In 1961, the Supreme Court established what has come to be known as the “exclusionary rule”. The case that established the rule was Mapp v. Ohio. This rule states that evidence seized in violation of the Fourth Amendment cannot be used as evidence against defendants in a criminal prosecution, state or federal.
To this day, some legal experts (pro prosecution) criticize the Mapp case on the ground that it unfairly “lets the criminal go free because the office has erred.” But supporters of Mapp argue that excluding illegally seized evidence is necessary to deter police from conducting illegal searches. According to this deterrence theory, the police won’t conduct improper searches if the resulting evidence is barred from the trial.
