retroactivity

Retroactivity refers to the application of a law, rule, or policy to actions or events that took place before its adoption. It is encountered in legal, philosophical, and policy contexts. In law, retroactivity is typically contrasted with prospectivity, where the law affects only future conduct. Retroactivity can be subdivided into substantive retroactivity, which alters rights or duties, and procedural retroactivity, which changes the mechanism for enforcing rights without altering substantive protections.

Ex post facto: In criminal law, retroactive application that increases punishment or makes conduct illegal after it was committed is prohibited by the ex post facto principle in many jurisdictions, including the U.S. Constitution.

Civil and administrative contexts: Retroactive laws or decisions may be allowed if they do not impose punishment and serve legitimate public purposes, such as correcting errors or addressing broad policy goals. Courts balance fairness, reliance interests, and legal certainty.

Judicial retroactivity: Courts determine whether a new rule applies to past cases. In the United States, Teague v. Lane restricts retroactive application of new constitutional rules to cases on collateral review, with narrow exceptions for settled, substantive rights or rules that would affect the accuracy of trial results.

Examples: A tax statute enacted to apply to the previous year; or a new interpretation of a statute that changes how it should be applied to past actions; a court’s decision recognizing a constitutional right may be applied retroactively in some circumstances.

Retroactivity is a debated area in comparative law, as jurisdictions balance legal certainty, fairness, and the need to adapt law to new knowledge.