A search is constitutional if it does not violate a person’s reasonable or legitimate

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 Interest Protected.—For the Fourth Amendment to apply to a particular set of facts, there must be a “search” and a “seizure,” occurring typically in a criminal case, with a subsequent attempt to use judicially what was seized.30 Whether there was a search and seizure within the meaning of the Amendment, and whether a complainant’s interests were constitutionally infringed, will often turn upon consideration of his interest and whether it was officially abused. What does the Amendment protect? Under the common law, there was no doubt. In Entick v. Carrington,31 Lord Camden wrote: “The great end for which men entered in society was to secure their property. That right is preserved sacred and incommunicable in all instances where it has not been taken away or abridged by some public law for the good of the whole. . . . By the laws of England, every invasion of private property, be it ever so minute, is a trespass. No man can set foot upon my ground without my license but he is liable to an action though the damage be nothing . . . .” Protection of property interests as the basis of the Fourth Amendment found easy acceptance in the Supreme Court32 and that acceptance controlled the decision in numerous cases.33 For example, in Olmstead v. United States,34 one of the two premises underlying the holding that wiretapping was not covered by the Amendment was that there had been no actual physical invasion of the defendant’s premises; where there had been an invasion—a technical trespass— electronic surveillance was deemed subject to Fourth Amendment restrictions.35

The Court later rejected this approach. “The premise that property interests control the right of the government to search and seize has been discredited. . . . We have recognized that the principal object of the Fourth Amendment is the protection of privacy rather than property, and have increasingly discarded fictional and procedural barriers rested on property concepts.”36 Thus, because the Amendment “protects people, not places,” the requirement of actual physical trespass is dispensed with and electronic surveillance was made subject to the Amendment’s requirements.37

The new test, propounded in Katz v. United States, is whether there is an expectation of privacy upon which one may “justifiably” rely.38 “What a person knowingly exposes to the public, even in his own home or office, is not a subject of Fourth Amendment protection. But what he seeks to preserve as private, even in an area accessible to the public, may be constitutionally protected.”39 That is, the “capacity to claim the protection of the Amendment depends not upon a property right in the invaded place but upon whether the area was one in which there was reasonable expectation of freedom from governmental intrusion.”40

Katz’s focus on privacy was revitalized in Kyllo v. United States,41 in which the Court invalidated the warrantless use of a thermal imaging device directed at a private home from a public street. The rule devised by the Court to limit police use of new technology that can “shrink the realm of guaranteed privacy” is that “obtaining by sense-enhancing technology any information regarding the interior of the home that could not otherwise have been obtained without physical ‘intrusion into a constitutionally protected area’ . . . constitutes a search—at least where (as here) the technology in question is not in general public use.”42 Relying on Katz, the Court rejected as “mechanical” the Government’s attempted distinction between offthewall and through-the-wall surveillance. Permitting all offthewall observations, the Court observed, “would leave the homeowner at the mercy of advancing technology—including technology that could discern all human activity in the home.”

Although the sanctity of the home has been strongly reaffirmed, protection of privacy in other contexts becomes more problematic. A two-part test that Justice Harlan suggested in Katz often provides the starting point for analysis.43 The first element, the “subjective expectation” of privacy, has largely dwindled as a viable standard, because, as Justice Harlan noted in a subsequent case, “our expectations, and the risks we assume, are in large part reflections of laws that translate into rules the customs and values of the past and present.”44 As for the second element, whether one has a “legitimate” expectation of privacy that society finds “reasonable” to recognize, the Court has said that “[l]egitimation of expectations of privacy by law must have a source outside of the Fourth Amendment, either by reference to concepts of real or personal property law or to understandings that are recognized and permitted by society.”45

Thus, protection of the home is at the apex of Fourth Amendment coverage because of the right associated with ownership to exclude others;46 but ownership of other things, i. e., automobiles, does not carry a similar high degree of protection.47 That a person has taken normal precautions to maintain his privacy, that is, precautions customarily taken by those seeking to exclude others, is usually a significant factor in determining legitimacy of expectation.48 Some expectations, the Court has held, are simply not among those that society is prepared to accept.49 In the context of norms for the use of rapidly evolving communications devices, the Court was reluctant to consider “the whole concept of privacy expectations” at all, preferring other decisional grounds: “The judiciary risks error by elaborating too fully on the Fourth Amendment implications of emerging technology before its role in society has become clear.”50

What seems to have emerged is a balancing standard that requires “an assessing of the nature of a particular practice and the likely extent of its impact on the individual’s sense of security balanced against the utility of the conduct as a technique of law enforcement.” Whereas Justice Harlan saw a greater need to restrain police officers through the warrant requirement as the intrusions on individual privacy grow more extensive,51 the Court’s solicitude for law enforcement objectives frequently tilts the balance in the other direction.

Application of this balancing test, because of the Court’s weighing of law enforcement investigative needs,52 and its subjective evaluation of privacy needs, has led to the creation of a two-tier or sliding-tier scale of privacy interests. The privacy test was originally designed to permit a determination that an interest protected by the Fourth Amendment had been invaded.53 If it had been, then ordinarily a warrant was required, subject only to the narrowly defined exceptions, and the scope of the search under those exceptions was “strictly tied to and justified by the circumstances which rendered its initiation permissible.”54 But the Court now uses the test to determine whether the interest invaded is important or persuasive enough so that a warrant is required to justify it;55 if the individual has a lesser expectation of privacy, then the invasion may be justified, absent a warrant, by the reasonableness of the intrusion.56 Exceptions to the warrant requirement are no longer evaluated solely by the justifications for the exception, e. g., exigent circumstances, and the scope of the search is no longer tied to and limited by the justification for the exception.57 The result has been a considerable expansion, beyond what existed prior to Katz, of the power of police and other authorities to conduct searches.

In United States v. Jones,58 the Court seemed to revitalize the significance of governmental trespass in determining whether a Fourth Amendment search has occurred. In Jones, the Court considered whether the attachment of a Global-Positioning-System (GPS) device to a car used by a suspected narcotics dealer and the monitoring of such device for twenty-eight days, constituted a search. Although the Court ruled unanimously that this month-long monitoring violated Jones’s rights, it splintered on the reasoning. A majority of the Court relied on the theory of common law trespass to find that the attachment of the device to the car represented a physical intrusion into Jones’s constitutionally protected “effect” or private property.59 While this holding obviated the need to assess the month-long tracking under Katz’s reasonable expectation of privacy test, five Justices, who concurred either with the majority opinion or concurred with the judgment, would have held that long-term GPS tracking can implicate an individual’s expectation of privacy.60 Some have read these concurrences as partly premised on the idea that while government access to a small data set—for example, one trip in a vehicle—might not violate one’s expectation of privacy, aggregating a month’s worth of personal data allows the government to create a “mosaic” about an individual’s personal life that violates that individual’s reasonable expectation of privacy.61 As a consequence, these concurring opinions could potentially have significant implications for the scope of the Fourth Amendment in relation to current and future technologies, such as cell phone tracking and wearable technologies that do not require a physical trespass to monitor a person’s activities and that can aggregate a wealth of personal data about users.62