Mapp marked the final incorporation of the Fourth Amendment into the Due Process Clause of the Fourteenth. It also extended the evidence Exclusionary Rule, originally discovered in Weeks v. U.S. in 1914, to state prosecutions.

Mapp's Cleveland, OH home was entered by several police officers in search of a fugitive alleged to be hiding there, as well as for gambling paraphernalia. They claimed to have a valid search warrant, but it was not produced during the search. While they didn't find what they were looking for, they did supposedly find obscene materials, and Mapp was tried and convicted for possession of obscenity. While the judge acknowledged that the search was illegal, he admitted the evidence discovered, using as his reasoning a Supreme Court decision in Wolf v. Colorado (1949), in which the Court stated that the Weeks Exclusionary Rule did not apply to the states.

Mapp's attorneys originally appealed the case on the basis of the obscenity charge. When the ACLU filed an amicus brief, it added that the abusive search procedure should also present an opportunity to review the Wolf decision -- an opportunity it obviously felt would be well received by the Court.

In deciding for Mapp, the Supreme Court was able to reach a plurality decision on the outcome, but unable to present a majority opinion on the logic for reaching it. As such, while the Mapp decision is clear in its effect -- that the Fourth Amendment, through the agency of the Due Process Clause of the Fourteenth, requires all illegally gained evidence to be tossed out of State and Federal court proceedings -- the arguments supporting the outcome do not provide a lot of comfort that the Court was clear in its constitutional vision.

In his plurality opinion, Justice Tom C. Clark -- who had always believed that Wolf would be found lacking -- reiterated the arguments originally made in Weeks, pointing out Justice Oliver Wendell Holmes' opinion that, without the Exclusionary Rule, the Fourth Amendment is reduced to "a form of words". He responded to then-Judge Benjamin Cardozo's concern that a "constable's blunder" would set a criminal free, with "the criminal goes free if he must, but it is the law that sets him free."

Justice Potter Stewart, who had taken the position in an earlier case that the Due Process Clause of the Fourteenth in and of itself should be sufficient to afford protection against illegal searches at the state level, would not join the opinion in Mapp because the exclusionary issue had not been properly briefed and argued in the case.

The three dissenting justices all resisted the incorporation doctrine generally and the idea of infringing the rights of state judicial systems.

One man -- Justice Hugo Black -- found himself as the swing vote in the case, but his was yet another line of logic. When Wolf was decided, it was Justice Black alone who wondered whether the Fourth Amendment required an exclusionary rule at all. In Mapp, he disagreed with the plurality assessment and opined that the Rule could only be justified by the Fourth and Fifth Amendment together. As a firm supporter of incorporation doctrine, he agreed to apply the Rule to the states, but not with the logic in arriving there.

Logically, all evidence should be admitted at court, and offending police officers or detectives held accountable for their actions. This logic was not lost on the Court in Mapp, but Justice Clark noted in his opinion that states without exclusionary rules had generally failed to develop alternative methods for preventing illegal searches, and that several state supreme courts had responded by mandating their own exclusionary rules.

Recently there has been some turbulence in the criminal justice community to roll the Exclusionary Rule back under certain conditions -- specifically as regards the 1966 Miranda decision -- regarding it as too much of an impediment to the pursuit of criminals. While there seems to be some support on the Court for the concept, how carefully the rollback is defined will of course determine whether it can pass constitutional muster. Reflecting our legal logic today, it seems foolish to punish the "thing" -- in this case the evidence -- rather than the actual perpetrator -- in this case the conductor of a clearly illegal search.