Search and Seizure Research Paper

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In any free society, the police must be constrained. The constraint can come from a variety of sources—politics, bureaucratic culture, administrative sanctions, and so forth. It need not necessarily come from the law. And in most Western democracies, it does not come from the law; outside the United States, police seem to be regulated, where they are regulated, mostly through nonlegal means.

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For much of its history—until 1961, to be precise—the same held true of the United States.

Since that date, however, American law has played a very large role in regulating the police and reining in police misconduct. And the chief source of legal restraint is the law of search and seizure.

That law has three key features. First, it is constitutional. The basic standards that limit police investigation of crime—the standards that define when police can search a home, or seize a suitcase believed to contain drugs, or arrest a suspect for some crime—derive from the Fourth Amendment to the federal constitution. Because judges are the prime interpreters of the constitution, this means search and seizure law is basically judge-made. Because constitutional law is binding on popularly elected legislatures and executives, it means search and seizure law cannot be altered by elected politicians, state or federal. In the United States, to a degree that probably has no parallel elsewhere, judges—especially Supreme Court Justices—decide what rules the police must follow. Congress, state legislatures, and the police themselves must live with the rules these judges and Justices create.

Second, its chief business is protecting privacy. The dominant focus of the law of search and seizure is to limit what police can see and hear, to limit their ability to invade spaces people prefer to keep private. That is not the only interest the law protects, but it clearly is the interest that the law protects most. Other concerns—the potential for police violence, the harm to individual liberty that comes from arrest or street detention, discriminatory treatment of black and white suspects—receive much less attention from judges and Justices in Fourth Amendment cases.

Third, it is police-focused. Government gathers information about people in a variety of ways, through a variety of agents. Grand juries subpoena witnesses and documents; prosecutors interview suspects; administrative agencies inspect wetlands and workplaces. These things receive only slight legal regulation; with few exceptions Fourth Amendment law ignores them. That law’s clear focus is on police searches and arrests. It is not too much to say that Fourth Amendment law is a kind of tort law for the police; it is the body of civil liability rules that limit day-to-day police activities. Police must therefore pay close attention to Fourth Amendment rules; other government officials can usually ignore them.

The Fourth Amendment: Origins, Text, and History


Like most of the rest of the Bill of Rights, the Fourth Amendment has its origins in seventeenth- and eighteenth-century English common law. Unlike the rest of the Bill of Rights, the Fourth Amendment’s origins can be traced precisely—it arose out of a strong public reaction to three cases from the 1760s, two decided in England and one in the colonies.

The two English cases are usefully treated as a pair. Both Wilkes v. Wood, 19 Howell’s State Trials 1153 (C.P. 1763), and Entick v. Carrington, 19 Howell’s State Trials 1029 (C.P. 1765), involved pamphleteers charged with seditious libel for criticizing the king’s ministers and, through them, the king himself. In both cases, agents of the king issued a warrant authorizing the ransacking of the pamphleteers’ homes and the seizure of all their books and papers. (An aside is necessary at this point: Warrants are means of giving government officials permission to search or arrest someone whom they otherwise might not be allowed to search or arrest. In American practice, warrants are issued only by judges or magistrates after reviewing an application from a police officer. In eighteenth-century England, warrants were sometimes issued by agents of the Crown on their own initiative.) These searches were duly carried out. Wilkes and Entick sued for damages, claiming that the warrants were void and that the searches pursuant to them were therefore illegal. Both Wilkes and Entick won, with powerful opinions issued by Lord Camden, the judge in both cases. These decisions made Camden a hero in the colonies; a number of towns and cities were named after him because of his opinions in Wilkes and Entick.

The third case was the Writs of Assistance Case (see Dickerson, 1939). British customs inspectors seeking to stamp out smuggling in colonial Boston were given blanket search warrants, called writs of assistance, that permitted them to search anyplace where they thought smuggled goods might be. (The writs also allowed the inspectors to compel private citizens to help them carry out the searches—hence the writs’ name.) Some Boston merchants, represented by James Otis, sued, seeking a holding that the writs were invalid. The merchants lost, but Otis’s argument, with its ringing defense of individual privacy, became famous and strengthened opposition to British rule. John Adams later said of Otis’s argument that ‘‘then and there the child Independence was born.’’

Historians generally agree that the Fourth Amendment was designed to affirm the results in Wilkes and Entick, and to overturn the result in the Writs of Assistance Case. Three principles seem to follow. First, the government should not be allowed to search without some substantial justification, some reason to believe the place being searched contains the evidence being sought. That was the problem with the writs of assistance—they authorized searches based on no more than the unsupported suspicion of the inspector. Second, searches, particularly of private homes, should not go beyond their justification. That was the problem with the searches in Wilkes and Entick—the authorities did not simply search for and seize illegal writings, but took all the books and papers in the suspects’ houses. Third, the government should not use blanket warrants to evade the first two principles. That was a problem in all three cases. English common law held it a trespass to invade someone’s home without some kind of authorization; the warrants in Wilkes and Entick and the writs of assistance looked like efforts to evade that common law right. This explains why, at the time of the Founding era, search warrants—now viewed as a protection against police overreaching—were seen as more of a danger than a safeguard.

