In law, the rules of statutory interpretation govern how judges and others in the legal system should read the language in laws, regulations, and ordinances. The primary rule of statutory interpretation is that a law should be read for its “plain meaning” first of all. If the “plain meaning” of the law leaves it confusing, ambiguous, or vague, however, other rules of statutory interpretation are called upon to clarify the law’s meaning.
Some of the more commonly-used rules of statutory interpretation include:
- The legislature meant what it said. The rules of statutory interpretation assume that the legislature intended to insert every single word it used into the law it wrote and that the legislature used the words it meant to use. No words are meaningless or superfluous.
- Words should be “of the same kind.” Known by its Latin name, “ejusdem generis,” this rule states that an ambiguous word should be given a precise meaning that fits with the other words around it, if possible. For instance, if a statute prohibits people from carrying “pistols, revolvers, derringers, or other dangerous weapons, the phrase “dangerous weapons” should mean the same kind of weapons as the others in the list – here, firearms or handguns.
- Words should be understood by comparing them to words around them. This rule, known by the Latin name “noscitur a sociis,” is similar to the “of the same kind” rule above (“noscitur a sociis” means “it is known by its associates.”
- The presence of one word excludes any that are not present. The Latin phrase for this rule is “Expressio unius est exclusio alterius,” sometimes shortened to “expressio unius.” This rule states that, if a certain word is included in a statute, a related word not included in the statute is presumed to have been left out intentionally. For instance, an ordinance that prohibits riding a bicycle “on public streets, curb lawns, or driveways” is assumed to allow riding a bicycle on public sidewalks, since it does not expressly state sidewalk-riding is prohibited.
- Laws should be read together with other laws that cover the same subject. This rule is known as “in pari materia,” and it states that a law on a certain subject can be compared with earlier laws on the same subject in order to determine what is meant by the later law. For instance, a statute that says bicycle helmets must be worn when riding “on any state road” can be read in pari materia with an earlier statute that defines “state roads” as any paved road called “State Road” followed by a number to determine on precisely which roads bicycle helmets must be worn.
In addition to these rules for examining the words in statutes, some rules of statutory interpretation deal with the contents of laws. These are called “substantive” rules of statutory interpretation. For instance, the “Charming Betsy” rule (named after the 1804 case involving a boat called the “Charming Betsy”) states that state and federal laws should not be read so that they conflict with international laws. Statutes should also be read so that they do not conflict with “fundamental values” and so that they do not violate state sovereignty. Also, courts are expected to read statutes so as to avoid a constitutional question when possible (assuming that the legislature meant to act constitutionally), and to avoid an absurd or obviously unjust result (assuming that the legislature meant to act rationally).
A law that makes no sense even after it has been analyzed under the rules of statutory interpretation may be declared constitutionally void for vagueness because not even a careful analysis of the law could lead the average person to understand what behavior the law governs and what the penalties, if any, are for breaking that law.
Criminal laws that have multiple interpretations must also be read according to the “rule of lenity.” The rule of lenity states that, if two readings of a criminal statute are equally possible under the rules of statutory interpretation, and the defendant’s actions are criminal under one of the two readings but not the other, the court should go with the reading that makes the defendant’s behavior non-criminal, as long as the defendant thought he was not breaking the law when he acted. Laws that require frequent application of the rule of lenity, however, are usually also declared void for vagueness because they are overly confusing.