Four models of Fourth Amendment protection.

• Author: Kerr, Orin S.

INTRODUCTION I. THE FOUR MODELS A. The Probabilistic Model B. The Private Facts Model C. The Positive Law Model D. The Policy Model E. The Relationship Among the Models II. THE CASE FOR MULTIPLE MODELS OF FOURTH AMENDMENT PROTECTION A. The Goals of the Reasonable Expectations of Privacy Test B. Why the Proxy Models Cannot Provide Exclusive Guides to Fourth Amendment Protection 1. The Probabilistic Model 2. The Positive Law Model 3. The Private Facts Model C. Why the Policy Model Cannot Provide an Exclusive Guide to Fourth Amendment Protection 1. Lower Courts and the Reasonable Expectation of Privacy Test 2. The Instability of the Policy Model in the Lower Courts D. The Case for Multiple Models 1. Supreme Court Selection Among the Four Models 2. Lower Court Use of the Four Models 3. The Need for Recognition of the Four Models CONCLUSION INTRODUCTION

The reasonable expectation of privacy test is the central mystery of Fourth Amendment law. According to the Supreme Court, the Fourth Amendment regulates government conduct that violates an individual's reasonable expectation of privacy. (1) But no one seems to know what makes an expectation of privacy constitutionally "reasonable." The Supreme Court has repeatedly refused to offer a single test. (2) The Court has noted that "concepts of real or personal property law" might be relevant, as well as "understandings that are recognized and permitted by society." (3) But the Court has elsewhere rejected property as a guide, (4) and no one knows when society might opt to "recognize" or "permit" something. Who is "society," and how do Supreme Court Justices know what it thinks? Although four decades have passed since Justice Harlan introduced the test in his concurrence in Katz v. United States, (5) the meaning of the phrase "reasonable expectation of privacy" remains remarkably opaque.

Among scholars, this state of affairs is widely considered an embarrassment. The Court's handiwork has been condemned as "distressingly unmanageable," (6) "unstable," (7) and "a series of inconsistent and bizarre results that [the Court] has left entirely undefended." (8) Treatises and casebooks struggle to explain the test. Most simply announce the outcomes in the Supreme Court's cases, (9) and some suggest that the only way to identify when an expectation of privacy is reasonable is when five Justices say so. (10) The consensus among scholars is that the Supreme Court's "reasonable expectation of privacy" cases are a failure. (11)

The chaos prompts an obvious question: why can't the Supreme Court settle on a single test for what makes an expectation of privacy "reasonable"? Many areas of law require courts to apply vague standards, such as whether a company was "negligent" (12) or a defendant's awareness of risk deviated from the standard of a reasonable person. (13) But the confusion over the reasonable expectation of privacy test is much deeper. Supreme Court opinions cannot even agree on what kind of test it is. Is it descriptive? Is it normative? Just what does it measure? The cases are all over the map, and the Justices have declined to resolve the confusion. (14)

This Article explains why the Supreme Court has not and cannot adopt a single test for what makes an expectation of privacy "reasonable." Because finding an expectation of privacy "reasonable" usually subjects the government's conduct to the warrant requirement, the doctrine needs to distinguish less troublesome police practices permitted without a warrant from more troublesome practices allowed only with a warrant or under special circumstances such as exigent circumstances or consent. The Supreme Court has not and cannot adopt a single test for when an expectation is "reasonable" because no one test effectively and consistently distinguishes the more troublesome police practices that require Fourth Amendment scrutiny from the less troublesome practices that do not.

There are two basic ways that courts could try to develop such a test, and neither approach works in practice. First, courts could identify a measurement that serves as a reliable proxy for whether a police practice requires regulation. Such proxies fail because the facts of police investigations prove too diverse; no one measurement accurately draws the line in all cases. Alternatively, courts could examine specific practices directly and decide whether they are troublesome enough to require Fourth Amendment regulation. This approach fails because it cannot be administered consistently by decentralized lower courts. Because each Fourth Amendment case involves a single discrete set of facts, courts must imagine each case as within a category of cases before determining whether that category of police practices is troublesome enough to require a warrant. This choice of category is entirely arbitrary, however, meaning that no two lower courts would be likely to agree on any given Fourth Amendment rule.

