Legal Definitions - ex post facto law

An ex post facto law is a legal term referring to a law that applies to events that occurred before the law was enacted, especially in a way that negatively affects a person's rights or status. In the United States, the Constitution specifically prohibits the government from passing ex post facto criminal laws. This means that a person cannot be punished for an action that was legal when they committed it, nor can they receive a harsher punishment than what was in effect at the time of the offense. The prohibition also prevents laws that make it easier to convict someone by changing the rules of evidence or procedure after the fact.

While ex post facto criminal laws are strictly forbidden, some laws that apply retroactively in civil (non-criminal) matters may be permissible under certain circumstances.

• Example 1: Criminalizing a Previously Legal Act

  Imagine a state passes a new law making it a felony to possess certain types of rare, exotic plants, effective January 1st. If the state then attempts to prosecute someone for possessing such a plant that they legally acquired and owned before January 1st, this would be an ex post facto law. The law is being applied retroactively to criminalize an action (possessing the plant) that was perfectly legal at the time it occurred, thereby negatively impacting the individual's rights.

• Example 2: Increasing Punishment Retroactively

  Sarah commits a specific type of fraud in March, at which time the maximum penalty for that crime is two years in prison. In June, before Sarah's trial, the legislature passes a new law increasing the maximum penalty for that same fraud to five years in prison. If the court were to sentence Sarah to three years in prison under the new five-year maximum, it would be applying an ex post facto law. The law retroactively increases the punishment for a crime committed before the new, harsher penalty was in effect, thereby negatively affecting Sarah's rights.

• Example 3: Changing Rules of Evidence to Disadvantage the Accused

  A state has a rule of evidence that prohibits certain types of digital forensic analysis from being used in court. John is accused of a cybercrime committed in July. In August, the state legislature changes the rule, making that specific digital forensic analysis admissible. If the prosecution then uses this newly admissible evidence against John in his trial for the July crime, it could be challenged as an ex post facto application of the law. The change in the rule of evidence, applied retroactively, makes it easier to convict John for an act committed when the old, more favorable rule was in place, thus disadvantaging him after the fact.