Notice that none of these three cases involved ordinary criminal law enforcement. None stemmed from the investigation of a murder, or a robbery, or a rape. Rather, each involved the investigation and prosecution of what might fairly be called dissidents—ordinary law-abiding citizens who disagreed strongly with the laws they were disobeying, and who enjoyed some substantial support among the citizenry. It is not at all clear from the Fourth Amendment’s history that James Madison and his contemporaries wished to restrict the investigation of ordinary crimes; indeed, it is not clear that they even thought about the investigation of ordinary crimes.

Notice, too, that none of these cases involved searches by people whom we would recognize today as police officers. Police forces did not exist in the eighteenth century, either in England or in the colonies. It follows that the Framers could not possibly have thought about how best to regulate them. The Fourth Amendment’s central role—reining in the police— is a role that it assumed much later. This point counsels in favor of a certain modesty when seeking to extract contemporary lessons from the Fourth Amendment’s historical context.

The Fourth Amendment’s Text

The Fourth Amendment, along with the other provisions of the Bill of Rights, was proposed by James Madison. The version that was ultimately ratified (Madison’s original version was slightly different) reads as follows:

The right of the people to be secure in their persons, houses, papers, and effects, against unreasonable searches and seizures, shall not be violated, and no Warrants shall issue, but upon probable cause, supported by Oath or affirmation, and particularly describing the place to be searched, and the persons or things to be seized.

The first clause—lawyers usually call it the ‘‘reasonableness clause’’—contains a simple prohibition: unreasonable searches and seizures are forbidden. It leaves the key term, ‘‘unreasonable,’’ undefined. The second clause, usually called the ‘‘warrant clause,’’ places a set of limits on the issuance of search or arrest warrants. Three limits are listed: the warrants must be supported by probable cause, they must define where the search is to take place, and they must define what the object of the search is—who or what is to be seized.

This text nowhere requires the government to get search or arrest warrants—the second clause limits the use of warrants, but never says when, if ever, the government must use them. So far as the text of the Fourth Amendment is concerned, the police apparently may search or seize without a warrant, as long as the search or seizure is reasonable. This is unsurprising given the Fourth Amendment’s origins. Madison and his contemporaries were chiefly concerned with preventing a recurrence of searches like the ones in Wilkes and Entick; the safest way to do that was to severely limit the use of warrants. Requiring them was apparently not on the Framers’ agenda.

Subsequent History to 1961

For a century and a half after it was ratified, the Fourth Amendment (like the rest of the Bill of Rights) applied only to the federal government; state and local police were not bound by it. During most of this period, federal criminal investigation and prosecution was rare—there was no F.B.I., and no army of federal prosecutors—so there was little opportunity for Fourth Amendment litigation. As a consequence, Fourth Amendment law basically lay dormant until Prohibition in the 1920s, which for the first time produced a large and active federal enforcement bureaucracy. By that time, three important changes had taken place. First, the Supreme Court had adopted the exclusionary rule (in Weeks v. United States, 232 U.S. 314 (1914)), which held that illegally seized evidence ordinarily could not be used in criminal trials. The source and rationale of that rule are discussed in a separate entry. Second, during the course of the nineteenth century search warrants had come to be seen as a way of limiting police authority, not as means by which the government could evade legal restriction. This is a natural development: once the Fourth Amendment placed stringent limits on warrants, requiring warrants became a good way to ensure that police had good reasons for searching. Accordingly, one sees frequent discussion in Prohibition-era cases of the importance of requiring police to get advance permission from a magistrate, in the form of a warrant, before searching. Third, probable cause had become the generally applicable legal standard for searches. ‘‘Reasonable’’ searches meant searches supported by probable cause—which meant, roughly, a fair likelihood that the evidence searched for would be found in the place searched.

Thus, by the end of the 1920s, Fourth Amendment law had assumed the following structure. Probable cause was required for all searches or arrests. A warrant, obtained in advance, was required at least for searches of homes, and possibly for many other searches as well. (Curiously, arrests for serious crimes were not thought to require warrants, a rule that still holds today.) And these rules were enforced primarily by an exclusionary rule, so that when the police violated the rules, any evidence they found would be inadmissible in a subsequent criminal trial.

