Initially, the applicability of the Fourth Amendment was governed by property concepts such as common law trespass. There needed to be an actual physical intrusion into a constitutionally protected area such as a home. The Supreme Court held that the tapping of a telephone wire (Olmstead v. United States, 1928) or a detechtaphone, a listening apparatus in the wall (Goldman v. United States, 1942), did not constitute a physical intrusion. On the other hand a spike mike inserted into the crevice of a heating duct constituted a physical penetration of the home and thus implicated the Fourth Amendment (Silverman v. United States, 1961).
As Fourth Amendment jurisprudence developed, however, the Court moved from an approach based on property law concepts to a more comprehensive theory of privacy. In Katz v. United States (1967), federal agents placed a listening device on the outside wall of a public telephone booth to listen to the defendant's conversation. Following common law trespass doctrine, the Ninth Circuit held that because there was no actual penetration of the booth, there was no trespass, and therefore no search. The Court reversed, ruling that because the Fourth Amendment “protects people, not places, ” it no longer made sense to rely on the antiquated property law concept of trespass.
Justice John Marshall Harlan's concurring opinion in Katz, relied on by later decisions of the Court, has become the basis for analyzing the rationale established by the decision. Harlon accepts the majority's ruling that the Fourth Amendment protects “people not places.” He asks, however, “[w]hat protection [does] it afford to those people?” He then suggests a twofold analysis. First, did the individual exhibit an actual expectation of privacy? Second, was the expectation of privacy one that would be deemed reasonable by society? According to