lex mitior
Lex mitior, Latin for “more lenient law,” is a legal doctrine providing that when the law changes between the commission of a criminal offense and the final judgment, the more favorable law to the defendant should apply. It allows retroactive application of laws that either decriminalize conduct or reduce penalties. Lex mitior generally applies only before a conviction becomes final, meaning individuals whose convictions are already finalized typically cannot invoke the doctrine to vacate or overturn them.
The doctrine is widely recognized in European legal systems, particularly under international human rights instruments such as Article 15(1) of the International Covenant on Civil and Political Rights and Article 7 of the European Convention on Human Rights. In contrast, lex mitior is not consistently applied in the United States. This inconsistency is not necessarily due to the U.S. Constitution’s Ex Post Facto Clause. In Calder v. Bull, 3 U.S. (3 Dall.) 386 (1798), Justice Samuel Chase stated that the prohibition on ex post facto laws does not extend to laws that mitigate punishment or decriminalize conduct, but only to those that impose new penalties, create or aggravate crimes, or alter evidentiary rules to facilitate conviction.
Lex mitior has been particularly relevant in recent years in the contexts of marijuana decriminalization and the reduction or abolition of the death penalty.
[Last reviewed in July of 2025 by the Wex Definitions Team]
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