Legal Definitions - Ex Post Facto Clause

The Ex Post Facto Clause refers to specific provisions in the U.S. Constitution that prohibit the government from passing "ex post facto" laws. The Latin phrase "ex post facto" literally means "from a thing done afterward." In legal terms, an ex post facto law is one that retroactively changes the legal consequences of actions that were committed before the law was enacted.

Essentially, this clause protects individuals by ensuring they cannot be punished for an act that was not a crime when they committed it, or receive a harsher punishment than what was in place at the time of the offense. It also prevents the government from making it easier to convict someone for a past crime by changing the rules of evidence or procedure after the fact.

Here are some examples to illustrate the Ex Post Facto Clause:

• Example 1: Retroactively Criminalizing an Act

  Imagine a city council passes a new ordinance making it illegal to use personal electric scooters on public sidewalks, imposing a significant fine. If someone used their electric scooter on a public sidewalk last week, before this new ordinance was enacted, they cannot be cited or fined under the new law. The Ex Post Facto Clause prevents the city from retroactively making their past, legal conduct into a crime.

• Example 2: Retroactively Increasing Punishment

  A state legislature decides to increase the maximum prison sentence for a specific type of environmental pollution from five years to ten years, effective immediately. If a company committed an act of pollution before this new law took effect, and their case goes to trial after the law changes, they can only be sentenced up to the original five-year maximum. The Ex Post Facto Clause protects them from receiving a harsher punishment that did not exist at the time their offense occurred.

• Example 3: Retroactively Changing Rules to Disadvantage the Accused

  Consider a state that passes a new law simplifying the requirements for prosecutors to introduce certain types of digital evidence in court, making it easier to secure convictions for cybercrimes. If an individual is on trial for a cybercrime committed before this new evidence rule was enacted, the prosecution cannot use this new, more lenient rule to introduce evidence against them that would have been inadmissible under the old rules. The Ex Post Facto Clause prevents the application of new procedural rules that would disadvantage the accused by making a conviction more likely for past actions.