What Is the Ex Post Facto Clause and How Does It Limit Criminal Law ?
The ex post facto clause serves three purposes.
First and foremost, the Framers intended it to create a "constitutional bulwark" against "the favorite and most formidable instruments of tyranny" (The Federalist Nos. 44 (James Madison) and 84 (Alexander Hamilton)) including laws which "inflicted greater punishment than the law annexed to the offence" (Calder v. Bull, supra, 3 U.S. at p. 389 (Chase, J.).) (See also 2 Records of the Federal Convention, supra, at p. 375.)
In addition, historical records and 200 years of Supreme Court precedent indicate a central concern of the Framers was that legislatures "give fair warning" when they increase the punishment for a crime. (The Federalist No. 44, supra; Calder v. Bull, supra, 3 U.S. at p. 388, Chase, J.; Miller v. Florida (1987) 482 U.S. 423, 430, 96 L. Ed. 2d 351, 107 S. Ct. 2446.)
Finally, the ex post facto clause was intended to stop "the practice of arbitrary imprisonments" suffered by the Colonists under British rule. (The Federalist No. 84, supra.)
These considerations remain relevant today. (See e.g., Weaver v. Graham, supra, 450 U.S. at pp. 28-29; People v. Frazer, supra, 21 Cal. 4th at p. 754.)
They suggest the following test for determining whether a law is proscribed by the ex post facto prohibition. Does the law penalize activities in the absence of fair warning? Does retroactive application of the law have no apparent purpose other than to increase the punishment for an act after it is committed? Is the law a consequence of legislative vindictiveness? (See Note, Ex Post Facto Limitations on Legislative Power (1975) 73 Mich. L. Rev. 1465, 1501.)
The concept of prior notice embodied in the ex post facto clause is among "the first principles of the social compact" (The Federalist No. 44, supra) and so rooted in American notions of justice (Landgraf v. USI Film Products (1994) 511 U.S. 244, 265, 128 L. Ed. 2d 229, 114 S. Ct. 1483) it is difficult to imagine ex post facto laws would be enforceable even without a specific constitutional ban (2 Records of the Federal Convention, supra, at p. 376).
Thus, while the Framers had good cause to fear a vindictive government, the most significant role of the ex post facto clause today is to give fair warning of the consequences of criminal behavior. (See, e.g., Miller v. Florida, supra, 482 U.S. at p. 430; Weaver v. Graham, supra, 450 U.S. at p. 28; Dobbert v. Florida (1977) 432 U.S. 282, 297, 53 L. Ed. 2d 344, 97 S. Ct. 2290.)