What is an ex-post facto law?

An ex-post facto law is specifically defined as a law that retroactively changes the legal consequences of actions that were committed before the enactment of that law. This means it criminalizes conduct that was legal at the time it was performed or increases the penalties for an act after it has been committed. This principle is vital to the rule of law and individual rights, as it helps protect citizens from arbitrary legislation that could unfairly punish them for past actions.

The other options do not describe an ex-post facto law. A law enacted for future conduct only pertains to regulations that govern actions yet to occur, which does not fit the definition at all. A law allowing penalties leading to imprisonment is a broader concept that can apply to various types of laws but does not specifically address the retroactive nature required for a law to be classified as ex-post facto. Finally, a law based on precedents refers to judicial decisions and the doctrine of stare decisis, rather than legislation that retroactively affects behavior.