The belief that diminished capacity was a legitimate defense was so widely held by the Michigan legal community that, before Carpenter, the Michigan State Bar's Criminal Jury Instructions included an instruction on the defense. Commentary accompanying the instruction noted that "[t]he defense of diminished capacity is available when the defendant's mental impairment leaves him or her unable to form the specific intent needed to commit the crime." Standing Comm. on Std.Crim. Jury Instructions, Mich. State Bar, 1 Mich.Crim. Jury Instructions 6.3, at 6-11 (2d ed. 1994) (This instruction was removed only after Carpenter was decided.).
It is indisputable that defendants were able to raise the defense prior to Carpenter, but the availability of the defense alone does not make its elimination unexpected. Indeed, in Rogers, the year-and-a-day rule had been available to defendants for nearly one hundred years, but the Supreme Court nevertheless concluded that its elimination was foreseeable because the rule never served as the basis for a decision in the state and many other states had abolished the rule. In Michigan, diminished capacity likewise never served as the basis for any court's decision. Even in Michigan v. Griffin, 433 Mich. 860, 444 N.W.2d 139 (1989), the Michigan Supreme Court's remand for an evidentiary hearing to determine if counsel was ineffective for failing to raise diminished capacity as a defense reemphasizes only the point that the defense was available. It does not indicate that the defense was so well established that its elimination was unexpected.
Can a state retroactively abolish a defense, even a "partial" defense such as diminished capacity? If it is purely a legislative change, clearly not. That is the heart of the constitutional prohibition of ex post facto laws. A statute enacted after the crime that makes previously legal conduct criminal or increases the punishment of previously illegal conduct cannot be applied. But how about a court's reinterpretation of a previously enacted statute? Tougher question. The Supreme Court has found a due process protection against court-made changes to substantive criminal law, but it is not exactly congruent with the ex post facto rule for statutes.