugging,²¹ this reading of Katz exemplifies Kerr’s commitment to physical analogies and the physical world. The central principle of his approach is determining which types of digital surveillance are roughly equivalent to physical invasions and which are not. Analogies to physical-world contexts are at the core of Kerr’s approach to most of the issues discussed in the book, from wiretapping (p. 140) to email (p. 142), login IP addresses (p. 143), message to/from data (p. 139), ride-sharing records (p. 166), and more.

B. Adjustment

Kerr also recognizes the perils of analogy. The digital world is fundamentally different from the physical world. The analogies we might draw between them are often imperfect and vague—and, occasionally, actively misleading. Kerr details the many ways that digital evidence and information have changed modern police investigations (ch. 2) and notes that these changes can disrupt and devalue physical-world analogies (p. 40). In these situations, Kerr asserts that courts can engage in “equilibrium-adjustment,” although this should only occur when the technology “has a transformative effect” on the preexisting rule (p. 68).

As discussed in the Introduction, the combination of analogy and adjustment can justify virtually any result that a court might reach. There is no clear line where technological change disrupts an analogy to such a degree that it is “transformative,” and equilibrium adjustment is necessary. Rather, it is an “intuitive” practical inquiry lacking any larger theoretical or guiding principle.²²

The clearest example of the indeterminacy of the analogy-adjustment approach, and of Kerr’s hostility to theory, is the book’s treatment of Carpenter v. United States.²³ In Carpenter, the Supreme Court held that the Fourth Amendment applied to cell phone location data, which revealed user location data more than one hundred times per day, potentially exposing a person’s activities, associations, and other sensitive details of their lives.²⁴ Kerr frames Carpenter as a prominent example of equilibrium adjustment, applied in a context where analogy is stretched to its breaking point. He notes that allowing police officers to constantly track an individual’s location digitally, on the basis of prior law allowing police officers to physically follow suspects, is not just maintaining the status quo. In the predigital world, police could not feasibly follow a suspect everywhere they went twenty-four hours a day for years or more (p. 148). Now, with the advent of cell phone location data, police can do this cheaply and easily. Allowing such tracking on the basis of the traditional rule would be a major expansion of government power, allowing for “new and more invasive surveillance” on a vast scale (p. 148).

The catch is that Kerr endorses this dramatic expansion of government power. He opposed and continues to oppose Carpenter and its equilibrium adjustment (pp. 68–69), preferring to rely on the problematic analogy to physical police surveillance (p. 148). His argument is essentially that Carpenter is one of the “difficult judgment calls” that equilibrium adjustment often presents, and that, in his view, the location tracking in Carpenter is insufficiently precise to justify equilibrium adjustment (p. 68). How precise it would have to be is left unsaid, but there is a larger ambiguity as to when technology is sufficiently transformative to justify equilibrium adjustment (p. 68), and when it merely expands government power and creates substantially more invasive surveillance (p. 148). Similar judgment calls arise in other areas, and there are few, if any, guidelines in the book to help determine when adjustment should occur.

C. Rejecting Theory

The book’s discussion of Carpenter is also emblematic of Kerr’s more overt rejections of Fourth Amendment theory. Kerr argues that Carpenter was and still is wrongly decided (pp. 68–69). He also defends the principle that people should have no Fourth Amendment rights in data they fail to conceal from third parties.²⁵ Yet he devotes multiple sections of the book to approvingly describing the rationales that led to Carpenter (pp. 63–65, 148–50) and devotes a lengthy chapter to how he would apply Carpenter to various novel questions in digital Fourth Amendment law (ch. 9). There is nothing wrong (indeed, there is much to be admired) in presenting the best arguments against one’s position. But the disconnect is nonetheless jarring. A more theoretically driven book, after telling us that Carpenter was wrongly decided and that only those who conceal their data from others should receive Fourth Amendment protection (pp. 69, 146–47), would presumably then tell us that the Supreme Court should reverse Carpenter for these reasons. This book takes a more pragmatic, but in some ways less convincing, approach. It narrowly interprets the case rather than calling for its abandonment and characterizes it as consistent with the book’s approach, despite Kerr’s ongoing opposition to the case and its reasoning.

