Ex Post Facto Clause: Retroactive Criminal Laws Barred

The ex post facto clause appears twice in Article I: section 9 forbids Congress to pass such laws and section 10 forbids the states. It bars a legislature from criminalising conduct after the fact and from increasing the punishment for conduct already completed. Its purpose is fair warning: people must be able to rely on the law as it stood when they acted.

This guide explains the four categories identified in Calder v. Bull and reaffirmed in Collins v. Youngblood, the three limits that keep the clause narrow, why retroactive civil liability is treated quite differently, and how to tell a genuinely civil penalty from a criminal one wearing a civil label.

The four Calder v. Bull categories

Justice Chase’s opinion in Calder v. Bull remains the controlling framework, and Collins v. Youngblood confirmed it as the test. A retroactive criminal law is unconstitutional if it does any of four things.

Criminalises an act that was innocent when it was done.
Aggravates a crime, making it greater than it was when committed.
Increases the punishment for a crime beyond what the law provided at the time of the offence.
Alters the rules of evidence so as to make conviction easier than the law required when the act occurred.

The fourth category is the one candidates forget. A statute that lowers the standard of proof, removes a corroboration requirement or shifts a burden, and then applies to past conduct, offends the clause just as surely as one that lengthens a sentence.

Exam tip: name the Calder category you are using. Saying which of the four applies is worth more than a general statement that the statute is retroactive.

Three limits that keep the clause narrow

Criminal or penal laws only

The clause reaches criminal and penal legislation. Retroactive taxes, regulatory charges and civil remedies fall outside it, however burdensome. Those measures are reviewed under rational basis and are generally sustained where no final judgment has already fixed the parties’ rights.

Legislative action only

The clause constrains legislatures. If a court announces a new interpretation and applies it to conduct that preceded the decision, that is not an ex post facto violation, although it may raise a due process question about fair warning. Keep the two arguments separate.

Retroactivity required

A statute that governs only conduct occurring after enactment is never ex post facto, no matter how severe it is. The clause is about reach backwards in time, not about severity.

Criminal or civil in substance

Because the clause is confined to criminal laws, everything can turn on characterisation, and the legislature’s label does not control. Courts examine the substance of the measure: its purpose, whether it has historically been regarded as punishment, whether it turns on a finding of scienter, and whether it is excessive in relation to any remedial aim. A forfeiture so punitive that it functions as criminal punishment can be caught even though the statute calls it civil.

Retroactive measure	Ex post facto violation?	Why
Criminalises conduct lawful when done	Yes	Calder category one
Adds three years to sentences already being served	Yes	Calder category three
Removes a corroboration requirement for past offences	Yes	Calder category four, easier conviction
Imposes a modest civil forfeiture on past offenders	No	Civil penalty, reviewed under rational basis
Applies a new sentencing rule only to future conduct	No	No retroactive reach
A court applies a new reading of an old statute	No	Judicial action, analysed under due process

Running the same fact pattern through the clause’s three limits.

Worked example: the longer sentence

A legislature raises the penalty for a possession offence from two years to five and provides expressly that the increase applies to all past violations, including people currently serving sentences. The statute is legislative, criminal and retroactive, and it increases punishment beyond what the law allowed at the time of the offence. That is Calder category three, and the increase cannot be applied to those offenders.

Change the penalty. The legislature instead imposes a five hundred dollar civil forfeiture on past offenders. The clause is not engaged because the penalty is civil, so the measure is reviewed under rational basis and will usually survive. It may still be attacked on due process or excessive fines grounds, but those are separate arguments.

Change the timing. The five-year term applies only to offences committed after enactment. There is no retroactivity and therefore no violation, even though the sentence more than doubled.

Ex post facto compared with bills of attainder

The two clauses sit side by side and guard against different evils. A bill of attainder is legislative punishment of specific, identified persons without any trial: the objection is that the legislature adjudicated guilt. An ex post facto law may apply to an open class and may be enforced through an ordinary trial: the objection is that the law changed after the conduct. A single statute can violate both, and each should be analysed on its own terms.

Common mistakes that cost points

Applying the clause to retroactive civil liability. Taxes, fees and civil remedies are outside it.
Applying the clause to courts. Judicial retroactivity is a due process problem, not an ex post facto one.
Confusing severity with retroactivity. A harsh but purely prospective statute is untouched.
Forgetting the fourth Calder category about evidentiary rules that ease conviction.
Accepting the statute’s own label. Substance controls, so a punitive civil measure can be criminal in effect.
Merging the analysis with bills of attainder instead of running both tests separately.
Overlooking that section 10 imposes the same ban on the states as section 9 does on Congress.
Failing to identify the date of the conduct, which is the fact that decides the whole question.

Frequently asked questions

Can a legislature lower a penalty retroactively?

Yes. The clause bars retroactive disadvantage, not retroactive leniency, so a reduction may be applied to past conduct.

Do sex-offender registration schemes violate the clause?

Courts have generally held that such schemes are civil and regulatory rather than punitive, and so are outside the clause, though the answer depends on how burdensome the particular scheme is in substance.

Does the clause protect against changes in procedure?

Only where the change makes conviction easier than the law required at the time of the offence. Neutral procedural adjustments are permitted.

Ex post facto problems in California sentencing, 2026

Both the federal Constitution and article I, section 9 of the California Constitution prohibit retroactive criminal legislation, and the categories from Calder v. Bull as restated in Collins v. Youngblood (1990) remain the test: criminalising past conduct, aggravating a crime, increasing punishment, or altering the rules of evidence to make conviction easier.

California’s frequent use of ballot initiatives makes retroactivity a live question rather than a historical one. Measures reducing penalties have generally been applied to people already sentenced, because reducing punishment raises no ex post facto concern and California has its own rule favouring retroactive application of ameliorative changes. Measures increasing penalties operate prospectively only, which is why the effective date of an initiative such as the 2024 measure raising penalties for certain theft and drug offences matters so much in Los Angeles County District Attorney charging decisions.

Recurring issues in Los Angeles County practice:

Ameliorative changes apply retroactively. Under the California rule associated with In re Estrada, a reduction in punishment reaches cases not yet final unless the enactment says otherwise.
Registration schemes are usually regulatory. Smith v. Doe (2003) treated sex offender registration as non-punitive, so retroactive application is generally permitted.
Residency restrictions can still fail. The California Supreme Court invalidated blanket residency restrictions as applied in a densely populated county, though on due process rather than ex post facto grounds.
Parole and credit rules are analysed for effect. A change that lengthens actual confinement can violate the clause even if framed as procedural.
Limitation periods cannot be revived. Reopening an expired criminal limitations period is unconstitutional under Stogner v. California (2003), itself a California case.
Civil commitment is outside the clause. Sexually violent predator proceedings are treated as civil, so retroactivity arguments generally fail.

In 2026, ask whether the change increases punishment and whether the case was final when it took effect. Read with bills of attainder, habeas corpus and murder and manslaughter.

Next steps

Study this clause with its Article I neighbours and with the wider retroactivity rules. Read Bills of Attainder: Legislative Punishment Explained and The Writ of Habeas Corpus: Testing Unlawful Detention, then compare the tiers of review in Levels of Scrutiny in Constitutional Law.

For primary sources, read Calder v. Bull and Collins v. Youngblood at Justia, the entry on ex post facto laws at Cornell LII, and the examination scope published by the State Bar of California.