These rules still applied only to the federal government. That state of affairs changed when, in Wolf v. Colorado, 338 U.S. 25 (1949), the Supreme Court held that the Fourth Amendment was part of the liberty protected by the Fourteenth Amendment’s due process clause against infringement by state and local officials. Twelve years later, in Mapp v. Ohio, 367 U.S. 643 (1961), the Supreme Court gave teeth to Wolf by imposing the exclusionary rule on the states. Henceforth local police, who are the primary enforcers of American criminal law, would be subject to the same search and seizure rules as F.B.I. agents, and to the same penalty for violating those rules.

It is not too much to say this worked a revolution in the way American police are themselves policed. Before 1961, local police were subject to state constitutional limits and could be sued for common law trespass (just like the offending officials in Entick and Wilkes). But these limits were illusory: state constitutions went unenforced, and common law claims against police officers virtually were never brought. Consequently, there was no working law of search and seizure, no body of rules that officers felt bound to obey, outside the federal system. Local police were restrained, if they were restrained at all, by local custom or politics. Law played no real part in their regulation.

This posed more of a danger to some suspects than to others. At the time Mapp was decided, it was widely (and surely correctly) believed that local police, especially in the South, treated black suspects much more harshly than white ones. And blacks could not protect their interests through the political process, because they were often either denied the right to vote or frozen out of governing coalitions. Although the opinions in Mapp do not make this point explicitly, it seems likely that one of the reasons—perhaps the primary reason—for the Supreme Court’s assertion of regulatory control over local police was the desire to protect black suspects from unfair treatment at the hands of nearly all-white police forces. In this way, Fourth Amendment law, which began as a tool for protecting upperclass pamphleteers and smugglers, had become a means of protecting a poor minority against oppression by police forces dominated by a middleclass (white) majority.

After 1961

The law Mapp imposed on local police forces was basically the same law that had been imposed on federal agents enforcing Prohibition in the 1920s: probable cause for searches and arrests, with warrants required for searches unless the police had a good excuse for not getting one. Perhaps because of a coincidence in timing—at about the time Mapp was decided, crime rates began skyrocketing, with the number of serious felonies trebling in the course of the next decade—these rules came to seem too burdensome for increasingly busy local police. (Rising crime also meant rising public hostility to the Supreme Court’s efforts to regulate the criminal process, which was seen as ‘‘handcuffing’’ police and prosecutors.) Beginning in 1968, the Supreme Court moved to relax these rules in two key ways. First, in Terry v. Ohio, 392 U.S. 1 (1968), the Court permitted police to ‘‘stop and frisk’’ suspects on the street based on reasonable suspicion of criminal activity, a lesser standard than probable cause. Terry involved suspicion of an about-to-be-committed robbery, but the Court soon applied its reasonable suspicion standard to past crimes and, most importantly, to drug crime. With these extensions, Terry meant that officers could briefly detain people, but not arrest them, based on fairly low-level suspicion of crime—the sort of suspicion that might come from spending time in the company of ‘‘known’’ drug dealers at places where drug trafficking is believed to be common.

The second change involved the warrant requirement. In a series of decisions stretching from the early 1970s to the early 1990s, the Court created or expanded various exceptions to the warrant requirement. For example, searches of cars were exempt, as were searches incident to arrest, as were inventory searches (these involved the inspection and cataloging of a suspect’s belongings when he is taken into custody). These various exceptions, taken together, meant that the warrant requirement applied to searches of houses and apartments, but almost never applied to anything else. For searches and seizures outside private homes, police were still bound by the probable cause or reasonable suspicion standards, but no advance permission to search was required.

The creation and expansion of ‘‘stop and frisk’’ doctrine and the contraction of the warrant requirement were both contentious; Fourth Amendment decisions in the 1970s and 1980s gave rise to some of the most heated arguments the Supreme Court has ever seen. Defenders of Fourth Amendment law’s classical structure, primarily Justices William Brennan and Thurgood Marshall, argued passionately that it was important to preserve probable cause, not the softer reasonable suspicion standard, as the primary standard for searches and seizures; they also argued for a broad warrant requirement to provide an extra check on police overreaching. But these arguments generally lost, and the structure that had emerged by the early 1990s is now fairly stable. The key characteristics of that structure are the subject of the next part.

The Current Structure of Search and Seizure Law

Search and seizure law today is built around three key questions. First, did the police ‘‘search’’ or ‘‘seize’’ anyone or anything? If not, the law leaves police action basically unregulated. If so, what justification must the police have— probable cause, reasonable suspicion, or (in rare cases) something else? Finally, what process must the police follow—must they seek permission in advance from a magistrate, or can they search first and defend themselves in a suppression hearing later?

