Ex Post Facto Laws

Ex Post Facto Laws are statutes that make an act punishable as a crime when such an act was not an offense when committed. Article I, section 10, clause 1 of the Constitution provides that no state shall pass any ex post facto law; Article I, section 9, clause 3 imposes the same prohibition upon the federal government. The Supreme Court early determined that these clauses prohibit laws with retroactive effect only in the field of criminal law and do not apply to statutes dealing with civil matters. Nonetheless, retroactive laws in the civil area may under certain circumstances violate the Contract or Due Process Clauses of the Constitution. The ban on ex post facto laws operates solely as a restraint on legislative power and has no application to changes in the law made by judicial decision.

Besides preventing the enactment of laws making acts criminal that were not criminal when committed, the Ex Post Facto Clauses also render invalid the retroactive application of laws that, while not creating new offenses, aggravate the seriousness of a crime. Moreover, a statute that prescribes a greater punishment for a crime already committed violates the clauses. A law that alters the rules of evidence so as to make it substantially easier to convict a defendant is likewise prohibited by the Constitution.

Edgar Bodenheimer