What does an ex post facto law do?

An ex post facto law is a legislative act that retroactively changes the legal consequences of actions that were committed before the law was enacted. Specifically, it makes an act that was legal at the time it was committed a crime under new law, thus punishing individuals for past behavior that was not considered illegal when it occurred. This concept is rooted in the principle of fairness and justice, ensuring that individuals are not held accountable for actions that were permissible at the time they were performed.

The correct choice highlights the essence of ex post facto laws: they are fundamentally about punishing individuals for actions that, at the time of commission, were not deemed criminal. The United States Constitution explicitly prohibits ex post facto laws in Article I, Section 9 for the federal government and Section 10 for state governments, underscoring the importance of this protection in maintaining a fair legal system.