The Definition Of ‘‘Search’’ and ‘‘Seizure’’

The most important of these questions may be the first one, for if a given police tactic is not a ‘‘search’’ or ‘‘seizure’’ within the meaning of the Fourth Amendment, the police are free to use that tactic when and on whom they wish, free of legal constraint.

‘‘Searches,’’ in Fourth Amendment law, are police tactics that infringe a ‘‘reasonable expectation of privacy.’’ A reasonable expectation of privacy is the kind of expectation any citizen might have with respect to any other citizen. A fair translation of this standard might go as follows: Police can see and hear the things that any member of the public might see and hear, without fear of Fourth Amendment regulation. Only when police cross that line, only when they see and hear things that members of the public would not be allowed to see and hear, has a ‘‘search’’ taken place.

A few examples might clarify the standard. Eavesdropping on telephone conversations is a ‘‘search.’’ Overhearing a conversation on the street is not. Climbing over a backyard fence is a ‘‘search.’’ Observing the same backyard from the window of an airplane is not. Hiding in the bushes outside a house and looking inside is a ‘‘search.’’ Standing on a public street and looking through open curtains into a living room is not. Opening a briefcase to inspect its contents is a ‘‘search.’’ Observing someone carrying a briefcase on the street is not.

When applying the reasonable-expectation-of-privacy standard, courts ask whether, at the moment the police officer observed the illegal behavior, he was in a position any member of the public might have been in. The duration and intensity of police observation does not matter. Thus, police officers can stake out a private home, taking up residence across the street and watching all comings and goings for a period of days or even weeks, and that behavior does not constitute a ‘‘search,’’ because at any given moment, any member of the public might have been looking. And police can follow a suspect’s movements along public streets, in shops or restaurants, and so forth; once again, such behavior is not a ‘‘search’’ (even though it amounts to something like stalking), because any member of the public might have seen any given transaction in public.

One other feature of the definition of ‘‘search’’ bears mention. Consensual transactions are not ‘‘searches,’’ even if consent was given under false pretenses. So if a police officer poses as a drug buyer, and a suspect lets him into the suspect’s house in order to sell him drugs, anything the officer sees and hears in the house is fair game—no Fourth Amendment ‘‘search’’ has taken place. The use of undercover agents is thus routinely permitted by search and seizure law, whether or not the police have good reason to suspect the person with whom the undercover agent is dealing of any crime. Also, if a police officer asks permission to look in a suspect’s car or briefcase, and the suspect says yes, once again no ‘‘search’’ has taken place. This last point is particularly important. Police officers exert a certain amount of force just by virtue of their status. For many, perhaps most, a request from a police officer will sound like a command; the tendency will be to say yes whether one wants to or not. Nevertheless, if the police officer asks, and the suspect says yes, that almost always amounts to consent. Only if the officer behaves unusually coercively— if he pulls his weapon, or grabs hold of the suspect, or the like—will a court find that the consent was involuntary.

All these rules sound complicated; in practice, they are relatively simple. In general, the police are ‘‘searching’’ when they are either committing some kind of trespass—grabbing a suspect’s briefcase and looking inside, breaking into a house or apartment, climbing over a backyard fence—or are engaged in some kind of electronic eavesdropping—for example, wiretapping a phone. Most of the rest of what police do to gather information falls outside the Fourth Amendment.

‘‘Seizures’’ are harder to define. The Supreme Court says that a suspect has been ‘‘seized’’ if a reasonable person in the suspect’s shoes would not feel free to leave. If the Court took its own language seriously, every conversation between a police officer and a citizen would be a ‘‘seizure.’’ After all, few people, when approached on the street by an officer, feel free to turn on their heels and walk away. The consequences of that position would be huge; the police would need some adequate justification for every interaction.

Not surprisingly, the law does not operate that way in practice. The working standard seems to be roughly the same as the standard for consent. The dispositive question is this: Did the police officer behave coercively (not counting the coercion that is inherent in a police officer questioning a suspect)? If so, the encounter is a ‘‘seizure.’’ If not, it is not. Compared to the definition of ‘‘search,’’ which has acquired a good deal of definition over the years, the definition of ‘‘seizure’’ remains remarkably vague and openended. Conversations on the street between police officers and citizens often begin as consensual conversations; at some point the encounter often becomes a ‘‘seizure.’’ Neither the officer nor the suspect—nor, for that matter, courts—know precisely when that point is.

Probable Cause and Reasonable Suspicion

When the police have searched or seized someone, the Fourth Amendment requires some justification. With rare exceptions, the justification takes one of two forms: either the police must have probable cause, or they must have reasonable suspicion. The following paragraphs explain what these standards mean, and to what cases each standard applies.