Kerr also dedicates an entire chapter (ch. 10) to critiquing—on almost entirely practical grounds—the mosaic theory of the Fourth Amendment. The mosaic theory is the idea that a large quantity of data collection might be a Fourth Amendment search, even if a smaller quantity of the same data collection might not.²⁶ This chapter is an especially strong one, and Kerr’s points are thoughtful, sound, and skillfully argued, as usual. But there is still much to be said for tying the concept of a Fourth Amendment search to violations of privacy, even if doing so forecloses bright-line rules. The point of the mosaic theory is that, while forty seconds of government video surveillance of the area around your home might not infringe your privacy to any meaningful extent, forty years of government video surveillance would. The same holds true for location tracking, communications metadata collection, and several other forms of surveillance. Many judges and scholars choose to recognize this reality, likely because their concept of the Fourth Amendment is grounded in privacy or government power.²⁷ As the next Part explores, decoupling the Fourth Amendment from these abstract values, even to serve the valuable goals of administrability and clarity, has its own downsides.

II. Anti-Theory and the Fourth Amendment’s Scope

There are real advantages to a pragmatic, narrowly focused approach to Fourth Amendment questions. As Kerr has said, such an approach has enabled him to influence the emerging body of digital Fourth Amendment law by writing in ways that are both comprehensible and useful to judges facing new issues.²⁸ By eschewing grand theory, he produces scholarship that meets judges where they are, without alienating them with academic-speak or asking them to dramatically change existing law. It also makes his work more accessible to a general audience. This is part of the reason why Kerr’s scholarship has been so influential and popular and why his informal writing on the Fourth Amendment is so enjoyable. It’s not that pragmatism is the secret of his success—Kerr’s secret is being remarkably productive and generating several decades’ worth of high-quality scholarship. Still, pragmatism helps amplify his influence and extend his reach.

One downside of a pragmatic, lightly theorized approach to Fourth Amendment law, however, is that it offers little constraint or guidance in novel situations. In the absence of an underlying theory of the Fourth Amendment and its fundamental values, new Fourth Amendment questions present tricky judgment calls about which Fourth Amendment rules feel intuitively correct. Kerr’s prescribed path of digital Fourth Amendment law, as laid out in the book, can appear as a series of particularized choices with no overarching theme.

To be sure, a similar critique could be (and has been) leveled against the Supreme Court for deciding Fourth Amendment cases on the basis of intuitive judgments alone, without any coherent theory of what the Fourth Amendment should do.²⁹ In an insightful article written nearly thirty years ago, Ronald Allen and Ross Rosenberg described the Court’s atheoretical approach, arguing that the accumulation of case-by-case Fourth Amendment decisions and the fact-specific knowledge they embody constitute a coherent, organic theory grown from the bottom up.³⁰ Kerr’s approach can be defended on similar grounds as creating a sort of one-man common law of the digital Fourth Amendment. In both the Supreme Court’s context and Kerr’s, the sheer accumulation of narrow choices tends to cohere into a discernable whole over time.³¹

But there are reasons why so many scholars have called for the adoption of a general theory of Fourth Amendment law.³² Among other concerns, the lack of constraint imposed by an atheoretical Fourth Amendment can permit courts’ biases or preferences to creep into their decisions. Kerr’s approach is an objective one, and it leads him to call for more Fourth Amendment protections in some cases and fewer protections in others. But over the course of the book, his preference for physical analogies and clear-cut rules can lead him to favor an oversimplified, overly narrow Fourth Amendment.

A. Devices

Kerr’s pragmatic approach plays out in different ways throughout the various sections of his book. In Part II, “Local Devices,” Kerr’s focus on the physical world provides a useful framework for evaluating searches that are similar to traditional searches but with a digital twist. One of the most compelling chapters in the book discusses inspections of digital devices at U.S. borders (ch. 7). Traditionally, the government is allowed to physically search people and property at the border without any cause.³³ Kerr examines the three rationales for this power—customs enforcement, immigration enforcement, and blocking contraband—and finds that all of them are tied closely to the physical nature of border crossing (p. 120). He argues that none of these rationales has purchase in the new context of digital data, at least so long as the device holder is a U.S. citizen. Citing Riley v. California,³⁴ which held that police could not warrantlessly search the digital contents of a cell phone incident to arrest, Kerr calls for an equilibrium adjustment to require probable cause to search a U.S. citizen’s digital device at the border (p. 125). He then punts on the exact nature of the rules that should govern noncitizens, specifying that only future cases can address issues like pretextual discrimination or whether lawful permanent residents should be treated like citizens for Fourth Amendment purposes (p. 128). This set of proposals leaves a great deal unanswered and highlights the absence of a broader theory, but it reflects a compelling, narrow-but-deep argument for modifying a traditional doctrine to suit the realities of the digital world.

