The application of the Fourth Amendment embraces two discrete questions. The first is whether the individual, by his conduct, has exhibited an actual (subjective) expectation of privacy, whether the individual has shown that he seeks to preserve something as private. The second question is whether the individual's subjective expectation of privacy is one that society is prepared to recognize as "reasonable," whether the individual's expectation, viewed objectively, is "justifiable" under the circumstances. Any claim of privacy must be "justifiable," "reasonable," or a "legitimate expectation of privacy" that has been invaded by government action.
After the victim of a robbery began receiving phone calls from the person who claimed to be the robber, the police installed a pen register, without a warrant, at the central telephone system in order to determine the identity of the numbers that petitioner, a suspect, was dialing. After the police discovered that petitioner had called the victim, they charged him with robbery. Petitioner alleged that use of the pen register constituted an illegal search within the meaning of U.S. Const. amend. IV. The trial court convicted the petitioner of robbery. His conviction was affirmed by the court of appeals, holding that there is no constitutionally protected reasonable expectation of privacy in the numbers dialed into a telephone system, and hence no "search" requiring a warrant under the Fourth Amendment with respect to the use of a pen register at a telephone company's offices.
Were petitioner's Fourth Amendment rights violated by the warrantless use of a pen register?
The Court determined that petitioner's U.S. Const. amend. IV rights were not violated. The Court found that petitioner did not have a legitimate expectation of privacy regarding the numbers he dialed on his phone because those numbers were automatically turned over to a third party, the phone company. The Court also ruled that even if petitioner did harbor some subjective expectation that the phone numbers he dialed would remain private, this expectation was not one that society was prepared to recognize as "reasonable."