Within the context of law, the mischief rule is a rule of statutory interpretation that attempts to determine the legislator's intention. Originating from a 16th century case in the United Kingdom, its main aim 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.
MeaningThe Mischief Rule is a certain rule that judges can apply in statutory interpretation in order to discover Parliament's intention. It essentially asks the question: By creating an Act of Parliament what was the "mischief" that the previous law did not cover?
This was set out in Heydon's Case  3 CO REP 7a. where it was stated that there were four points to be taken into consideration when interpreting a statute:
1.What was the common law before the making of the act?
2.What was the "mischief and defect" for which the common law did not provide?
3.What was the remedy the parliament hath resolved and appointed to cure the disease of the commonwealth?
4.What is the true reason of the remedy?
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 rule was illustrated in the case of Smith v Hughes  2 All E.R. 859, where 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.
 UseThis rule of construction 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.
This rule has often been used to resolve ambiguities in cases in which the literal rule cannot be applied. In Smith v Hughes the mischief approach gave a more sensible outcome than that of the literal approach.
 Advantages1.The Law Commission sees it as a far more satisfactory way of interpreting acts as opposed to the Golden or Literal rules.
2.It usually avoids unjust or absurd results in sentencing
 Disadvantages1.It is seen to be out of date as it has been in use since the 16th century, when common law was the primary source of law and parliamentary supremacy was not established.
2.It gives too much power to the unelected judiciary which is argued to be undemocratic.
3.In the 16th century, the judiciary would often draft acts on behalf of the king and were therefore well qualified in what mischief the act was meant to remedy. This is not often the case in modern legal systems.
4.The rule can make the law uncertain, susceptible to the slippery slope.
 References1.^ http://legal-directory.net/english-law/interpretation-mischief-rule.htm