Retroactivity in law is the application of a given norm to events that took place or began to produce legal effects, before the law was approved. Most countries are guided by the general principle of irretroactivity of law, which severely restricts this kind of ex-temporary application.
Until the middle of the 20th century, common-law systems in the United States operated on the premise that the law was natural and the courts only found the law. This presumption led to all decisions being retroactively applied to all prior cases that involved the issue in question. However, with the development of legal realism and the proliferation of legislatively enacted law, the courts shifted to a role of interpreting law that had been "made" rather than a role of "finding" natural law. Complete retroactivity was abolished by the Supreme Court in Griffith v. Kentucky, 479 U.S. 314 (1987).
Currently in the United States there are four forms of retroactivity (or preclusion of retroactive application) when a new rule is developed on appeal:[1]
In many countries, including Mexico, there is an exception to the rule of irretroactivity in criminal or penal law. Mexico recognizes the irretroactivity of law as a general principle; however, when it comes to penal law, the law itself states – and has done so since 1857 – that the laws can only be applied retroactively, if the law that was in vigor when the crime took place is more favorable to the processed person, or said person chooses to have that law applied to him.