Ex Post Facto Laws 

Fundamentals of Criminal Law by Adam J. McKee

A law enacted to punish behavior after the behavior occurs is known as an ex post facto law. Such laws are prohibited by The vast majority of state constitutions have a similar prohibition.  

Article 1, Section 9, of the United States Constitution. For example, Article 2, Section 17 of the Arkansas Constitution prohibits such laws as well. The idea is that it is unfair to punish people for acts that they had no way of knowing were prohibited.

Term of Art:  Retroactive

The term retroactive is used in law to signify that a law takes effect from a past date.

An ex post facto law, then, punishes an individual retroactively.  The basic logic of the prohibition is that such laws are an affront to fundamental fairness.  Generally speaking, there are three types of ex post facto laws. First, a law is ex post facto if it punishes behavior that occurred before the law was in effect. Second, ex post facto laws may increase the punishment for the offense after the criminal act occurred. Third, a law can be ex post facto if it increases the chance of conviction after the criminal act occurred.

It is important to note that changes benefiting a criminal defendant are not considered ex post facto and may constitutionally be applied retroactively. In the preceding example, if the state amended the murder statute to shorten the statute of limitations, this change actually benefits defendants by making it more difficult to convict them. Thus this amendment would be constitutional.

References and Further Reading

Ex Post Facto Laws.” West’s Encyclopedia of American Law.

Retroactivity of Judicial Decisions.” Encyclopedia of the American Constitution.

Modification History

File Created:  07/12/2018

Last Modified:  09/05/2023

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This work is licensed under an Open Educational Resource-Quality Master Source (OER-QMS) License.

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