An ex post facto law (Latin for "from a thing done afterward"), also known as a retrospective law, is a law that is retroactive, i.e. that affects facts or legal relationships that have existed prior to the enactment of the law. Ex post facto is a term from the Latin and means "after the deed." In reference to criminal law, it is an attempt to criminalize an action that was not a crime at the time it was committed. A law may have an ex post facto effect without being technically ex post facto. For example, when a law that repeals another law, the repealed legislation will no longer apply to situations even when those situations arose before the law was repealed. The principle of prohibiting these kinds of laws is also known as Nullum crimen, nulla poena sine praevia lege poenali.

Generally speaking, ex post facto 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 retrospective legislation, though some have suggested that judge-made 'law' is retrospective as a new precedent applies to events that happened prior to the judicial decision. In some nations that follow the Westminister system of government, such as the United Kingdom, ex post facto laws are technically possible as parliamentary supremacy allows the parliament to pass any law it wishes. However, in a nation with an entrenched bill of rights or written constitution this may prohibit ex post facto legislation.

On occasion ex post facto laws are seen as necessary for the public good. This is reflected in some international law documents (such as the International Covenant on Civil and Political Rights) which allow trials under retrospective laws in the prosecution of certain crimes against humanity.

Ex post facto laws internationally

• Australia - Australia has no constitutional prohibition on ex post facto laws, though courts interpret statues with a strong presumption that they do not apply retrospectively. Retrospective laws designed to combat tax avoidance were passed in the early 1980s by the Fraser government.

• France - any ex post facto criminal law may only be applied if it benefits the accused person (for instance, if weaker sentences are now applicable but were not previously applicable); otherwise, prohibited by the Declaration of the Rights of Man and of the Citizen.

• Indonesia - Article 28I of the Indonesian constitution prohibits trying citizens under retrospective laws in any circumstance. This was tested in 2004 when the conviction of one of the Bali bombers under retrospective anti-terrorist legislation was quashed.

• United Kingdom - Ex post facto laws are permitted under the doctrine of the 'sovereignty of parliament'; in criminal law, however, ex post facto laws are prohibited by article 7 of the European Convention on Human Rights.

• United States - Prohibited by Article I section 9 (applying to federal law) and section 10 (applying to state law) of the U.S. Constitution. 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 case of 1798, in which Justice Chase established four categories of unconstitutional ex post facto laws. See also Fourteenth Amendment, Bouie v. City of Columbia, Rogers v. Tennessee

• Republic of Ireland - Prohibited by Article 15.5.1 of the Constitution of Ireland.

• Sweden - Retroactive penal sanctions and other retroactive legal effects of criminal acts, and retroactive taxes or charges due the State are prohibited by chapter 2, article 22, point 5 of the Instrument of Government (Regeringsformen).

European Convention on Human Rights

Most European nations, and all European Union nations, are bound by the European Convention on Human Rights. Article 7 of the convention prohibits ex post facto laws subject to two exceptions. It also prohibits a heavier penalty being imposed than was applicable at the time when an act was committed. The exceptions are:

• Acts illegal under international law at the time of their commission.
• Acts criminal according to "the general principles of law recognised by civilised nations".

See also

• Nulla poena sine lege - the principle that no-one may be punished for an act which is not against the law.