The same can be said for another strong chapter of the book, discussing how to administer warrants for digital evidence stored in a computer or similar device (ch. 6). Warrants for physical searches must specify the place to be searched and the things police officers will look for when they search.³⁵ Translating these requirements to inspections of a computer hard drive is difficult, because hard drives can contain billions of pages worth of text, and evidence may be located anywhere on the hard drive. In addition, much of what agents uncover will have nothing to do with the original warrant or the probable cause of a crime on which it is based. Some courts have declined to impose any restrictions on hard drive searches (p. 96), while others have sharply restricted them by tossing out evidence found in a comprehensive search (p. 96). Still others have addressed these issues by limiting, ex ante, where officers can search on a hard drive, imposing various conditions on digital searches via detailed protocols (pp. 96–97).

Kerr deftly proposes a more administrable middle-ground approach that would allow police to search the entire hard drive for evidence of the crime specified in the warrant but would forbid the government from using any incidental evidence they find of other crimes in court (p. 101). Through a creative series of adjustments, Kerr manages to fashion something like the prior, physical-world rule while also recognizing that hard drive searches differ too much from physical searches to rely on a straightforward analogy. Equilibrium adjustment is at its most effective in these areas, where it can approximate the function of traditional physical doctrines by tweaking the law to adapt to relatively simple contextual changes. Adjustment may work, for example, when the size of a searchable area goes from relatively small (in the physical world) to large (in the digital world). But even here, adjustment is an intuitive, ad hoc method, creating new rules that function in fundamentally different ways from prior rules, with few guidelines or hard limits. And more profound or qualitative changes present real problems for the analogy-adjustment approach.

B. Networks

The further Kerr’s book gets from traditional searches and their digital analogues, the less practical its analogies and adjustments become. Part III of the book concerns networks and the data they generate. In other words, it deals with the internet, where so much of modern communication and activity takes place and where enormous quantities of personal information are generated every day. Here, where analogies to physical contexts are often distant or misleading, Kerr tends to undervalue privacy and underestimate the harms caused by police surveillance.

1. Inside and Outside Data

Some network data is plainly protected by the Fourth Amendment. The contents of emails and other messages, as well as documents or photos stored on online servers, are uncontroversially protected so long as they are not exposed to third-party service providers.³⁶ Kerr writes that these forms of “content” data are basically equivalent to the “inside” of a house or item and, accordingly, receive traditional Fourth Amendment protections (p. 137). Virtually everything else is “outside” data, which is traditionally not protected by the Fourth Amendment (p. 137). Accordingly, “non-content” forms of digital data, including location data, to/from message data, subscriber information, ride-sharing records, credit card data, app usage data, and much more, would not be protected under Kerr’s framework for digital information.³⁷ This is not an indefensible or unreasonable view, and it would protect the most sensitive forms of internet data from warrantless searches. But this view was controversial over fifteen years ago, when Kerr first developed it, and it has become less defensible and more destructive of personal privacy with virtually every new technological and cultural development of the past two decades.³⁸

New forms of non-content information arise with remarkable frequency in the internet era, and the extent to which non-content information can reveal sensitive aspects of a person’s life has also increased sharply in recent years.³⁹ The surge in internet use, the ubiquity of cell phones and video cameras, the ability of machine-learning and other analytic tools to derive sensitive information from metadata, and the proliferation of new forms of online data have all combined to make non-content data extremely prevalent. It has also become more similar to content data in that it is remarkably revealing of the details of our lives.⁴⁰ Whether and to what extent to protect this data is the central question of digital Fourth Amendment law.

Kerr’s answer to this question is consistent with the basic analogy to inside versus outside information—non-content information is analogous to outside information and is thus not protected (pp. 137–38). Further, content data exposed to third-party service providers might not be protected either (pp. 170, 193). Carpenter complicates this picture, but only very little. Kerr offers a test of sorts for when Carpenter’s rationales should be applied to a new form of non-content data (pp. 157–62). But this test is so strict that virtually nothing is protected under it.

