Ex Post Facto: Definition, Laws & Example

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  • 0:00 What Is Ex Post Facto Law?
  • 1:03 Constitutional Prohibition
  • 1:42 The Reason for Ex Post Facto
  • 2:25 When Does Ex Post Facto Apply?
  • 2:52 Examples of Ex Post Facto
  • 4:15 Lesson Summary
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Lesson Transcript
Instructor: Temante Leary

Temante has taught college Business, Law and Criminal Justice courses. He has a master's degree in business and a juris doctorate (law) degree.

In this lesson, we will learn the definition of ex post facto. We'll learn the ways in which a law can be considered to be ex post facto. In addition, we will look at some examples of ex post facto laws.

What Is an Ex Post Facto Law?

Due process, or fairness, is one of the fundamental principles of United States democracy and the United States Constitution. Laws that violate your due process rights are thus considered to be unconstitutional. An ex post facto law is considered to be unconstitutional.

The term ex post facto in Latin means ''after the fact.'' An ex post facto law is a law that is passed after the fact that criminalizes an action that was legal when it was committed.

So in other words, let's say last week you accidentally spilled your coffee on the mayor of your city (we'll just say that you're that clumsy). Yesterday, he managed to get the city council to pass a law making that an illegal action and today, the cops are at your door to arrest you for the incident. The new law would be an ex post facto law as it applies to you because it wasn't illegal to spill your coffee when you did it, but now there is a law making it illegal, and they are trying to arrest you for it.

Constitution Prohibition

The Constitution prohibits ex post facto laws in two sections of Article I. In Article I, Section 9, Clause 3 of the United States Constitution it provides, ''No . . . ex post facto Law shall be passed,'' and in Article I, Section 10, Clause 1, it provides, ''No state shall pass any . . . ex post facto Law.'' The unconstitutionality of ex post facto laws in the United States has been well established, beginning with the Framers of our Constitution. You are protected under the Constitution from ex post facto laws being enforced like the mayor and police tried to do in the previous example.

The Reason for Ex Post Facto

The Framers of the Constitution wanted to ensure that you were not unfairly punished for actions that were legal when you committed them. Today, most democratic nations ban ex post facto laws or otherwise discourage them because they simply aren't fair. If you were to be arrested for spilling your coffee on the mayor in the previous example, that would not be fair.

The main idea of ex post facto is you should not expect to be punished for doing something that was legal when you did it just because later a law is passed making whatever you did illegal. You are entitled to know what you can and cannot do, and the ability to change that after the fact denies you due process and would give the government too much power.

When Does Ex Post Facto Apply?

In general, a statute retroactively alters the criminal law if it:

(1) makes criminal an act that was not a crime when committed

(2) proscribes greater punishment for a crime after its commission

(3) decreases the amount of evidence required for a conviction

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