Legal Definitions - ex post facto

Ex post facto is a Latin term meaning "from a thing done afterward." In legal contexts, it refers to a law that retroactively changes the legal consequences of actions that were committed before the law was enacted. Most commonly, an ex post facto law is a criminal statute that:

• Punishes conduct that was legal when it occurred.
• Increases the punishment for a crime after it was committed.
• Removes a defense that was available to a person at the time they committed an alleged offense.

The United States Constitution explicitly prohibits both federal and state governments from passing ex post facto laws. This prohibition is a fundamental principle of fairness, ensuring that individuals can understand what conduct is illegal and what the consequences will be at the time they act, without fear of being retroactively penalized by new laws.

Examples:

• Criminalizing Past Legal Behavior:

  Imagine a city where, for many years, it was perfectly legal to operate a short-term rental (like an Airbnb) in residential zones without any special permits. Sarah owned a home and rented out a spare room to tourists for several years, always following existing regulations. Last year, the city council passed a new ordinance making all short-term rentals illegal in residential zones and included a clause stating that anyone who operated such a rental in the past five years could be fined and face legal action.

  How it illustrates ex post facto: This new ordinance would be an ex post facto law because it retroactively criminalizes an activity (operating a short-term rental) that was entirely legal when Sarah engaged in it. She could not have known she was committing an offense at the time she was renting out her room.

• Increasing Punishment Retroactively:

  Consider a state where, in 2018, the maximum penalty for a first-time conviction of misdemeanor shoplifting was a $500 fine and no jail time. David was convicted of shoplifting in 2018, paid his fine, and completed his sentence. In 2022, the state legislature, concerned about rising theft rates, passed a new law that retroactively increased the penalty for all past misdemeanor shoplifting convictions to include a mandatory 30 days in jail, even for those who had already completed their original sentence.

  How it illustrates ex post facto: This new law would be an ex post facto law because it attempts to make the punishment for a crime (misdemeanor shoplifting) more burdensome *after* David had already committed the crime and served his original, lesser sentence. The law cannot retroactively apply a harsher penalty.

• Removing an Available Defense:

  Suppose in 2017, a state law allowed for a "duress" defense in certain non-violent crimes, meaning if a person committed a crime because they were under immediate threat of severe harm from another individual, they could argue they were not fully responsible. Emily was forced by an armed assailant to participate in a minor property crime in 2017 and was subsequently charged. Before her trial in 2018, the state legislature amended the law, completely eliminating the duress defense for all crimes, and specified that this change applied to all pending cases, including Emily's.

  How it illustrates ex post facto: This new law would be an ex post facto law because it deprives Emily of a legal defense (the duress defense) that was explicitly available to her under the law at the exact time she committed the alleged act. She should be tried under the laws and defenses that were in place at the time of the incident.