Mental retardation as a mitigating factor ≠ mental retardation constitutionally barring execution.

These are 2 different issues, the unanimous Supreme Court said today. And Justice Ginsburg, the author of the opinion, schools the Sixth Circuit in the law of issue preclusion:

[M]ental retardation for purposes of Atkins, and mental retardation as one mitigator to be weighed against aggravators, are discrete issues. Most grave among the Sixth Circuit’s misunderstandings, issue preclusion is a plea available to prevailing parties. The doctrine bars relitigation of determinations necessary to the ultimate outcome of a prior proceeding. The Ohio courts’ recognition of Bies’ mental state as a mitigating factor was hardly essential to the death sentence he received. On the contrary, the retardation evidence cut against the final judgment. Issue preclusion, in short, does not transform final judgment losers, in civil or criminal proceedings, into partially prevailing parties.

Ouch.