The reasonable expectation of privacy is crucial in distinguishing a legitimate, reasonable police
search and seizure from an unreasonable one. A "search" occurs for purposes of the Fourth Amendment when the Government violates a person's "reasonable expectation of privacy". In
Katz v. United States,
Justice Harlan issued a
concurring opinion articulating the two-prong
test later adopted by the
U.S. Supreme Court as the test for determining whether a police or government
search is subject to the limitations of the
Fourth Amendment: • Governmental action must contravene an individual's actual, subjective expectation of privacy • Expectation of privacy must be reasonable, in the sense that
society in general would recognize it as such To meet the first part of the test, the person from whom the information was obtained must demonstrate that they,
in fact, had an actual, subjective expectation that the
evidence obtained would not be available to the public. In other words, the person asserting that a search was conducted must show that they kept the evidence in a manner designed to ensure its privacy. The first part of the test is related to the notion "
in plain view". If a person did not undertake reasonable efforts to conceal something from a casual observer (as opposed to a snoop), then no subjective expectation of privacy is assumed. The second part of the test is analyzed
objectively: would society at large deem a person's expectation of privacy to be reasonable? If it is plain that a person did not keep the evidence at issue in a private place, then no
search is required to uncover the evidence. For example, there is generally no search when police officers look through garbage because a reasonable person would not expect that items placed in the garbage would necessarily remain private. An individual has no legitimate expectation of privacy in information provided to third parties (
third-party doctrine). In
Smith v. Maryland, 442 U.S. 735 (1979), the
Supreme Court held individuals have no "legitimate expectation of privacy" regarding the telephone numbers they dial because they knowingly give that information to telephone companies when they dial a number. Therefore, there is no search where officers monitor what phone numbers an individual dials, although the
Congress has enacted laws that restrict such monitoring. The Supreme Court has also ruled that there is no objectively reasonable expectation of privacy (and thus no search) when officers hovering in a
helicopter 400 feet above a suspect's house conduct surveillance. The
U.S. Court of Appeals for the Sixth Circuit held in 2010 that users did have a reasonable expectation of privacy in the contents of their e-mail in
United States v. Warshak, although no other court of appeals has followed suit. == In cyberspace ==