How did Mapp v. Ohio extend the exclusionary rule to state criminal proceedings?

In Mapp v. Ohio (1961), the U.S. Supreme Court extended the exclusionary rule—which prohibits the use of evidence obtained in violation of the Fourth Amendment—to all state criminal proceedings.12

Prior to this decision, the Court had held in Wolf v. Colorado (1949) that while the right to privacy was protected against state intrusion, the exclusionary rule itself was not a mandatory remedy for state courts. In Mapp, the Court utilized the incorporation doctrine, reasoning that because the Fourth Amendment’s protection against unreasonable searches and seizures is guaranteed against the states through the due process clause of the Fourteenth Amendment, the exclusionary rule must also apply.12

The Court concluded that the rule is the only effective way to compel respect for constitutional privacy rights. Without it, the Fourth Amendment would be reduced to a mere "form of words," rendering the right to privacy unenforceable. Consequently, the Court held that evidence seized unconstitutionally by state police is inadmissible in state trials.2