Ex Post Facto Punishment
For a system to be fair to all parties, it must have the essential element of finality. The rule against Ex Post Facto punishment is rooted in the theory that a defendant must be given notice of the punishment in order for due process to have taken place. Therefore, a defendant may not be retro-actively punished and any legislation that occurs after the offense can not be used against the defendant.
In certain cases a court may determine that a piece of legislation that has ex post facto effects, does not violate this principle if the effects are regulatory and not punitive in nature.
International Covenent on Civil and Political Rights, Article 14, Section 7- "No one shall be liable to be tried or punished again for an offence for which he has already been finally convicted or acquitted in accordance with the law and penal procedure of each country."
Examples of Prohibitions on Ex Post Facto Punishment
"No bill of attainder or ex post facto Law shall be passed."- Article I, section 9, US Constitution.
- 92. Restriction on retrospective legislation.
- Parliament shall not pass any law to alter the decision or judgment of any court as between the parties to the decision or judgment.