[Latin, "After-the-fact" laws.] Laws that provide for the infliction of punishment upon a person for some prior act that, at the time it was committed, was not illegal.
Ex post facto laws retroactively change the RULES OF EVIDENCE in a criminal case, retroactively alter the definition of a crime, retroactively increase the punishment for a criminal act, or punish conduct that was legal when committed. They are prohibited by Article I, Section 10, Clause 1, of the U.S. Constitution. An ex post facto law is considered a hallmark of tyranny because it deprives people of a sense of what behavior will or will not be punished and allows for random punishment at the whim of those in power.
The prohibition of ex post facto laws was an imperative in colonial America. The Framers of the Constitution understood the importance of such a prohibition, considering the historical tendency of government leaders to abuse power. As ALEXANDER HAMILTON observed, "[I]t is easy for men ? to be zealous advocates for the rights of the citizens when they are invaded by others, and as soon as they have it in their power, to become the invaders themselves." The desire to thwart abuses of power also inspired the Framers of the Constitution to prohibit bills of attainder, which are laws that inflict punishment on named individuals or on easily ascertainable members of a group without the benefit of a trial. Both ex post facto laws and bills of attainder deprive those subject to them of DUE PROCESS of law?that is, of notice and an opportunity to be heard before being deprived of life, liberty, or property.
The Constitution did not provide a definition for ex post facto laws, so the courts have been forced to attach meaning to the concept. In Calder v. Bull, 3 U.S. (3 Dall.) 386, 1 L. Ed. 648 (1798), the U.S. Supreme Court provided a first and lasting interpretation of the Ex Post Facto Clause. The focus of the Calder case was a May 1795 resolution of the Connecticut legislature that specifically set aside a March 1793 probate court decree. The resolution allowed the defeated party in the probate contest a new hearing on the matter of the will. The Court in Calder ruled that the Connecticut resolution did not constitute an ex post facto law because it did not affect a vested property right. In other words, no one had complete ownership of the property in the will, so depriving persons of the property did not violate the ex post facto clause. The Court went on to list situations that it believed the clause did address. It opined that an ex post facto law was one that rendered new or additional criminal punishment for a prior act or changed the rules of evidence in a criminal case.
In Calder, the Court's emphasis on criminal laws seemed to exclude civil laws from a...