The failure of any one test to consistently distinguish police practices needing Fourth Amendment regulation from those that do not has led to the mixed system that exists today. Although the courts speak of a single "reasonable expectation of privacy" test, the one label masks several distinct but coexisting approaches. Four approaches predominate, together reflecting four different models of Fourth Amendment protection. The first three rely on proxies. The probabilistic model considers the likelihood that the subject's information would become known to others or the police. The lower the likelihood, the more likely it is that a reasonable expectation of privacy exists. The private facts model asks whether the government's conduct reveals particularly private and personal information deserving of protection. This approach focuses on the information the government collects rather than how it is collected. The positive law model considers whether the government conduct interferes with property rights or other legal standards outside the Fourth Amendment. When courts apply the positive law model, an expectation of privacy becomes reasonable when it is backed by positive law such as trespass. The fourth and final model, the policy model, reflects the direct approach. Courts applying the policy model focus directly on whether the police practice should be regulated by the Fourth Amendment.

Scholars and students of Fourth Amendment law find the current approach frustrating because the courts routinely mix and match the four models. Most Supreme Court opinions feature multiple models to varying degrees, and they often switch from model to model without recognizing the change. It's easy to see why the current approach is so widely condemned: at the Supreme Court level, the Justices pick from different arguments and can seem to justify almost any result by picking the model that best suits it. And yet there is no recognized meta-theory to the models, no single rationale that explains when some models should be used and others should not be. The result is a body of law that seems chaotic and confused and in need of major reworking.

But appearances can be deceiving. What at first looks like conceptual confusion turns out to be a much-needed range of approaches. Specifically, the use of multiple models has a critical advantage over the use of a single model: it facilitates a decentralized Fourth Amendment in which different models apply in different settings depending on which model best identifies practices in need of constitutional regulation in that setting. Lower courts can then incorporate the Supreme Court's choice of model through analogy, resulting in the predominance of particular models in particular types of cases. Indeed, it turns out that the Supreme Court's cases reflect this dynamic already, at least to a modest degree. The Court's emphasis on particular models seems to correlate reasonably well with the contexts in which those models accurately help identify police practices in need of constitutional regulation.

Greater awareness of the four models could facilitate this goal considerably. The appearance of confusion in the Supreme Court's cases partly reflects the incorrect assumption that there must be a single test for when an expectation of privacy is reasonable. Greater awareness of the need for multiple approaches can help judges select models in each case to better accomplish the goals of the reasonable expectation of privacy test. At the Supreme Court level, Justices should pick models by considering which models best identify practices in need of regulation in that setting. Lower court judges should in turn apply Supreme Court precedents with the Court's choice of model explicitly in mind.

The Article will proceed in two Parts. Part I introduces the four models, and it explains how they work and the Supreme Court cases in which each model appears. Part II explains why the Court has embraced all four models instead of one, and how a greater recognition of the models can help courts better use them to accurately and consistently identify which police practices should count as a Fourth Amendment "search."

1. THE FOUR MODELS

   The reasonable expectation of privacy test distinguishes investigative steps that the Fourth Amendment regulates from investigative steps that it does not regulate. If government conduct violates a reasonable expectation of privacy, then that conduct is a "search" and is legal only if justified by a search warrant or a specific exception to the warrant requirement such as consent or exigent circumstances. (15) On the other hand, if government action does not implicate a reasonable expectation of privacy, then the Fourth Amendment does not regulate it and investigators can take that step at any time without constitutional limitation. (16) As a result, the reasonable expectation of privacy test defines the line between unregulated investigative steps that can be used at any time from special investigative steps that must...