Does the U.S. Constitution forbid the government from using illegally obtained evidence against a criminal suspect in court? The U.S. Supreme Court has ruled that it does.

In the landmark 1914 case of Weeks v. United States, the Supreme Court announced the far-reaching legal doctrine that has come to be known as the exclusionary rule, which generally bars the use in court of such illegally obtained evidence. Weeks arose after federal officials kicked down the door of a criminal suspect, scoured his home without a search warrant, and discovered a number of incriminating documents, which were later used against him at his federal trial. The Supreme Court said such evidence must be tossed out.

"If letters and private documents can thus be seized and held and used in evidence against a citizen accused of an offense," the Court ruled, "the protection of the Fourth Amendment declaring his right to be secure against such searches and seizures is of no value, and, so far as those thus placed are concerned, might as well be stricken from the Constitution."

Five decades later, in Mapp v. Ohio (1961), the Court extended the exclusionary rule to criminal trials held at the state level. "Presently, a federal prosecutor may make no use of evidence illegally seized," the Court observed, "but a State's attorney across the street may, although he supposedly is operating under the enforceable prohibitions of the same [Fourth] Amendment." The Mapp decision put an end to that federal-state discrepancy. "The fruits of an unconstitutional search," the Court declared, are now "inadmissible in both state and federal courts."

Today the exclusionary rule stands as a centerpiece of Fourth Amendment law and as a treasured doctrine among civil libertarians. Yet the rule has its critics. Prominent among them is the respected liberal Yale University law professor Akhil Reed Amar, who has denounced the exclusionary rule as "despicable and dispensable."

In Amar's view, the exclusionary rule should be scrapped because it "draws no strength from the deeply rooted American ideal of protecting innocent Americans from erroneous convictions." Instead, as he wrote in his 2012 book, America's Unwritten Constitution, "the rule perversely benefits the guilty…precisely because they were guilty—precisely because reliable evidence of their guilt had surfaced." Amar insists that "reliable evidence" of guilt should be admitted at trial, no matter how the government happened to obtain it.

He raises an interesting point. By excluding evidence of guilt, the rule does, by design, benefit the guilty. Isn't our system of justice supposed to be designed to protect the innocent?

But questions of guilt and innocence in such matters are not always as simple as Amar would have you believe. Consider the events that led to Mapp v. Ohio. In 1957, Cleveland police were hunting for a bombing suspect and wanted to search a particular two-family residence. Dollree Mapp lived on the top floor of that residence and she absolutely refused to let the authorities enter her home without a search warrant.

After a stand-off of several hours, the police forced the door and proceeded to ransack Mapp's home without a warrant. Among other things, the officers rifled through her dresser, her chest of drawers, her photo album, and her personal papers. Needless to say, the police failed to turn up any bombing suspects hidden inside the photo album or squeezed inside a dresser drawer.

But the police did discover some pornographic materials, and those materials happened to be illegal under state law at that time. Mapp was thus charged with possessing pornography and later convicted of that offense.

Mapp was guilty of possessing pornography. But at the same time, she was entirely innocent of harboring (or being) a bomber, the ostensible reason the police illegally invaded and pillaged her home in the first place. So perhaps the Supreme Court's 1961 ruling in Mapp's favor did protect an innocent American from government malfeasance after all.

As a replacement for the exclusionary rule, Amar suggests other ways to deter government violations of the Fourth Amendment, such as "tort suits against abusive officers, punitive damages to deter future misconduct, [and] proto-class-action devices enabling small-fry search victims to band together."

In theory, that sounds promising. When the cops violate your rights, it would be good to sue the cops for every cent. Unfortunately, there are a few practical impediments. One is a little thing called qualified immunity, which makes it extremely difficult for most victims of police misconduct to actually sue the police for their misdeeds. Another is a little thing called absolute immunity, which basically makes it impossible for most victims of prosecutorial misconduct to sue prosecutors for their misdeeds.

For better or worse, the exclusionary rule is one of the few protections now available against unconstitutional government searches.