Retrospective Law, also referred to as ex post facto laws, are laws which change the legal consequences of an action after it actually occurred. This has the unfair effect of imposing a penalty on a person even though he or she acted in a perfectly legal manner at the time in question.
Retrospective criminal laws are prohibited by global and regional human rights treaties. Laws should be prospective, as well as sufficiently precise and adequately accessible, so a person can know whether in advance whether their conduct is or is not legal.
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