An ex post facto law:

Question

Quizplus · Accepted Answer

An ex post facto law is a criminal law that is written to apply retroactively, meaning it punishes someone for something they did before the law was enacted. This is prohibited by the Constitution, as it violates due process and fairness. Choice A is incorrect because it describes a prospective law, not a retroactive one. Choice B is incorrect because it describes a conflict of laws issue, not an ex post facto issue. Choice D is incorrect because federal common law is not necessarily criminal law, and even if it were, it would not necessarily be ex post facto.

An Ex Post Facto Law