Calder v. Bull (ex post facto laws)

[A18]

Calder v. Bull,  3 U.S. 386 (1798)

How do you define an ex post facto law?

[J. J.]

Constitutionally sound. 

[Emsworth]

The original understanding of "ex post facto law" encompassed only laws relating to crimes. Sir William Blackstone, in his Commentaries on the Laws of England, defines an ex post facto law thus: "when after an action (indifferent in itself) is committed, the legislator then for the first time declares it to have been a crime, and inflicts a punishment upon the person who has committed it."
Obviously, at the very least, an "ex post facto law" would include a law that retroactively criminalizes an action. I would also include (like Justice Chase): a law that retroactively increases the punishment for a crime; a law that retroactively aggravates the crime; and a law that retroactively changes the evidentiary standards for conviction. Thus, a law that retroactively restores the death penalty, or that retroactively reclassifies an offense from manslaughter to murder, or that retroactively imposes a lower burden of proof on the prosecution, would also be consdiered an ex post facto law.

Thus, for example, a law retroactively invalidating a contract would not be an ex post facto law (it would be unconstitutional, yes, but not an ex post facto law).