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COPYRIGHT 2007 Stanford Law School
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."
I. 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 be used only sparingly and in specific circumstances.
But what makes an expectation of privacy constitutionally "reasonable"? This Part argues that the Supreme Court's decisions include four equally viable answers to the question. There are four different models of Fourth Amendment protection--four relatively distinct categories of argument used to justify whether a reasonable expectation of privacy exists. (17) Two are normative and two are descriptive. Two are macro-scale and two are micro-scale. (18) Most opinions mix and match the four approaches, relying on multiple models in each opinion. As a result, observers often don't see the distinct types of claims. This Part clarifies the four existing models, and demonstrates that the Supreme Court sometimes embraces and sometimes rejects each of the four models as a guide to Fourth Amendment protection.
A. The Probabilistic Model
The first model of the Fourth Amendment is what I term the probabilistic model. According to this approach, a reasonable expectation of privacy depends on the chance that a sensible person would predict that he would maintain his privacy. The inquiry is descriptive rather than normative: it tries to assess the likelihood that a person will be observed or a place investigated based on prevailing social practices. (19) Under the probabilistic approach, a person has a reasonable expectation of privacy when the odds are very high that others will not successfully pry into his affairs. As those odds drop, the individual's expectation of privacy becomes less and less reasonable. (20) As a result, the Fourth Amendment protects citizens against unexpected invasions of privacy. When government conduct collects evidence in a way that interferes with customs and social expectations, revealing what a reasonable person might expect would remain hidden, it violates a reasonable expectation of privacy
Bond v. United States offers an example of the probabilistic approach. (21) A border patrol agent boarded a bus at the Texas-Mexico border and conducted a brief search for narcotics by walking the length of the bus and squeezing soft luggage placed in the overhead compartment. A squeeze of the defendant's canvas bag revealed what appeared to be a "brick-like" object stored inside, and the agent then opened the bag and found drugs. (22) In an opinion by Chief Justice Rehnquist, the Court held that the officer's "probing tactile examination" of the defendant's luggage violated his reasonable expectation of privacy. (23) The key was that the agent's probing had exceeded the usual handling common among bus passengers:
When a bus passenger places a bag in an overhead bin, he expects that other passengers or bus employees may move it for one reason or another. Thus, a bus passenger clearly expects that his bag may be handled. He does not expect that other passengers or bus employees will, as a matter of course, feel the bag in an exploratory mariner. (24)
The officer's conduct was a search because it was contrary to the reasonable expectations of bus passengers.
Minnesota v. Olson is another useful example. (25) Olson helped rob a gas station, and after the robbery he returned to a friend's duplex apartment where he was staying as an overnight guest. The police searched the apartment without a warrant and found Olson hiding in a closet. In an opinion by Justice White, the Court held that Olson had a reasonable expectation of privacy in the apartment. Such a rule "merely recognizes the everyday expectations of privacy that we all share," (26) Justice White explained. "Staying overnight in another's home is a longstanding social custom," (27) and the customary practice is for hosts to respect the privacy interests of their guests: "The host may admit or exclude from the house as he prefers, but it is unlikely that he will admit someone who wants to see or meet with the guest over the objection of the guest." (28) Olson had a reasonable expectation of privacy in the apartment because social customs and norms made it reasonable for him to expect others would not be admitted there.
The majority and dissenting opinions in California v. Ciraolo provide another illustration. (29) Ciraolo was growing marijuana in his backyard, and constructed a 10-foot fence around the property to block others from seeing it. The police borrowed an airplane, flew over the property at 1,000 feet, and took photographs of the marijuana plants growing in the backyard. The majority opinion by Chief Justice Burger suggested that the surveillance did not violate Ciraolo's reasonable expectation of privacy because aerial observation is common: "In an age where private and commercial flight in the public airways is routine, it is unreasonable for respondent to expect that his marijuana plants were constitutionally protected from being observed with the naked eye from an altitude of 1,000 feet." (30) In dissent, however, Justice Powell disagreed on the likelihood of observation: he argued that the chances were so low that Ciraolo's expectation was reasonable. (31) Although the two opinions disagreed on the outcome, they agreed on the proper inquiry: both opinions considered the likelihood that the suspect's property would be subject to observation by others.
