The Model Penal Code did not expressly adopt the rule of lenity. Instead, it requires that criminal statutes be “construed according to the fair import of their terms.” In cases involving ambiguous language, however, it directs courts to construe statutory language to further both the general purposes of the criminal law and the specific purposes of the statute under consideration.
The United States Supreme Court has consistently struck down criminal laws that are so vague that ordinary people could not reasonably determine their meaning and application from the language of the statute or that confer excessive discretion on law enforcement authorities to arrest or prosecute or on judges and juries to determine what conduct is prohibited. The “void for vagueness” doctrine is based on the due process clauses of the Fifth Amendment when a federal statute is involved, and on the Fourteenth Amendment when a state statute is involved. It helps ensure that the American criminal law implements the principle of legality.
The doctrine ensures that criminal statutes provide fair notice of what behavior is forbidden. It requires the legislature to define the elements of the crime clearly in advance rather than require the judiciary to do so retroactively. The vagueness doctrine also prevents police from arbitrarily choosing which persons they will arrest. Finally, it helps ensure a consistent and equal application of the criminal law.
Void for vagueness does not preclude the legislature from passing a criminal law to accomplish a legitimate law enforcement goal. It simply requires the legislature to use clear and focused language. Of course, it is not always clear when a law is too indefinite so as to be unconstitutional. Courts are more likely to strike down laws as unconstitutionally vague when they are very general in scope, are overly broad or too readily reach innocent behavior (especially if the First Amendment is involved), and confer very broad discretion on police officers to arrest whom they choose (especially if racial discrimination appears to be involved). Thus, in Papachristou v. City of Jacksonville, the Supreme Court struck down a broadly worded vagrancy ordinance because it gave the police “unfettered discretion” to decide whom to arrest. Justice Douglas noted: “The rule of law, evenly applied to minorities as well as majorities, to the poor as well as the rich, is the great mucilage that holds society together.” More recently, in City of Chicago v. Morales, the Court agreed with the Illinois state supreme court that a Chicago ordinance that prohibited criminal street gang members from “loitering” with other gang members or non-members was unconstitutionally vague because it failed to give ordinary citizens adequate notice of what conduct is criminal and conferred too much enforcement discretion on police officers.