Probable cause has never received a clear definition in the cases; the Supreme Court has said, unhelpfully, that an officer has probable cause to arrest when ‘‘the facts and circumstances within [the officers’] knowledge and of which they had reasonably trustworthy information [are] sufficient in themselves to warrant a man of reasonable caution in the belief that an offense has been or is being committed’’ Brinegar v. United States, 338 U.S. 160, 175–76 (1949). What that means in practice seems to be, roughly, more likely than not. Thus, probable cause to arrest requires enough information to show that the suspect probably committed the crime. Probable cause to search requires enough information to show that evidence of crime can probably be found in the place to be searched.

One issue that regularly arises in the cases is whether police can rely on tips from informants to establish probable cause. Informants are sometimes ordinary citizens who call into police stations with useful information; more commonly they are criminals themselves who report information about other criminals in return for some favors—sometimes leniency, sometimes cash—from the police. Informants are common in police work, and especially in the policing of drug markets. (The leading study of search warrants shows that jurisdictions with the most drug cases also make the most use of informants’ tips.) Such tips are clearly useful to the police; they equally clearly pose significant problems. Informants may have some incentive to frame rivals, or to concoct whatever information will get them the greatest reward from the police. And since the identity of the informant is almost always confidential—for obvious reasons, informants tend to insist on anonymity—there is some risk that police will make up phony informants to justify their own hunches. For these reasons, the Supreme Court in the 1960s and 1970s placed serious restrictions on the use of informants’ tips to generate probable cause.

Those restrictions were largely removed in 1983, in Illinois v. Gates, 462 U.S. 213 (1983). The Court in Gates held, basically, that informants’ tips were to be treated like any other information. And the Court specifically approved findings of probable cause where an informant’s tip is partially corroborated—as where the informant says a suspect will be driving a certain car at a certain time and place and will be carrying drugs, and the officer confirms that the suspect was indeed driving that car at that time and place.

Like probable cause, reasonable suspicion is not well defined in the cases. Indeed, the only thing one can confidently say about it is that reasonable suspicion means something less than probable cause. Just how much less is unclear. A good indication of the kind and level of information required is the following common fact pattern. A police officer, on foot or in his car, turns a corner on a city street in an area known as a locale for drug trafficking. A young man standing on a street corner sees the officer, turns, and runs in the other direction. The officer knows nothing about the young man other than that he was in a place where crime was common, and that he ran when he saw the police. Does the officer have reasonable suspicion?

In Illinois v. Wardlow, 120 S.Ct. 673 (2000), the Supreme Court said the answer is yes. Likewise, if police see a suspect in a place where drugs are often sold, and if the suspect has a series of brief conversations with people the police know to be drug users, most courts would say the police have reasonable suspicion. In these cases the police have some ground for suspecting criminality, but not a great deal. Reasonable suspicion is meant to capture that intermediate condition. To what cases do these standards apply? Probable cause is the governing standard for arrests, and for searches of homes, cars, or personal effects. For arrests, the standard is probable cause to believe the suspect has committed a crime; for searches it is, with one important exception, probable cause to believe evidence will be found in the place sought. The exception is for searches ‘‘incident to arrest.’’ When the police make a valid arrest, they are permitted to search the suspect’s person, any belongings he has with him, and his car; if the arrest occurs at home, they may search the area within his immediate control. These searches incident to arrest are legal if the arrest is legal. Thus, probable cause to arrest authorizes not only arrests but also a good many searches.

Reasonable suspicion is the governing standard for brief stops, as where an officer detains a pedestrian for a few minutes or pulls over a car in order to ask the driver a few questions. Reasonable suspicion of the presence of a weapon is the standard for very brief searches, such as a pat-down or frisk of a suspect’s outer clothing. In practice, reasonable suspicion of the presence of a weapon tends to follow from reasonable suspicion of criminal activity. At least for serious crimes, judges tend to find that suspicion of the crime entitles the police to frisk the suspect in order to detect any weapons. The officer is, of course, entitled to check the contents of the pockets if this frisk turns up anything that might be a weapon. Naturally, the line between lumps that might be weapons and lumps that might be evidence of crime (drugs, for example) is a fine one. As a result, frisks for weapons tend to turn into frisks for either weapons or evidence.

The authority to frisk, or conduct a brief search, based on reasonable suspicion extends beyond the suspect’s clothing. If the suspect is in a car, the officer is entitled to briefly look through the car’s interior. If the suspect is carrying a purse or briefcase, the officer can briefly look inside, long enough to ensure that it does not contain a gun. And any evidence the officer finds during the course of such inspections is admissible.

Thus, given reasonable suspicion, officers can briefly detain pedestrians or pull over drivers of cars. Given reasonable suspicion of the kind of crime often associated with weapons, officers can briefly look around the car’s interior, frisk a suspect’s outer clothing, and open any containers (such as a purse or bag) that the suspect might reach to check for weapons. More extensive searches and seizures require probable cause.

