What is Ex Post Facto?

Tricia Christensen
Tricia Christensen

Ex post facto is Latin for "after the fact," or something changed after an act. It often refers to laws enacted after a crime or action has already been taken. This would render some people guilty of breaking laws applied after the fact, even if their actions were not considered illegal at the time they were taken.

Woman standing behind a stack of books
Woman standing behind a stack of books

In many cases, an ex post facto or retroactive law does not allow a court to find someone guilty of behavior that was against a law prior to the law’s establishment. More often, this type of law might toughen punishments on crimes, and if the law is changed before a suspect comes to trial, he or she might be subject to tougher punishments than previously expected.

Often however, applying tougher laws to previously convicted criminals in an ex post facto sense is not usually allowed in the US. This can disturb victims of criminals that seem to be released too early after a crime. Although public sentiment may side with the victim, a criminal’s sentence generally cannot be changed as a result of such a law.

Countries with a well-established bill of rights are least likely to allow for ex post facto laws to allow for the prosecution of those whose actions were legal prior to the law change. As well, those who have been prosecuted are not likely to have their sentences changed if punishment changes as the result of this type of law.

There are a few exceptions in US law. Laws requiring the registration of sex offenders may be applied ex post facto to criminals convicted before establishment of the law. Proponents for this type of application argue that such laws further protect the safety of the general public. Opponents of applying registration laws after the fact argue that the initial punishment of those convicted did not stipulate for registration, and it is thus outside of the rights of the court to require such.

Sometimes a law can be changed ex post facto that makes a formerly illegal act, legal. For example, amnesty to draft dodgers of the Vietnam war was granted, thus rendering their failure to register for the draft, or respond to being called up, no longer an illegal act. As a result, some people who had lived in hiding after failing to respond appropriately to the US Military Services were now no longer criminals and could resume normal lives.

Tricia Christensen
Tricia Christensen

Tricia has a Literature degree from Sonoma State University and has been a frequent wiseGEEK contributor for many years. She is especially passionate about reading and writing, although her other interests include medicine, art, film, history, politics, ethics, and religion. Tricia lives in Northern California and is currently working on her first novel.

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Discussion Comments



If a nation were to maintain rigid procrustean laws throughout history, without any amendments, we would have a very stiff system that would not allow for new ideas and new opportunities. With new times come a new set of constraints which must be imposed. Some laws are added, some taken away, because societies are always changing. Simply because a legal system may be slightly flawed or liquid does not mean it should be ignored. The costs of such a measure are far beyond the benefits of civil obedience.


Often these amendments to laws seem quite arbitrary. If the rules and regulations of a nation are so prone to change, what basis do we have for a solid legal system?

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