Law enforcement—and indeed the entire criminal justice system—is governed by procedural law. Procedural law is a body of laws for how things should be done at each step of the criminal justice process. Procedural law includes court procedures, such as rules of evidence, and police procedures, which involve such things as search and seizure, arrest, and interrogation (Fagin, 2007).
Rules of Evidence
Rules of evidence stipulate the requirements for introducing evidence and define the qualifications of an expert witness and the nature of the testimony he or she may give (Fagin, 2012). Rules of evidence affect police officers’ conduct because collecting evidence is part of their job. The rules state that evidence gathered through immoral, illegal, or unconstitutional means should not be used as evidence in a trial.
Evidence is declared inadmissible under the exclusionary rule, and the rule prohibits the use of evidence or testimony obtained in violation of the Constitution. The origins of the exclusionary rule can be traced back to a 1914 case heard by the U.S. Supreme Court. The case was Weeks v. United States (1914), and the court ruled that evidence against Freemont Weeks, who was accused of transporting lottery tickets through the mail, was obtained without a search warrant. Weeks took action against the police and petitioned for the return of his private possessions. The conclusion of the Supreme Court was unanimous. The court held that the seizure of items from Weeks’s residence directly violated his constitutional rights. The court also held that the government’s refusal to return Weeks’s possessions violated the Fourth Amendment. To allow private documents to be seized and then held as evidence against citizens would have meant that the protection of the Fourth Amendment, declaring the right to be secure against such searches and seizures, would be of no value whatsoever. This was the first application of what eventually became known as the exclusionary rule (Bohm and Haley, 2012).
In 1961, in Mapp v. Ohio, the Supreme Court extended the exclusionary rule by requiring that state courts use the rule—just as it was used in federal court proceedings. Mapp v. Ohio (1961) was a case involving a woman by the name of Dolree Mapp. The police received a tip from an informant that there was evidence at her home related to a bombing suspect. The police arrived at Ms. Mapp’s house and asked for permission to search her house. Ms. Mapp refused them permission and they left. Later, they returned, and this time they said they had a search warrant and they waived a blank piece of paper at her.
The police proceeded to search her home. They did not find any evidence of a bombing suspect, but they did find some obscene material. Ms. Mapp was arrested and was convicted in state court of possession of obscene materials (Fagin, 2007). Ms. Mapp appealed her conviction on the basis that her Fourth Amendment rights had been violated. The Supreme Court ruled that the same standards as in Weeks v. United States applied to defendants in state courts. That is, illegally obtained evidence would be inadmissible. As a result, Ms. Mapp’s conviction was reversed.