Special Cases—Police Use of Force and Group Seizures

There remain two important categories of cases where neither probable cause nor reasonable suspicion applies. The first is police use of force, where the standard is higher than probable cause. The second is group seizures such as roadblocks, where the standard is lower than reasonable suspicion.

At common law, the police could use whatever force necessary to apprehend suspects and to bring them under control. If suspected of a felony, and if he fled, the suspect could be killed. These doctrines were still in force as late as the 1980s; not until 1985 did the Supreme Court decide that a higher standard was required for police use of force against suspects.

In that case, Tennessee v. Garner, 471 U.S. 1 (1985), the police shot and killed a fleeing burglary suspect; there was no reason to believe the suspect was armed or had threatened serious physical harm to anyone. The Court concluded that the shooting was unreasonable, and hence violated the Fourth Amendment. The Court ruled that deadly force was permissible only given some immediate danger of death or serious physical injury. Immediate danger is presumed if the police have probable cause to believe the suspect has caused or threatened death or serious injury, and if the suspect has been warned to stop but refuses to do so. The standard for using nondeadly force is both less demanding and less certain: the Supreme Court says only that officers must behave reasonably, taking account of the danger the suspect poses, the kind of crime he is suspected of committing, and the amount of force necessary to bring him under control.

These standards governing police use of force receive much less attention in the courts than the standards governing police frisks or house searches, because use-of-force claims are litigated much less frequently. The reason has to do with remedies. The primary remedy for Fourth Amendment violations is the exclusionary rule; the huge majority of Fourth Amendment claims arise as efforts by criminal defendants to suppress illegally obtained evidence. But when police beat or shoot a suspect, they are not looking for evidence. Thus, excessive force claims only rarely lead to efforts to suppress evidence; the exclusionary rule is essentially irrelevant to these claims.

Instead, excessive force claims arise either as claims for damages by the victim (or the victim’s next-of-kin), or as criminal prosecutions of the offending officers. The latter happen only very rarely. Damages claims are more frequent, but still amount to a very small fraction of the number of exclusionary rule claims. Which explains why the law of excessive force is so much less developed than, say, the law of car searches or street stops.

Group searches and seizures are the second category of cases that fall outside the probable cause and reasonable suspicion standards. Here the law is both complex and unclear. In United States v. Martinez-Fuerte, 428 U.S. 543 (1976), the Supreme Court permitted the use of highway checkpoints near the border to check for illegal immigrants; at these checkpoints all cars were stopped and all drivers seized, even though the government had no reason to suspect any particular driver of harboring illegal aliens. Similarly, in Michigan Highway Department v. Sitz, 496 U.S. 444 (1990), the Court ruled that officers may set up roadblocks to catch drunk drivers, even though the officers have no ground for suspecting any particular driver of any wrongdoing. But in Indianapolis v. Edmond, 121 S.Ct. 447 (2000), the Court barred the use of roadblocks designed to catch drug violators. For now, the government can apparently seize (and perhaps search) all members of a group in pursuit of goals other than ordinary criminal law enforcement (border control in Martinez-Fuente, highway safety in Sitz). But it cannot use power as a routine crime control tactic.

Drugs may fall on either side of this boundary. In Edmond, drug checkpoints were banned. But the result has been different in a number of cases involving drug testing. Government agencies sometimes seek to require periodic testing of employees, without any showing that any given employee was reasonably suspected of drug use or other misconduct. These testing programs have sometimes prevailed in the courts and sometimes not; the usual approach is to ask whether there is some ‘‘special need’’ to test the category of employees in question. The idea is similar to the idea behind Martinez-Fuerte and Sitz: In support of these testing programs, the government often claims suspicionless testing is permissible as long as many people are tested; that is, as long as no one person is singled out. If that argument were applied to tactics like group fingerprinting or DNA testing, it would considerably expand the scope of police authority. For now, Edmond is holding the line against such an expansion. But the line is unstable. The government can argue, with some force, that its interest in fighting drug crime is at least as strong as its interest in catching drunk drivers. If so, it seems odd that the police would be granted substantially greater power in the latter setting than in the former.

The Role of Substantive Law

Probable cause and reasonable suspicion both refer to a level of probability, a likelihood that some asserted fact— the suspect committed a crime, or evidence of crime will be found in a particular place—is true. The asserted fact always involves crime. It follows that the meaning of probable cause or reasonable suspicion depends on what counts as a crime. To put it another way, to say that the police have probable cause to arrest a given suspect is to say they have reason to believe that the suspect probably violated some criminal statute. Whether the claim is right depends in part on just what behavior criminal statutes forbid. If enough criminal statutes forbid enough conduct, the police will have probable cause to arrest a large portion of the population.

