Also known as: retroactive laws · ex post facto law · retrospective law
Written by attorneys · grounded in primary & secondary sources — see below
A law that reaches back to alter the legal consequences of conduct completed before its enactment. Such a measure violates the Ex Post Facto Clauses when it criminalizes previously lawful acts or increases punishment for past conduct.
Roberto Reyes raised rents on his Franklin apartments four years before the legislature acted. The new statute declares those increases felonies and lets the district attorney prosecute using old lease records. Reyes moves to dismiss, arguing the law attaches criminal liability to conduct that was lawful when performed.
Titan Industries exceeded carbon limits during years when no federal ceiling existed. Congress later passes the Carbon Accountability Act making those emissions criminal and refers the company for prosecution. Titan moves to dismiss the charges on the ground that the statute imposes new criminal liability for completed conduct.
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Roland Rhodes stabbed a victim who lingered in a coma for months before dying. At the time of the attack, state precedent required death within a year and a day for murder liability. The state supreme court later abolishes the rule and applies the new standard to Rhodes's case, allowing the murder charge to proceed.
Riley Rivera bought a truck whose design later fails a newly announced strict-liability test. The manufacturer argues that applying the new standard to vehicles sold years earlier would impose retroactive liability for conduct that complied with the law at the time of sale.
Rosalind Reed's will was denied probate under the rules in force at the testator's death. The state legislature later passes a resolution setting aside the decree and ordering a new hearing under changed procedures. The disappointed heirs challenge the resolution as an impermissible retroactive alteration of vested rights.
Ryan Roberts is detained and charged before a military commission under rules that did not exist when the alleged conduct occurred. The government seeks to try him under the later-enacted procedures. Roberts petitions for habeas relief, contending the new rules cannot govern his completed acts.
A statute violates the Ex Post Facto Clauses when it makes conduct criminal that was lawful when performed or increases punishment for past acts. Courts invalidate such laws because they deny fair notice and allow legislatures to punish completed conduct after the fact.
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The Ex Post Facto Clauses target criminal penalties. Civil measures that function as punishment may still fall within the prohibition when they impose new disabilities or liabilities on past conduct without a nonpunitive purpose.
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No. A state may not enact legislation that retroactively criminalizes possession, rent increases, or emissions that were lawful when they occurred. Such statutes are invalid even when they address serious public harms like environmental damage or housing costs.
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Yes. When a statute raises maximum or mandatory minimum sentences and expressly applies them to prior offenses, it imposes greater punishment for completed conduct and is therefore an unconstitutional ex post facto law.
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…aside a decree of a court and granting a new trial to be had before the same court is not void under the Constitution as an ex post facto law. The Legislature of Connecticut, on the second Thursday of May, 1795, passed a resolution which set aside a decree of the Court of Probate for Hartford County made 21 March 1793…