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Term used to designate action taken to change the effect given to a set of circumstances. This action relates back to a prior time and places this new effect upon the same set of circumstances existing at that time. Ex post facto laws are prohibited by the U.S. Constitution.

Not every change in a convicted person's situation violates the Ex Post Facto Clause. A law implicates the Ex Post Facto Clause only if it criminalizes conduct that was not a crime when it was committed, increases the punishment for a crime beyond what it was at the time the act was committed, or deprives a person of a defense available at the time the act was committed. Collins v. Youngblood, 497 U.S. 37, 42-43 (1990). Courts have held that legislation may lawfully impose new requirements on convicted persons if the statute's 'overall design and effect' indicates a 'non-punitive intent.' United States v. Huss, 7 F.3d 1444, 1447 (9th Cir.'93).