Something much like this was true before the late 1960s. Loitering statutes made it a crime to hang around on street corners; vagrancy statutes made it a crime to be able-bodied but unemployed, or to be a ‘‘rogue’’ or ‘‘vagabond.’’ These open-ended prohibitions plausibly covered a large fraction of ordinary street behavior, at least by people the police might wish to arrest or search. Consequently, the police had something close to blanket authority to arrest or search a large portion of the population. The probable cause standard mattered little.

In the late 1960s and early 1970s, most loitering and vagrancy statutes were declared unconstitutional on the ground that they were unacceptably vague. States and cities responded with a wave of statutes and ordinances criminalizing loitering with intent to commit acts of prostitution or drug use; some jurisdictions went farther, passing laws forbidding loitering in the presence of members of gangs that themselves were involved in various sorts of criminal activity. In Chicago v. Morales, 119 S.Ct. 1849 (1999), the Supreme Court struck down one such law, declaring that it was, like older loitering and vagrancy laws, too vague, and that it infringed on individuals’ liberty to wander about on public streets free of official interference.

Morales was not a Fourth Amendment decision; the Court relied on the due process clause of the Fourteenth Amendment instead. But the Court’s decision may have a large impact on the scope of Fourth Amendment protection. Again, if ‘‘crime’’ covers enough territory, police will have reasonable suspicion or probable cause with respect to most people, and Fourth Amendment standards will, as a practical matter, cease to operate. Morales may suggest that, at least with respect to pedestrians, the Court will not permit that state of affairs to recur.

The story is different when it comes to automobile traffic. In many states, moving violations—speeding, changing lanes without using a turn signal, running a stop sign, and the like— are, technically, crimes. Since such rules are not strictly enforced in most places (to the extent that speeding is the norm on many roads), most drivers are, technically, committing crimes most of the time, which gives the police authority to stop a large fraction of drivers.

That authority can be exercised strategically. If police believe a given suspect is transporting drugs, but they lack the kind of support needed to satisfy the probable cause or reasonable suspicion standards, they can wait for the suspect to run a stop sign, or speed, or violate some other traffic regulation, then pull the suspect over, arrest him for the traffic offense, and search him and his car incident to the arrest—all the while looking for drugs. In Whren v. United States, 517 U.S. 806 (1996), the Supreme Court held that this sort of pretextual police search is permissible. As long as the police have probable cause to believe the suspect is doing something the state defines as a crime, an arrest is legal, and if an arrest is legal, so is a search incident to arrest.

Whren and Morales are thus in some tension. Whren gives police near-blanket authority to stop vehicles; Morales seems to forbid near-blanket authority to stop pedestrians. In both cases, the primary determinant of the scope of police authority is not Fourth Amendment law, but the law that defines crimes.

The Warrant Requirement

All police searches and seizures are subject to legal challenge, but the challenge ordinarily comes after the fact. The officer searches, the defendant moves to suppress evidence found in the search, and the court holds a hearing to determine whether the search was legal. Where a search or arrest warrant is required, by contrast, judicial evaluation of the search or arrest happens in advance. The officer fills out a brief warrant application, with a sworn affidavit stating the facts in support of the application; a magistrate reviews the application, sometimes questioning the officer, and then decides whether to issue the warrant. If the warrant is issued and the search turns up evidence, the defendant can still seek to suppress it, but the court will give substantial deference to the magistrate’s judgment. Thus, Fourth Amendment law knows two procedures for testing the legality of a search: the warrant process before the search, and the suppression hearing after.

In form, the law requires the first of these procedures—warrants—unless some special exception applies. In practice, the exceptions swallow the rule. Warrants are not required for (among other things) arrests outside the suspect’s home, searches incident to arrest (which, remember, cover the suspect’s clothing, car, and belongings at the time of arrest), searches of cars whether or not anyone is arrested, and brief stops or frisks of suspects. These categories encompass the large majority of searches and seizures.

Warrants are required for wiretaps, for searches of homes, and for arrests that take place in a home. Even here, there is an exception for cases where ‘‘exigent circumstances’’ exist— where getting a warrant is impractical because of the danger that suspects will flee or evidence will be destroyed.

The warrant requirement thus applies chiefly when police wish to enter a private home. Elsewhere, police generally are allowed to search first, and face judicial scrutiny afterward. Fourth Amendment law purports to have a warrant requirement with a few narrow exceptions. The true rule is no warrant requirement, with an exception for homes and wiretaps.

