The Fourth Amendment to the U.S. Constitution protects individuals from unreasonable searches. Generally, law enforcement searches are considered to be reasonable when they are conducted pursuant to a warrant based upon probable cause. However, if the activity is not a "search" for purposes of the Fourth Amendment, a warrant is not required. Accordingly, courts have struggled to define what activities constitute a "search."
Prior to 1967, this determination turned on whether there had been physical intrusion. Surveillance without trespass was considered outside the scope of the Constitution (Olmstead v. United States, 1928). In Katz v. United States (1967), the U.S. Supreme Court abandoned the physical intrusion approach and expanded the protection of privacy rights. In Katz, the FBI, acting without a warrant, placed a "bug" on the outside of a public phone booth and recorded Mr. Katz's conversations. The Court held that the Fourth Amendment "protects people, not places" and found that what an individual "seeks to preserve as private" should be afforded constitutional protection. Justice Harlan, in concurrence, articulated a two-part test for whether a "search" has occurred. First, the individual must have a subjective expectation of privacy. Second, this expectation must be one that society recognizes as reasonable. The Court has applied this test in numerous cases since Katz, finding that frisks of persons (Terry v. Ohio, 1968) and luggage searches (Bond v. United States, 2000) constitute "searches" but that rummaging through garbage (California v. Greenwood, 1988), aerial surveillance of a home (California v. Ciraolo, 1986), and even enhanced aerial photography of an industrial complex (Dow Chemical Co. v. United States, 1986) do not.
Technology and privacy
Most recently, the Court considered the effect of advances in technology on privacy expectations (Kyllo v. United States, 2001). In Kyllo, an application for an arrest warrant was based on thermal images of a home demonstrating an abnormal quantity of infrared radiation (associated with grow lights used to cultivate marijuana). In considering whether use of the thermal imaging device was a "search," the Court recognized that subjective expectations of privacy may be diminished as technology advances: "It would be foolish to contend that the degree of privacy secured to citizens by the Fourth Amendment has been entirely unaffected by the advance of technology." Nevertheless, the Court held that some minimum expectation of privacy is presumed to be reasonable and that even the availability of sense-enhancing technology will not erode constitutional protection of this sphere. Accordingly, the Court concluded that "[w]here, as here, the Government uses a device that is not in general public use, to explore details of the home that would previously have been unknowable without physical intrusion, the surveillance is a search and is presumptively unreasonable without a warrant" (Kyllo v. United States, 2001).
The role of psychology
The Court's requirement that there be a "reasonable expectation of privacy" makes relevant empirical evidence of societal expectations of privacy (see Kagehiro & Taylor, 1988; Slobogin & Schumacher, 1993). Psychological research has identified a number of variables that influence the perceived intrusiveness of law enforcement actions. For example, people rate searches of their own property and roving suspicionless searches (e.g., random drug tests) as more intrusive than searches of others and searches with known evidentiary objectives. Moreover, some theorize (Slobogin & Schumacher, 1993) that individuals perceive searches as less intrusive where there is a strong presumption of guilt (e.g., when a suspect is handcuffed and detained), when the search relates to a more severe crime and when law enforcement motives are beneficent or protective (e.g., airport frisks in relation to a known terrorist threat).
In many areas, the Supreme Court's holdings have been consistent with research findings about what individuals find more intrusive (e.g., "bugging" a phone) and less intrusive (e.g., fly-overs). However, the research findings run counter to the Court's holdings in other areas. For example, research participants rate employer-conducted drug testing, the use of undercover agents or dogs, and requests that bus passengers consent to searches as highly invasive, but the Court has not recognized a substantial privacy interest in these situations.
A fertile area for future research will be to examine additional factors that may influence expectations of privacy. Research could examine, for example, the extent to which perceptions of intrusiveness are influenced by the nature of any measures taken to safeguard privacy, the nature of the place being searched, or the degree to which the search requires a physical intrusion. In addition, psychologists could investigate reactions to technological developments such as those raised in Kyllo. Whether, and in what ways, expectations of privacy change in response to technological developments, and whether such expectations depend on the degree to which the technology is available or the extent to which the technology enhances human senses are interesting questions to be pursued.