Bond, Ciraolo, and Olson are just three among many Supreme Court cases relying on the probabilistic model. (32) They teach that a reasonable expectation of privacy is a descriptive expectation based on norms and prevailing social practices that others will not observe what the individual seeks to protect as private. Whether an expectation of privacy is reasonable depends on the expectations of a reasonable person. Much like Learned Hand's famous negligence formula in Carroll Towing measured the probability that a harmful event might occur, (33) so a reasonable person might measure the probability that particular information will be revealed. (34)
So far, so good. But there's a wrinkle: for every case in which the Court endorses the probabilistic model, you can find several others flatly rejecting it. In many cases, the Supreme Court has dismissed the probabilistic model as simply incorrect as a matter of basic Fourth Amendment law. Consider the Supreme Court's recent decision in Illinois v. Caballes. (35) Caballes was stopped for speeding, and the officer brought a drug-sniffing dog to the scene. When the dog alerted the officer to the presence of drugs in the trunk, the officer searched the trunk and found marijuana. The Supreme Court held that the use of the dog to alert for the presence of drugs was not a search. According to Justice Stevens' majority opinion, the chance that the police would find out about the drugs in the trunk was completely irrelevant to the Fourth Amendment inquiry: "[T]he expectation 'that certain facts will not come to the attention of the authorities' is not the same as an interest in 'privacy that society is prepared to consider reasonable."' (36) As the Court emphasized twenty years earlier in United States v. Jacobsen, "The concept of an interest in privacy that society is prepared to recognize as reasonable is, by its very nature, critically different from the mere expectation, however well justified, that certain facts will not come to the attention of the authorities." (37)
The Supreme Court has also rejected the probabilistic model in its many cases on misplaced confidences. In these cases, the defendants gave private information to a friend or business associate on the assumption that such information would remain a secret. The friend then gave the information to the police, either because he was a confidential informant, (38) he was wearing a wire on the government's behalf, (39) or investigators served him with a subpoena. (40) In all of these cases, the Court held that providing information to the third party eliminated any reasonable expectation of privacy no matter how unlikely it was that the friend would betray the suspect's confidence. As the Court summarized in United States v. Miller:
[T]he Fourth Amendment does not prohibit the obtaining of information revealed to a third party and conveyed by him to Government authorities, even if the information is revealed on the assumption that it will be used only for a limited purpose and the confidence placed in the third party will not be betrayed. (41)
The defendant assumes the risk, no matter how small, that the information will end up in the hands of the police. (42)
Finally, consider United States v. Ross, which involved the search of a paper bag found in the suspect's car. (43) The Court's opinion emphasized that the ease of opening the bag was completely irrelevant to whether it supported a reasonable expectation of privacy. According to the Court, "the central purpose of the Fourth Amendment" foreclosed a distinction between containers that were "worthy" and "unworthy" of protection:
For just as the most frail cottage in the kingdom is absolutely entitled to the same guarantees of privacy as the most majestic mansion, so also may a traveler who carries a toothbrush and a few articles of clothing in a paper bag or knotted scarf claim an equal right to conceal his possessions from official inspection as the sophisticated executive with the locked attach6 case. (44)
Whether a reasonable person would expect privacy in his bag is irrelevant, much like every home receives protection regardless of whether it is an isolated mountain retreat or a frequently burglarized apartment in an urban neighborhood.
In sum, the probabilistic model provides only an occasional guide to whether an expectation of privacy is reasonable. Sometimes the Court relies on it, but often the Court flatly rejects it and looks elsewhere.
B. The Private Facts Model
The second model of Fourth Amendment protection is what I call the private facts model The private facts model focuses on the information the government collects, and considers whether that information is private and worthy of constitutional protection. If the government obtains information that is particularly private, then the acquisition of that information is a search; if the information collected is not private or does not otherwise merit protection, then no search has occurred. The key question becomes what information the government collected rather than how it was obtained or whether the government's conduct was unexpected.
United States v. Jacobsen offers a helpful example. (45) In Jacobsen, a cardboard box sent via Federal Express broke open during delivery. A white powder seeped out, and an FBI agent performed a chemical field test of the powder to determine if the powder was cocaine. The field test returned a positive result, leading to criminal charges against the package recipient. In an opinion by Justice Stevens, the Supreme Court held that a field test for narcotics could not violate a reasonable expectation of privacy. A field test "could disclose only one fact," (46) whether the powder was cocaine. But that fact could not be a "private" fact, the Court concluded. (47) If the test returned a negative result, then "such a result reveals nothing of special interest." (48) And if the test returned a positive result, then it merely revealed that a crime had been committed because the possession of cocaine is a crime. (49) Because the field test could only reveal evidence of a crime, "and no other arguably 'private' fact," it could not violate any expectation of privacy that was constitutionally "legitimate.' (50)
Dow Chemical Co. v. United States reflects a similar approach. (51) The Environmental Protection Agency (EPA) hired a commercial photographer to take aerial photographs of a chemical plant to identify violations of environmental protection laws. The owner of the chemical plant, Dow Chemical, brought a civil suit claiming that the photography violated the Fourth Amendment. In an opinion by Chief Justice Burger, the Court rejected the Fourth Amendment challenge based in part on the limited information the photography revealed: "[T]he photographs here are not so revealing of intimate details as to raise constitutional concerns. Although they undoubtedly give EPA more detailed information than naked-eye views, they remain limited to an outline of the facility's buildings and equipment." (52) The photographs revealed some information but did not reveal anything important or intimate. Therefore it was not a search.
Of course, like the other models, the private facts model works both ways: it can be used either to deny Fourth Amendment protection or to justify it. Consider United States v. Karo, in which Drug Enforcement Agency (DEA) agents placed a tracking device inside a can of chemicals used to extract cocaine from materials imported into the United States. (53) The tracking device revealed the location of the can, and this showed that the can had been brought inside a private home. In an opinion by Justice White, the Court held that using the device to obtain information about the inside of a home violates...
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