What is an ex post facto law?

The four categories

From Calder v. Bull (1798), a law is ex post facto if it:

1. criminalises an act that was innocent when done;
2. makes a crime more serious than it was when committed;
3. increases the punishment for a crime after it was committed;
4. alters the rules of evidence to make conviction easier for past conduct.

What the clause does not cover

Civil and regulatory laws generally fall outside it, which is why sex-offender registration was upheld in Smith v. Doe (2003) as regulatory rather than punitive. Procedural changes that do not disadvantage the defendant are allowed, and a law that reduces punishment retroactively is permitted. Judicial decisions are analysed under due process rather than the ex post facto clause, but unforeseeable expansions of criminal liability by courts face a similar limit.

Where it comes up in practice

Sentencing enhancements and guideline amendments applied to earlier conduct; extensions of a statute of limitations after the original period has already expired, which the Supreme Court held unconstitutional in Stogner v. California (2003); and changes to parole and good-time credit rules that lengthen actual time served.

The state dimension

Many state constitutions contain their own ex post facto clauses, and several state supreme courts read them more broadly than the federal one — striking down retroactive registration schemes that survived federal review.