The Latin phrase ex post facto means “from a thing done afterward.” In law, it refers to a criminal statute that retroactively punishes conduct that was legal at the time it was committed. The United States Constitution expressly prohibits such laws under:
- Article I, § 9 – Prohibits Congress from passing ex post facto laws.
- Article I, § 10 – Prohibits states from passing ex post facto laws.
What Constitutes Punishment
In Beazell v. Ohio, 269 U.S. 167 (1925), the Supreme Court defined the scope of the constitutional ex post facto prohibitions, holding: “It is settled, by decisions of this Court so well known that their citation may be dispensed with, that any statute which punishes as a crime an act previously committed, which was innocent when done, which makes more burdensome the punishment for a crime, after its commission, or which deprives one charged with crime of any defense available according to law at the time when the act was committed, is prohibited as ex post facto.”
Courts have applied this rule across criminal procedure. In California Department of Corrections v. Morales, 514 U.S. 499 (1995), California amended a law allowing the Board of Prison Terms to defer parole hearings for up to three years for prisoners convicted of multiple homicides. Morales, who was already incarcerated, claimed the change violated the ex post facto prohibition. The Supreme Court held that the amendment was constitutional because it did not increase Morales’s sentence or alter his substantive opportunity for parole. A procedural modification to the parole process does not violate ex post facto prohibitions.
Retroactive Judicial Decisions
The ex post facto clauses apply to legislative acts creating new laws retroactively criminalizing behavior or conduct, not judicial rulings. Courts may sometimes announce new legal rules without applying them retroactively to the case before them to avoid ex post facto concerns.
Year and a Day Rule
The Year and a Day Rule is a common law doctrine that barred a homicide conviction if the victim died more than one year and one day after the defendant’s act. In Rogers v. Tennessee, 532 U.S. 451 (2000), the defendant-petitioner stabbed a man who died 15 months later. Tennessee’s appellate court upheld the conviction and abolished the rule. Rogers argued that this retroactive abolition violated the ex post facto prohibition. The Supreme Court disagreed, holding that the change was a permissible exercise of judicial lawmaking. The Court cited Bouie v. City of Columbia, 378 U.S. 347 (1964), which held that due process forbids the retroactive application of judicial interpretations that are “unexpected and indefensible.” The Court found that abolishing the Year and a Day Rule was neither unexpected nor indefensible and that ex post facto limitations apply only to legislative acts.
See also: Koch v. Vill. of Hartland, 43 F.4th 747 (7th Cir. 2022); Reingold, Paul. "Wrong Turn on the Ex Post Facto Clause." Kimberly Thomas, co-author. Cal. L. Rev. 106, no. 3 (2018): 593-630.
[Last reviewed in October of 2025 by the Wex Definitions Team]