In this landmark decision, the Supreme Court held that the exclusionary rule was binding on the states whenever evidence was obtained in an unreasonable search and seizure, thereby completing the “incorporation” of the Fourth Amendment into the due process clause of the Fourteenth Amendment.

In an earlier landmark case, Weeks v. United States[c]Weeks v. United States (1914), the Supreme Court held that evidence obtained contrary to the principles of the Fourth Amendment would be inadmissible in federal criminal trials. The purpose of this controversial exclusionary rule was to protect people from police misconduct. Since the Fourth Amendment was not binding on the states at that time, the ruling had no impact on state trials, in which most criminal prosecutions took place.

Justice Charles Whittaker was among the justices who dissented in Mapp because they objected to applying the incorporation doctrine.

(Collection of the Supreme Court of the United States)

In another landmark case, Wolf v. Colorado[c]Wolf v. Colorado (1949), the Court unanimously required that the states must respect the “core” principle of the Fourth Amendment, which is freedom from unreasonable searches and seizures. By a 6-3 vote, however, the Court also held that the exclusionary rule was not binding on the states because it was not “an essential ingredient” of the amendment. The decision resulted in the so-called “silver platter doctrine,”Silver platter doctrine which permitted federal prosecutors to use evidence obtained illegally by the states, until 1960.

Seven police officers, claiming to have a warrant but never showing it, broke into the home of Dolly Mapp in Cleveland, Ohio. An informant had told them that they would find gambling paraphernalia and a fugitive wanted for a recent bombing. Although they found neither, they did find pornographic books and magazines. These items were used as evidence to convict Mapp of possessing illegal pornographic materials. Ohio’s high court concluded that the incriminating evidence had been obtained illegally, but while referring to Wolf, it nevertheless allowed the evidence to be used. When the case was appealed to the U.S. Supreme Court, an amicus brief of the American Civil Liberties Union advocated that Wolf be overturned, although Mapp’s attorney argued the case entirely on the issue of obscenity.

The majority of the justices decided to ignore the less important obscenity question. Five justices agreed that the exclusionary rule should be applied to the states. Speaking for the five, Justice Tom C. ClarkClark, Tom C.;Mapp v. Ohio referred to the rule as a “deterrent safeguard,” without which the right might exist in theory, but not in reality. Observing that half of the sates already had adopted the exclusionary rule, he wrote that there were no other practical means to prevent the police from conducting unreasonable searches. Concerning an earlier justice’s complaint, “the criminal is to go free because the constable has blundered,” he answered, “The criminal goes free if he must, but it is the law that sets him free.” Justice Potter StewartStewart, Potter;Mapp v. Ohio refused to join the majority because he wanted to base the decision on the First Amendment. The three dissenters rejected the incorporation doctrine, which in their view intruded upon the states’ sovereignty.

Clark, Tom C.

Due process, procedural

Douglas, William O.

Exclusionary rule

Ferguson v. City of Charleston

Fourteenth Amendment

Fourth Amendment

Hudson v. Michigan

Incorporation doctrine

Search warrant requirement