In theory, this regime offers an extra measure of protection for house searches. Forcing police to ask permission in advance of a search, and requiring that permission come from a neutral magistrate, should weed out potential searches based on thinly supported police hunches. In practice, it is unclear how much the warrant requirement accomplishes. Most reviews of warrant applications are cursory, and magistrates only hear from one side—the police officer. Not surprisingly, most applications are granted. That, one might think, suggests warrants are something of a sham, a process by which discretionary decisions by police officers are rubber-stamped by magistrates. On the other hand, the leading study of the warrant process suggests the large majority of searches pursuant to warrants turn up evidence of crime. That, one might suppose, suggests warrants work, that they serve as an effective screen. The truth may be somewhere in between these two views, or it may be different in different jurisdictions.

Two Problems

The basic structure of search and seizure law seems fairly stable; large changes are unlikely, at least in the near future. In assessing that structure, it is helpful to focus on two issues. The first goes to the interests the law of search and seizure protects. The second goes to an interest that, for the most part, the law of search and seizure ignores.


The dominant focus of the law of search and seizure is protecting privacy. ‘‘Privacy’’ here has a particular meaning—it is not some generalized right to be let alone; rather, it is the interest in being free from observation, the interest in not being seen or heard. That the law protects privacy in this limited sense is shown by the cases that define ‘‘searches,’’ which cover only police conduct that permits officers to see or hear things that ordinary citizens would not be able to see or hear.

Notice that the interest in liberty receives less protection. Searches of private homes require probable cause and a warrant. Probable cause, with no warrant, suffices for an arrest outside the home—even though an arrest can lead to detention in the police station. The interest in being free from police violence receives, if not less protection, less attention, which may amount to the same thing: The number of excessive force claims brought against police officers is but a small fraction of the number of suppression motions based on allegedly illegal car searches.

The law’s focus on privacy sits uncomfortably together with its focus on regulating the police. Administrative agencies like the Internal Revenue Service arguably invade people’s privacy more than the police—think about the kinds of information people must supply on their tax forms. If one really wished to protect privacy, then, a natural way to do so would be to regulate with some care what questions the IRS can ask and how it can ask them. Yet Fourth Amendment law has almost nothing to say about those topics. At the same time, it has a great deal to say about questions like when the police can inspect the inside of a paper bag, or look inside the glove compartment of a car—trivial privacy invasions, one might think, compared with tax forms. In other words, search and seizure law protects privacy, but only when the police infringe it. That seems an odd way to protect privacy.

And protecting privacy may be an odd way to regulate the police. Police do two things that other government agents—grand juries, prosecutors, or administrative agencies—do not. Police arrest people, which means removing them from their homes and locking them up. And police beat, sometimes shoot, people as a means of obtaining and maintaining control over them. If one were to imagine a body of law whose goal was specially to regulate the police, one might expect that law to focus on those two things: on regulating police ability to deprive suspects of their liberty and, perhaps especially, on limiting police ability to injure or kill suspects. Fourth Amendment law does some of that. But it focuses more on privacy interests, on searches of homes and cars and paper bags, and less on other, perhaps more important goals.

Race Discrimination

One of those goals might be to eliminate police discrimination on the basis of race. African Americans suffer a disproportionate share of arrests and prison sentences. Much of that disproportion flows from differences in crime rates across population groups, but some of the disproportion may be a consequence of discriminatory targeting of suspects by the police. Perhaps surprisingly, Fourth Amendment law does little to stop that sort of discrimination. Given the breadth of criminal law, police have probable cause to arrest or reasonable suspicion to stop a large portion of the population—when it comes to vehicular traffic, a large majority of the population. Within this pool of potential suspects, police can target whom they wish; Fourth Amendment law basically says nothing about their exercise of enforcement discretion. So if police officers stop large numbers of black drivers, ostensibly for speeding but primarily to check for drugs, and stop few whites, the black drivers have no legal claim.

This is true notwithstanding the fact that a number of courts forbid the use of race as a factor in police ‘‘profiles.’’ Such profiles are common in drug investigations; they basically list factors common to drug couriers in particular markets at particular times. Officially, race is a forbidden factor, but officers can easily take race into account without acknowledging that they do so, and for now, the law tolerates that.

Another form of potential discrimination involves the targeting of some kinds of crimes, and some neighborhoods, more severely than others. In the late 1980s and early 1990s, urban crack markets received more police attention than suburban markets in cocaine powder. Most crack defendants were black; most cocaine powder defendants were white. One could argue that the strong tilt against crack was, on balance, a good thing; one could also argue that it was socially harmful, in part because of the racial composition of the two pools of defendants. Whichever answer is right, the current law of search and seizure leaves the question wholly to the police.

There may be no good alternative to that position. Courts are poorly positioned to direct the allocation of police resources across crimes and neighborhoods, and any serious effort to eliminate discriminatory policing would require precisely that. Still, it seems strange that Fourth Amendment law—the body of law most clearly devoted to regulating the police—has so little to do with what may be the most serious regulatory problem in the world of policing: stamping out race discrimination.


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