The mischief rule is one of three rules of statutory interpretation traditionally applied by English courts. The other two are the "plain meaning rule" (also known as the "literal rule") and the "golden rule".
The main aim of the rule is to determine the "mischief and defect" that the statute in question has set out to remedy, and what ruling would effectively implement this remedy. In applying the mischief rule, the court is essentially asking what part of the law did the law not cover, but was meant to be rectified by Parliament in passing the bill.
The rule was first laid out in a 16th-century ruling of the Exchequer Court.
Conway v Rimmer is a rule of construction that judges can apply in statutory interpretation in order to discover Parliament's intention. In applying the rule, the court is essentially asking what the mischief was that the previous law did not cover, which Parliament was seeking to remedy when it passed the law now being reviewed by the court.
The mischief rule is of narrower application than the golden rule or the plain meaning rule, in that it can only be used to interpret a statute and, strictly speaking, only when the statute was passed to remedy a defect in the common law.
Legislative intent is determined by examining secondary sources, such as committee reports, treatises, law review articles and corresponding statutes.
The application of this rule gives the judge more discretion than the literal and the golden rule as it allows him to effectively decide on Parliament's intent. It can be argued that this undermines Parliament's supremacy and is undemocratic as it takes law-making decisions away from the legislature.
The way in which the mischief rule can produce more sensible outcomes than those that would result if the literal rule were applied is illustrated by the ruling in Smith v Hughes  2 All E.R. 859. Under the Street Offences Act 1959, it was a crime for prostitutes to "loiter or solicit in the street for the purposes of prostitution". The defendants were calling to men in the street from balconies and tapping on windows. They claimed they were not guilty as they were not in the "street". The judge applied the mischief rule to come to the conclusion that they were guilty as the intention of the act was to cover the mischief of harassment from prostitutes.
[F]or the sure and true interpretation of all statutes in general (be they penal or beneficial, restrictive or enlarging of the common law), four things are to be discerned and considered:
1st. What was the common law before the making of the Act.
2nd. What was the mischief and defect for which the common law did not provide.
3rd. What remedy the Parliament hath resolved and appointed to cure the disease of the commonwealth.
And, 4th. The true reason of the remedy;
And then the office of all the Judges is to make such construction as shall suppress the mischief, and advance the remedy, and to suppress subtle inventions and evasions for continuance of the mischief, and pro privato commodo, and to add force and life to the cure and remedy, according to the true intent of the makers of the Act, pro bono publico.
In the century in which it was created, and for some time thereafter, the mischief rule was used in a legislative environment very different from the one which has prevailed in the past two centuries. As Elmer Driedger notes,
[S]ixteenth-century common law judges ... looked upon statutes as a gloss upon the common law, even as an intrusion into their domain. Hence, statutes were viewed from the point of view of their effect upon the common law, as adding to it, subtracting from it or patching it up....
Then also, in the time of Heydon's Case, the judges paid more attention to the "spirit" of the law than to the letter. Having found the mischief they proceeded to make mischief with the words of the statute. They remodelled the statute, by taking things out and putting things in, in order to fit the "mischief" and "defect" as they had found them.
Modern courts continue to apply the rule in a more restricted manner, and generally with a greater regard for the integrity of the statutes which they are interpreting. Driedger puts it this way: "[T]o this day, Heydon's Case is frequently cited. The courts still look for the 'mischief' and 'remedy', but now use what they find as aids to discover the meaning of what the legislature has said rather than to change it." Driedger goes on to argue that this modern use of the mischief rule ought to be understood as one of the components of what he characterized as the "modern" method of statutory construction, rather than a stand-alone rule serving (as it formerly had), as an alternative to the methods of construction proposed by the plain meaning rule and the golden rule.