Kerr’s test would apply Carpenter to protect non-content data only if (1) the data is a new kind of data with no predigital equivalent; (2) the data reveals private personal information that is unlikely to be of legitimate state interest; and (3) the person’s disclosure of the data to a third-party service provider was essentially unavoidable (pp. 157–62). In the book, the only form of data that passes muster under this test is web surfing logs, such as a list of IP addresses visited by a web user (p. 164). Kerr’s discussion of this is admirable—web surfing logs should be protected, and protecting them is not yet a consensus position among courts.⁴¹ That said, web surfing logs are also somewhat unique. Not only is internet use inescapable in modern life, but web surfing logs easily reveal the contents of many internet communications––that is, the contents of the websites you visited at the times you visited them.⁴²

Other forms of non-content data less directly tied to content fare worse under Kerr’s approach. Kerr would deny Fourth Amendment protection to login IP addresses, the addresses with which devices identify themselves when they access the internet (p. 163). These addresses can link a device to a particular place, like a user’s home or the Wi-Fi network where they log in. For laptops or desktops, this is unlikely to reveal much. But for cell phones, these login IP addresses might be used to track a person’s location, potentially allowing police to trace them wherever they go. Depending on how a carrier runs its network, cell phones may be assigned a new login IP address as they move from cell tower to cell tower or when their internet activity changes.⁴³ A person’s cell phone login IP addresses could reveal their historical location data, obtained whenever they were using the internet.⁴⁴

Kerr would withhold protection from IP login data because it is “not new” and can be loosely analogized to a phone number or a home address (p. 163). It is somewhat like a phone number, albeit a phone number that can change every time you travel, use the internet, or pass by a familiar Wi-Fi network, and potentially reveals your movements and activities. But the fact that login IP addresses are sort of like something else is not a good enough reason on its own to deny them constitutional protection.

2. Inescapability

The same is true for ride-sharing records, which Kerr would not protect for several reasons, including that they are somewhat similar to paper logbooks used by taxi drivers.⁴⁵ The use of ride-sharing apps like Uber is also voluntary and not inescapable, Kerr argues, which makes the associated data ineligible for Fourth Amendment protection (p. 166). The concept of voluntary, potentially escapable disclosures to third parties is a major theme of this part of the book. It essentially calls for a robust application of the principle that information disclosed to third parties is unprotected with a small carveout for truly unavoidable aspects of modern life, such as website use. Like the classic third-party doctrine, which held that people have no expectations of privacy when they disclose information to any third-party service provider regardless of escapability,⁴⁶ it precludes Fourth Amendment protections for a large swath of modern online data. Unlike the classic doctrine, it is conceptually vague. Almost any form of data disclosure is escapable in theory, yet a great many are difficult to escape in practice if one is to participate in the modern world. Further, inescapability analysis elides individual differences, and technologies that are avoidable for most people may be unavoidable for others, including many disadvantaged populations.⁴⁷

Kerr argues that ride-sharing apps are avoidable because there are several other ways of traveling (p. 166). Most people could drive their car, take a bus, or walk rather than taking an Uber. Yet for some individuals, ride-sharing apps may be as essential as cell phones or internet access.⁴⁸ For disabled persons not living near public transportation, ride-sharing services may be the only viable means of transportation.⁴⁹ For people who cannot afford a car, these services may be essential for getting to appointments or job interviews.⁵⁰ Majority-Black neighborhoods often rely heavily on ride-sharing services, in part because those services engage in less racial discrimination than traditional taxi services.⁵¹ Moreover, many other forms of public or private transportation also involve location tracking, making the choice to take them rather than ride-sharing services irrelevant for privacy purposes.⁵² As in many modern contexts, disclosing one’s personal information is often less optional than it appears.

