What is an ex post facto law?

Why the Constitution bans them

The principle is basic fairness: people should be able to know the rules before they act. A government that could punish past conduct by passing new laws could target its opponents after the fact. The framers considered the protection important enough to write it into the original Constitution — Article I, Section 9 for Congress and Section 10 for the states — before the Bill of Rights existed.

The four categories

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

1. criminalizes 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

The clause applies only to criminal punishment. 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, and why retroactive tax laws, zoning changes and new licensing rules are usually allowed. Procedural changes that do not disadvantage the defendant — such as a new rule for how juries are selected — are permitted. A law that reduces punishment can apply retroactively; whether it does is up to the legislature.

Judicial decisions are analyzed under due process rather than the ex post facto clause, but a court's unforeseeable expansion of criminal liability faces a similar limit (Bouie v. City of Columbia, 1964; Rogers v. Tennessee, 2001).

How courts decide whether a law is punishment

When a legislature labels a law civil, courts look at whether its effects are so punitive that the label should be ignored. They ask whether the law imposes a physical restraint, has historically been seen as punishment, serves a legitimate non-punitive purpose, and is excessive in relation to that purpose. Registration, residency restrictions and civil commitment of sex offenders are the areas where this test is fought most often.

Where it comes up in practice

• Sentencing changes. Applying amended sentencing guidelines that raise the range to earlier conduct violates the clause (Peugh v. United States, 2013).
• Statutes of limitations. Reviving a prosecution after the original limitations period has already expired is unconstitutional (Stogner v. California, 2003). Extending a period that has not yet run is allowed.
• Parole and good-time credits. Changes that lengthen actual time served, such as canceling earned credits, can violate the clause (Lynce v. Mathis, 1997), while changes to how often parole hearings occur often do not.
• Evidence rules. Lowering the evidence needed to convict for past offenses is barred (Carmell v. Texas, 2000).

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. A claim that fails under the US Constitution may still succeed under the state constitution, so both should be raised.

If a new law is applied to your old case

Check the date of the conduct, the version of the law then in effect, and whether the new law is criminal punishment or a civil rule. A criminal defense or appellate lawyer can tell you whether an ex post facto challenge is available. It usually must be raised in the trial court or at sentencing to be preserved for appeal.

A simple illustration

Suppose a state raises the maximum sentence for burglary from five years to ten on July 1. A person who committed a burglary in May can be sentenced to no more than five years, even if the trial takes place in December. If instead the state had lowered the maximum, it could choose to give the May offender the benefit of the change — but it is not required to.