In Calder v. Bull, the Supreme Court recognized four types of laws that cannot be applied retroactively consistent with the Ex Post Facto Clause, including “[e]very law that alters the legal rules of evidence, and receives less, or different, testimony, than the law required at the time of the commission of the offence, in order to convict the offender.” But, in its opinion in Carmell v. Texas, the Court determined that ordinary rules of evidence do not violate the Clause because they (1) are “evenhanded, in the sense that they may benefit either the State or the defendant in a given case;” and (2) “do not at all subvert the presumption of innocence….”
Federal Rules of Evidence 413 and 414 as well as state counterparts, however, are neither evenhanded nor consistent with the presumption of innocence. Instead, these rules can only be used to benefit the prosecution, and they subvert the traditional presumption of innocence maintained by the propensity character evidence proscription. Accordingly, courts across the country have erred in finding that the retroactive application of these rules does not violate the Ex Post Facto Clause.
Colin Miller, Bullshit!: Why the Retroactive Application of Federal Rules of Evidence 413-414 and State Counterparts Violates the Ex Post Facto Clause, 4 Neb. L. Rev. Bull. 3 (2012), http://lawreviewbulletin.unl.edu/?p=1060.