Also known as: ex post facto prohibitions · ex post facto clause · ex post facto law · retroactive law prohibition
Written by attorneys · grounded in primary & secondary sources — see below
A constitutional prohibition barring legislatures from enacting criminal laws that retroactively criminalize conduct, increase punishment for past acts, or alter rules of evidence to the defendant's disadvantage.
Emma Erickson operated a manufacturing plant in State A for fifteen years under permits that imposed no carbon limits. After new federal standards took effect, State A enacted a statute making any prior excess emissions a criminal offense subject to fines and imprisonment. Emma faces prosecution for conduct completed years earlier. The statute cannot apply to her because it retroactively converts lawful emissions into a crime.
Ethan Evans was investigated for securities fraud in 2015, but the statute of limitations expired before charges were filed. In 2020 Congress extended the limitations period and directed that the new period revive all previously time-barred cases. Prosecutors now indict Ethan under the revived period. The extension violates the prohibition because it reopens the possibility of punishment for conduct that could no longer be prosecuted.
Select any source to read its text and confirm it supports the definition.
Elijah Edwards sold counterfeit goods in 2018 when the maximum prison term was five years. Congress later raised the maximum to fifteen years and expressly made the harsher term apply to all prior offenses. At sentencing the judge imposes the new fifteen-year term. The retroactive increase in punishment cannot stand under the prohibition.
Ezra Eastman challenged a state resolution that reopened a probate decree after the original judgment had become final. The resolution granted a new hearing on a will that had already been disallowed. The court upheld the resolution because it altered only civil remedies and did not impose or increase criminal punishment for past conduct.
Enzo Eastwood was convicted of an offense committed before revised federal sentencing guidelines took effect. The district court applied the new guidelines and imposed a longer term than the prior range allowed. The court of appeals vacated the sentence because the retroactive application increased punishment for conduct completed before the guidelines existed.
Eric Espinoza was charged with murder after the victim died more than a year and a day after the assault. The state supreme court abolished the common-law year-and-day rule and applied the abolition to Eric's case. The U.S. Supreme Court upheld the conviction because the change was reasonably foreseeable and did not constitute an unexpected judicial expansion of criminal liability.
It forbids any criminal statute that retroactively makes previously lawful conduct a crime, increases the punishment for past conduct, or changes the rules of evidence to make conviction easier.
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Yes. The clause targets criminal or penal legislation. Purely civil regulatory measures are generally outside its scope unless they are so punitive in purpose or effect that they function as punishment.
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No. Extending a limitations period after it has already expired and applying the extension to revive a previously barred case violates the prohibition because it reopens the possibility of punishment for past conduct.
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Yes. Any statute that expressly applies harsher maximum or mandatory minimum sentences to conduct completed before its enactment increases punishment retroactively and is therefore invalid.
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No. The clause restricts only legislative action. Due process, however, prevents courts from retroactively interpreting criminal law in an unexpected and indefensible manner.
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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…