A similar issue arises with geofence searches, where police ask Google or some other company for a record of every phone in a certain location at a certain time, generally surrounding some reported crime.⁵³ There is an ongoing controversy about the parameters of the warrants that officers typically obtain in order to get geofence data.⁵⁴ Kerr argues that they do not need a warrant in the first place because this historical location data is unprotected by the Fourth Amendment (p. 167). The crux of his argument is that cell phone users, during phone or app setup, generally give permission to Google Maps or some similar app to access their location information, and this permission terminates user rights in such data for as long as they use the app. I have argued against this approach elsewhere.⁵⁵ But the broader implication of Kerr’s argument is that very little of the information people generate on their cell phones would receive Fourth Amendment protection because the vast majority of it is disclosed to one app or another after users give permission during phone setup.⁵⁶ Users receive very little information about data collection, lack the ability to understand what information exists, and generally blindly agree to whatever permissions are required to get the apps operating.⁵⁷ Allowing the government to access massive quantities of detailed, often sensitive app data based on a vague permission screen the user clicked years ago when they bought their phone is hardly an effective way to adjust the Fourth Amendment to the new realities of the digital world.

Even Google searches (or Bing or Yahoo!) may not be protected under Kerr’s approach to voluntary disclosure. Such searches can be highly personal and revealing, as a look through your own search history will demonstrate.⁵⁸ Indeed, search terms are the content of your communications with Google, rather than non-content information, although they can also be glimpsed in the URLs of the Google results pages.⁵⁹ But it is unclear, Kerr contends, whether the use of search engines is truly inescapable in the modern world. He writes that a person “may feel perfectly able to participate in modern society without searching all that often or even at all” (p. 170). This assertion might be questioned on its own terms. Roughly 90 percent of Americans own a smartphone, and presumably those Americans regularly search for things using their phones.⁶⁰ Even on personal computers, the median user performs fifty-three Google searches per month, likely a small fraction of the searches they perform across all devices.⁶¹ In the modern world, avoiding searches entirely would be extremely difficult. But in any event, a test that cannot produce a coherent answer as to whether government officials should be able to read your Google searches at any time without cause is not a useful one. The version of the Fourth Amendment it reflects would be too narrow, and too focused on the minutia of inescapability, to maintain relevance in the digital age.

Numerous other forms of digital data—from the video recorded by advanced government surveillance cameras to personal data generated by normal online activity—would be unprotected under the rationales of Kerr’s approach. Things like the to/from data of email and text messages; dating app usage data; social media app usage data; DNA and other biometric data held by third parties such as genealogy services or health tracking apps; credit card and electronic wallet purchases; subscriber information; video obtained by video camera networks, facial-recognition-enabled cameras, and pole cameras aimed at houses; license plate tracking data; and several other forms of digital data would be wholly unprotected by the Fourth Amendment. Taken together, such a lack of protection against modern forms of surveillance threatens to leave people with little security against pervasive government monitoring and an ever-shrinking sphere of intimacy and privacy.

3. Buying Data

Moreover, those few forms of online metadata that Kerr would protect may be vulnerable to an end-run around Carpenter’s protections. Many forms of online data, including web surfing logs, are collected and sold by data brokers, generally for marketing purposes.⁶² In the book’s last chapter, Kerr argues that the government can lawfully buy data without triggering the Fourth Amendment, avoiding the need for a warrant or other legal process (p. 190). The basis of this argument is a distant analogy to the physical world. Kerr argues that a company in possession of a person’s cell phone location data is similar to a roommate who shares an apartment with them. Like a roommate consenting to a police search of a shared apartment, the companies can consent to police viewing anyone’s cell phone data (pp. 191–92). This is a questionable analogy—data brokers and cell phone companies do not see your personal data, have no privacy interest in it, and do not mingle their data with yours.⁶³ They are more like landlords, who own an apartment but do not live in it and who generally cannot consent to a police search of it.⁶⁴ But rather than resolving the issue on the basis of which remote analogy to apartment living is best, courts should simply ask whether the government is taking action to obtain private, Fourth Amendment-protected data. The protected data that many government agencies have purchased in the years following Carpenter is often deeply revealing and is not generally available to, or accessed by, the public.⁶⁵ By purchasing such data, the government is conducting a Fourth Amendment search, and a warrant is required.⁶⁶

In a late section of the book, Kerr argues against equilibrium adjustment to prohibit warrantless data buying on the ground that rights in digital data are not obviously weaker than rights in physical property (pp. 196–98). He notes that people often give consent to searches of their physical property because “saying no to a person of authority who is standing right in front of you is much harder than you might think” (p. 197). Kerr writes that these “consent searches give officers an important way to search homes without a warrant” (p. 197). There are few digital equivalents to this, he argues, and so allowing the government to buy otherwise protected data on the grounds that companies consent to such purchases does not disturb the general equilibrium of police investigations (pp. 197–98).

