John Paul Stevens

“ In Part IV-A the Court decides that a rule that the Eighth Amendment prohibits the execution of a mentally retarded person ought to apply retroactively. Assuming retroactivity is pertinent, I agree that the first exception to Justice Harlan's nonretroactivity doctrine "should be understood to cover not only rules forbidding criminal punishment of certain primary conduct but also rules prohibiting a certain category of punishment for a class of defendants because of their status or offense," ante, at 330, and that this claim lies within that exception. ”