An ex post facto law (Latin: "from a thing done afterward") applies retroactively to conduct that occurred before the statute existed. Most constitutional democracies and the core international human rights instruments prohibit such laws in the criminal sphere, treating the principle of non-retroactivity as a cornerstone of the rule of law and legal certainty.
The prohibition appears in several foundational texts:
- U.S. Constitution, Article I, Section 9 (restricting Congress) and Section 10 (restricting the states), which bar ex post facto laws. The Supreme Court in Calder v. Bull (1798) limited the clause to criminal matters and identified four categories: criminalizing past acts, aggravating a crime after the fact, increasing punishment, and altering rules of evidence to a defendant's disadvantage.
- European Convention on Human Rights, Article 7 ("No punishment without law").
- International Covenant on Civil and Political Rights, Article 15.
- American Convention on Human Rights, Article 9.
- The Rome Statute of the International Criminal Court, Articles 22–24, codifying nullum crimen sine lege and nulla poena sine lege.
A widely debated exception concerns crimes already recognized under international law at the time of commission. The Nuremberg Tribunal (1945–46) and Tokyo Tribunal addressed ex post facto objections by holding that aggressive war and crimes against humanity were already proscribed under customary international law. ICCPR Article 15(2) explicitly preserves prosecution for acts that were "criminal according to the general principles of law recognized by the community of nations" at the time.
Non-criminal retroactivity — for example, retroactive tax adjustments or civil regulatory changes — is generally permitted, though often constrained by due process or legitimate-expectations doctrines. In civil-law systems, the related maxim lex retro non agit expresses the same baseline presumption.
For MUN delegates and IR researchers, the concept frequently surfaces in transitional justice debates, sanctions design, and ICC jurisdictional disputes.
Example
When Germany prosecuted former East German border guards in the 1990s for shootings at the Berlin Wall, defendants invoked Article 7 of the European Convention; the European Court of Human Rights in *Streletz, Kessler and Krenz v. Germany* (2001) rejected the ex post facto challenge.
Frequently asked questions
In criminal matters, almost never under modern human rights treaties. The main recognized carve-out, in ICCPR Article 15(2), permits prosecution for acts that were already crimes under general principles of international law when committed.
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