Viewing warrantless, intimidation-based searches of homes as a basis for justifying digital Fourth Amendment law demonstrates the inadequacy of the analogy-adjustment framework. The costs and benefits of consent doctrine in the physical world exist in their own unique context, and the fact that requests for consent often permit the police to violate privacy through intimidation is a lamentable downside of traditional law, not a reason to reproduce its inequities in a novel situation. New issues present new balances between rights and government power, between privacy and police effectiveness. Trying to import or recreate old balances in novel contexts often leads to distortion, fossilization of what currently exists, or worse.

III. A Modern Fourth Amendment

If analogies to previous contexts fail us as guides to the digital Fourth Amendment, to what should we turn? This Part discusses some alternative responses to the puzzles of digital Fourth Amendment law.

First, there are other, more protective approaches available for addressing the particular issues discussed in The Digital Fourth Amendment. I have set out some of these above, and there is additional work arguing that the Fourth Amendment should extend, at least in some cases, to ride-sharing records,⁶⁷ geofence data,⁶⁸ Google search terms,⁶⁹ and sensitive data purchased from data brokers.⁷⁰ These are frontier issues, and some of them present close questions. However, Kerr’s categorical dismissal of Fourth Amendment rights in most of these forms of data underprotects privacy in the modern era and could open the floodgates to pervasive government surveillance.

Second, on a more theoretical level, courts and scholars might focus less on adjustment and more on equilibrium. That is, they might focus more directly on a balance between privacy and law enforcement effectiveness rather than starting from loose analogies to the physical world. Kerr touches on this balance briefly in the book (pp. 44–45) and has written more extensively about it elsewhere.⁷¹ His analysis might benefit from a more overt balancing of these interests, which may, for example, counsel that some forms of internet data shared with third parties merit Fourth Amendment protection, including protection against government purchases of such data. It might also motivate more focus on the potential large-scale surveillance of civilians, even if such surveillance has not yet occurred or has occurred but has not yet appeared in litigated cases or media reports.⁷²

There is a substantial literature on privacy-based theories of the Fourth Amendment, and a smaller but still ample literature on balancing privacy against competing values.⁷³ Balancing privacy interests against government interests without reference to physical-world rules may offer a means for courts and scholars to effectively address the novel questions of the digital Fourth Amendment.⁷⁴ Of course, even given the literature fleshing out privacy theory and normative Fourth Amendment balancing, it is necessarily a less concrete process than drawing an analogy to some physical context. Yet it offers no less constraint than the analogy-adjustment approach, which can support virtually any result.

Embracing direct balancing or anything like it would require Kerr to be comfortable with Fourth Amendment subdoctrines that are not bright-line rules and that measure how much a given surveillance tactic implicates privacy, rather than categorically ruling types of digital surveillance in or out depending on their resemblance to physical phenomena. This is likely a price worth paying. Although clarity is a substantial virtue in Fourth Amendment law, it is not the only virtue. Rules that provide sufficient clarity to precisely guide police officers in virtually every case may be profoundly deficient in other respects, like inadequately protecting privacy or protecting it in an arbitrary manner.⁷⁵

Further, courts need not overtly balance privacy and government interests in every case, and new tests and rules are likely to arise to deal with particular contexts. These subsidiary doctrines can provide both guidance and constraint, even as they are grounded in a more direct assessment of privacy and the harms of surveillance. After Carpenter, for example, empirical data shows that lower courts have focused on three particular factors discussed by the Supreme Court: the revealing nature of the data sought by the government, the amount of the data, and whether the data was voluntarily disclosed.⁷⁶ I have argued elsewhere that these factors should constitute a Carpenter test for novel search questions and that they should be weighed against each other, with none of them considered determinative on its own.⁷⁷ For instance, rather than restricting Fourth Amendment protections to only those forms of data that are not voluntarily disclosed, courts should consider the voluntariness of disclosure alongside the other factors discussed in Carpenter.⁷⁸ After all, the Carpenter opinion itself largely deemphasized voluntariness, confining its discussion of whether Carpenter’s disclosure was voluntary to a single paragraph in a lengthy opinion that largely focused on other considerations.⁷⁹

