Ex Post Facto Law Definition

Learn what ex post facto law is, why it’s prohibited, and examples of how it’s been used throughout history.

What is an Ex Post Facto Law?

An ex post facto law is a law that retroactively changes the legal consequences of actions that were committed before the enactment of the law. In essence, it punishes someone for an act that was not a crime at the time it was committed. This type of law is generally prohibited in order to protect individuals from arbitrary and oppressive laws.

Types of Ex Post Facto Laws

  • Retrospective Criminal Law
  • Ex Post Facto Civil Law
  • Retroactive Taxation

Examples of Ex Post Facto Laws

One famous example of an ex post facto law is the case of Calder v. Bull in 1798. The U.S. Supreme Court held that a Connecticut state law violated the Constitution by retroactively changing the legal consequences of an action. Another example is the Nazi Holocaust restitution program, which retroactively provided compensation to victims of the Holocaust.

Case Studies

In the case of Collins v. Youngblood in 1990, the U.S. Supreme Court ruled that retroactive changes in parole eligibility did not violate the ex post facto clause of the Constitution. However, in the case of Weaver v. Graham in 1981, the court ruled that retroactive changes to the length of a prisoner’s sentence violated the ex post facto clause.


According to a study by the American Bar Association, ex post facto laws have been used in over 50 countries around the world. The study found that these laws are most commonly used in cases involving war crimes, terrorism, and organized crime.

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