Ex post facto law

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 For example, when a law repeals a previous law, the repealed legislation no longer applies to the situations it once did, even if such situations arose before the law was repealed.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.

In the United States, the federal government is prohibited from passing ex post facto laws by clause 3 of Article I, section 9 of the U.S. Constitution and the states are prohibited from the same by clause 1 of Article I, section 10. This is one of the very few restrictions that the United States Constitution made to both the power of the federal and state governments prior to the Fourteenth Amendment. Over the years, when deciding ex post facto cases, the United States Supreme Court has referred repeatedly to its ruling in the Calder v. Bull, 3 U.S. 386 (1798), in which Justice Samuel Chase established four categories of unconstitutional ex post facto laws. The case dealt with Article I, section 10, since it dealt with a Connecticut state law.