Third, courts and scholars might look to an alternative set of theories of the Fourth Amendment that emphasize its role in preventing government authoritarianism.⁸⁰ As concerns about authoritarianism have begun to loom large in American politics, these theories have become increasingly relevant.⁸¹ Typically, these theories focus on concepts like power, security, and autonomy, rather than privacy itself.⁸² Under these theories, unchecked surveillance and law enforcement methods are important not only for how they facilitate monitoring of people’s lives but also as a means of disempowering civilians and subjecting them to government oppression.⁸³

The prescriptions of the antiauthoritarian scholars tend to be more forward-looking and prophylactic, focusing on preventive judicial oversight of surveillance rather than waiting for a clear privacy violation to emerge.⁸⁴ Relatedly, they tend to call for courts to imagine what the effect on a population’s liberty or security would be if the surveillance at issue were generalized and pervasively implemented.⁸⁵ At the same time, risks of creeping authoritarianism might be effectively addressed through less aggressive means than a warrant requirement, with the focus placed on judicial scrutiny rather than high barriers to privacy intrusion.⁸⁶

As an example, allowing the government to obtain basic subscriber information about an anonymous internet user presents relatively few privacy problems. The information generally consists of only a person’s name, address, and payment information (p. 143). But deanonymizing internet users without cause might pose real threats to free speech. Anonymous speech criticizing elected officials or otherwise expressing opinions has long been protected on First Amendment grounds.⁸⁷ An antiauthoritarian approach to the Fourth Amendment might require the government to document some articulable suspicion of a crime before obtaining a user’s subscriber information in order to prevent large-scale abuses of a generally nonsensitive surveillance technique.⁸⁸

Courts might take a similar approach to semi-pervasive facial recognition camera networks, which are currently operating in many cities across America.⁸⁹ While these systems may not yet violate civilians’ privacy to a sufficient extent to implicate Carpenter, they do pose substantial concerns about selective enforcement, discrimination, and government monitoring of large populations.⁹⁰ The low cost and scalability of these systems can also promote the consolidation of power in a central authority, allowing a relatively small number of officers to monitor a substantial area.⁹¹ Courts could exert oversight of such systems without necessarily requiring a warrant backed by probable cause for their use.⁹² The proactive nature of the antiauthoritarian theories can be seen as a response to the unique scale of potential surveillance in the digital era. Even relatively noninvasive digital surveillance techniques can pose risks of authoritarian abuse if deployed at scale—and many modern techniques can easily be deployed at a massive scale.⁹³

In an age when law enforcement is increasingly directed toward expressive acts and disfavored groups,⁹⁴ robust Fourth Amendment protections, grounded in core constitutional values, are ever more crucial. Modern Fourth Amendment law should be capable of preserving privacy in the digital age with little weight given to how judges addressed searches in increasingly distant physical contexts. And courts seeking to preserve citizen autonomy should bear in mind the lessons of antiauthoritarian Fourth Amendment scholarship, with an eye on the uncertain future and the alarming capabilities of modern surveillance technologies.

Conclusion

Orin Kerr’s The Digital Fourth Amendment sets out a full account of modern Fourth Amendment law by America’s leading Fourth Amendment scholar. As a book-length overview of Fourth Amendment law—accessible, clear, and informative—it has no equal. Kerr’s individual arguments are deeply considered, and his proposals for device-based searches and seizures are often compelling. The book’s flaws as a broader framework for modern Fourth Amendment law are the product of its largely analogical, anti-theoretical approach. It seeks to bring rules governing the physical world and grounded in the unique normative considerations of that world to bear on digital data, despite profound differences between the physical and digital contexts. In doing so, Kerr overlooks the dramatically greater reach and scale of modern surveillance, with all its attendant risks for personal privacy. The pragmatism and administrability of its proposals are appealing and worthy of consideration in every context. But a Fourth Amendment law capable of addressing the complex questions posed by modern technology requires more.

* Associate Dean for Research and Faculty Development, Professor of Law, University of Utah S.J. Quinney College of Law. Thanks to Jennifer Granick, Ronald Allen, and Orin Kerr for helpful comments and suggestions.