
The United States Constitution prohibits the federal and state governments from passing ex post facto laws, which are laws that retroactively criminalize actions that were not illegal when committed. The US Constitution's Article 1, Section 9, Clause 3 (federal laws) and Article 1, Section 10 (state laws) contain these prohibitions, which are designed to protect people from legislative determinations of guilt and preserve the Constitution's separation of powers. The US Supreme Court has repeatedly referred to its ruling in Calder v. Bull (1798), which established four categories of unconstitutional ex post facto laws, to decide ex post facto cases. The Court has also held that the prohibition applies only to criminal matters and not civil matters.
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What You'll Learn
- The US Constitution prohibits ex post facto laws
- Ex post facto laws are prohibited by both federal and state governments
- Ex post facto laws cannot be applied to crimes committed outside US jurisdiction
- Ex post facto laws cannot be applied retrospectively
- Ex post facto laws are distinct from bills of attainder

The US Constitution prohibits ex post facto laws
The US Constitution's prohibitions on ex post facto laws can be found in Article 1, Section 9, Clause 3 (for federal laws) and Article 1, Section 10 (for state laws). These provisions ban the enactment of ex post facto laws by the federal government and the states, respectively. The Supreme Court has interpreted these clauses to mean that legislatures are prohibited from enacting laws that impose criminal liability or increase criminal punishment retroactively.
The prohibition on ex post facto laws is closely related to the prohibition on bills of attainder, which are legislative actions that determine guilt or impose criminal punishment on specific persons or groups without a judicial trial. In some cases, a single legislative action may violate both the ex post facto and bill of attainder prohibitions. The prohibitions on ex post facto laws and bills of attainder serve to protect against legislative overreach and preserve the Constitution's separation of powers.
While the US Constitution prohibits ex post facto laws, there have been interpretations and applications of this prohibition that are worth noting. For example, the Supreme Court has held that the prohibition on ex post facto laws does not apply to crimes committed outside the jurisdiction of the United States against the laws of a foreign country. Additionally, in the case of California Dep't of Corrections v. Morales, the Supreme Court held that an amendment impacting someone currently imprisoned did not violate the ex post facto prohibition as long as it did not increase the punishment attached to the respondent's crime.
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Ex post facto laws are prohibited by both federal and state governments
Ex post facto laws are prohibited by both the federal and state governments in the United States. The US Constitution's Article 1, Section 9, Clause 3, and Article 1, Section 10, prohibit the federal and state governments from enacting ex post facto laws, respectively. These provisions are among the few restrictions imposed on both levels of government before the Fourteenth Amendment.
The prohibition against ex post facto laws is closely related to the ban on bills of attainder, which are legislative actions that determine guilt and impose punishment without a judicial trial. Both prohibitions serve to protect against legislative overreach and preserve the separation of powers. The ex post facto clause specifically prevents the retroactive criminalization of actions, ensuring that people are not punished for acts that were legal when committed.
The US Supreme Court has interpreted the ex post facto clause in several cases, including Calder v. Bull (1798), where Justice Samuel Chase established four categories of unconstitutional ex post facto laws. These categories include criminalizing previously lawful actions, increasing the severity of an offense, increasing punishment, and altering rules of evidence to favour conviction. The Court has also clarified that the ex post facto prohibition applies only to legislative decisions and not to judicial decisions.
Despite the prohibition, there have been instances where retroactive laws have been upheld. For example, in California Dep't of Corrections v. Morales (1995), the Supreme Court held that an amendment affecting parole hearings for prisoners convicted of multiple homicides did not violate the ex post facto clause as it did not increase the punishment for the respondent's crime. Similarly, in Smith v. Doe, the Court upheld Alaska's "Megan's Law," which required released sex offenders to register with the police and allowed for public notification, as a non-punitive civil regulatory scheme.
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Ex post facto laws cannot be applied to crimes committed outside US jurisdiction
The United States Constitution expressly forbids ex post facto laws, which are laws that retroactively alter the legal consequences of actions committed before the law's enactment. In other words, these laws criminalize actions that were not illegal when they were carried out.
The US Constitution's prohibition of ex post facto laws applies to both the federal government and the states. The Constitution's framers included this prohibition to protect people from legislative determinations of guilt and to reinforce the separation of powers.
The US Supreme Court has ruled that the prohibition on ex post facto laws does not apply to crimes committed outside US jurisdiction and against the laws of a foreign country. For instance, in Neely v. Henkel (1901), the Supreme Court held that the constitutional prohibitions on ex post facto laws do not extend to such cases.
