ex post facto

Definition of ex post facto

Etymology

From Latin ex ("from") + post ("after") + facto, ablative of factum ("deed")

Adjective

ex post facto (not comparable)

1. Retroactive
2. (law) Formulated or enacted after some event, and then retroactively applied to it.
   
   * 1998, Daniel E. Troy, Retroactive Legislation‎, page 56:
   Coupled with the Constitution's proscription of ex post facto laws is a similar prohibition against bills of attainder.
   
   * 2009 Saturday, March 21, Jim McTague, "Would a Tax on Bonuses Be Constitutional?", Barron's:
   
   That the tax would take effect after some of the payments were made also raises issues. Says former Attorney General Richard Thornburg, also in an e-mail: "Such legislation could well run afoul of constitutional restrictions on bills of attainder, ex post facto laws, laws impairing the obligations of contract, unauthorized takings of property and the like."

Further reading

An ex post facto law (from the Latin for "from after the action") or retroactive law, is a law that retroactively changes the legal consequences (or status) of actions committed or relationships that existed prior to the enactment of the law. In reference to criminal law, it may criminalize actions that were legal when committed; or it may aggravate a crime by bringing it into a more severe category than it was in at the time it was committed; or it may change or increase the punishment prescribed for a crime, such as by adding new penalties or extending terms; or it may alter the rules of evidence in order to make conviction for a crime more likely than it would have been at the time of the action for which a defendant is prosecuted. Conversely, a form of ex post facto law commonly known as an amnesty law may decriminalize certain acts or alleviate possible punishments (for example by replacing the death sentence with life-long imprisonment) retroactively.

A law may have an ex post facto effect without being technically ex post facto. For example, when a law repeals a previous law, the repealed legislation no longer applies to the situations it once did, even if such situations arose before the law was repealed. The principle of prohibiting the continued application of these kinds of laws is also known as Nullum crimen, nulla poena sine praevia lege poenali, particularly in European continental systems.

Generally speaking, ex post facto penal laws are seen as a violation of the rule of law as it applies in a free and democratic society. Most common law jurisdictions do not permit retroactive criminal legislation, though new precedent generally applies to events that occurred prior to the judicial decision. Ex post facto laws are expressly forbidden by the United States Constitution. In some nations that follow the Westminster system of government, such as the United Kingdom, ex post facto laws are technically possible as the doctrine of parliamentary supremacy allows Parliament to pass any law it wishes. However, in a nation with an entrenched bill of rights or a written constitution, ex post facto legislation may be prohibited.