The Court has interpreted the federal and state Ex Post Facto Clauses to ban legislatures from enacting laws that impose criminal liability or increase criminal punishment retroactively. This means that ex post facto laws cannot be applied to increase the punishment for a crime committed outside US jurisdiction. The Supreme Court has also held that a law that "mollifies" a criminal act is not an ex post facto law but is merely retrospective.
In summary, while ex post facto laws are prohibited in the US, they cannot be applied to crimes committed outside US jurisdiction. The Supreme Court has upheld this interpretation, ensuring that the constitutional prohibition on ex post facto laws does not extend beyond US borders.
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Ex post facto laws cannot be applied retrospectively
Ex post facto laws are expressly forbidden by the United States Constitution in Article 1, Section 9, Clause 3 (with respect to federal laws) and Article 1, Section 10 (with respect to state laws). The US Constitution prohibits the federal and state governments from passing ex post facto laws, i.e., laws that retroactively create a crime where none existed before.
In criminal law, ex post facto laws may criminalize actions that were legal when committed; they may aggravate a crime by bringing it into a more severe category than it was in when committed; they may change the punishment prescribed for a crime, as by adding new penalties or extending sentences; they may extend the statute of limitations; or they may alter the rules of evidence in order to make conviction for a crime likelier than it would have been when the deed was committed.
The US Supreme Court has held that the constitutional prohibitions on ex post facto laws do not apply to crimes committed outside the jurisdiction of the United States against the laws of a foreign country. The Court has also held that the prohibitions on ex post facto laws apply only to criminal matters, not civil matters. In the 1798 case of Calder v. Bull, the Court enumerated four ways in which a legislature may violate the Ex Post Facto Clauses' prohibition on imposing retroactive criminal liability: (1) making criminal an action taken before enactment of the law that was lawful when it was done; increasing the severity of an offense after it was committed; increasing the punishment for a crime after it was committed; and altering the rules of evidence after an offense was committed so that it is easier to convict an offender.
The prohibition against ex post facto laws serves to protect people from legislative determinations of guilt and reinforces the Constitution's separation of powers. The prohibition against ex post facto laws is closely related to the prohibitions of bills of attainder, which are legislative actions that determine guilt or impose criminal punishment on specific persons or groups without a judicial trial.
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Ex post facto laws are distinct from bills of attainder
The US Constitution prohibits the federal and state governments from enacting ex post facto laws, which are laws that retroactively criminalise actions that were not illegal when they were committed. The Constitution also prohibits bills of attainder, which are laws that determine guilt and impose punishment on specific individuals or groups without a trial. While these two types of laws are closely related, they are legally distinct.
Ex post facto laws and bills of attainder are prohibited by the US Constitution to prevent legislative overreach and preserve the separation of powers. The Constitution only permits the judiciary to determine whether someone is guilty or innocent, and it bans Congress and state governments from passing bills of attainder. The prohibition against bills of attainder reinforces the foundational idea in the American justice system that the government cannot punish someone unless they receive due process, typically in the form of a trial.
Ex post facto laws are prohibited by the Constitution's Ex Post Facto Clauses, which are found in Article I, Sections 9 and 10. These clauses ban the federal and state governments from enacting laws that impose criminal liability or increase criminal punishment retroactively. The Supreme Court has interpreted the Ex Post Facto Clauses to mean that people must have notice of the potential criminal penalties for their actions at the time they commit them.
Bills of attainder, on the other hand, are prohibited by the Constitution's Bill of Attainder Clause, also found in Article I, Section 9. This clause bans Congress from passing bills of attainder that "legislatively determine guilt and inflict punishment upon an identifiable individual without provision of the protections of a judicial trial." In other words, a bill of attainder is a law that punishes a specific person or group without them having the opportunity to defend themselves in a trial.
In some cases, a single legislative action may violate both the ex post facto and bill of attainder prohibitions. For example, in Cummings v. Missouri (1866), the Court held that a post-Civil War amendment to the Missouri Constitution violated both prohibitions because it required certain professionals to swear they had never been disloyal to the United States, effectively punishing a specific group of people (those disloyal to the Union) without a trial.
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Frequently asked questions
An ex post facto law is a law that retroactively changes the legal consequences of actions committed before the law was passed.
No, the US Constitution prohibits Congress and state governments from passing ex post facto laws.
The prohibition on ex post facto laws protects people from legislative determinations of guilt and reinforces the Constitution's separation of powers.
Examples of ex post facto laws include laws that criminalize previously innocent actions, aggravate a crime by bringing it into a more severe category, or change the punishment for a crime by adding new penalties.











































