Two months ago, the U.S. Supreme Court handed down a decision of sweeping legal consequence, relying not on the Constitution's text but on national public opinion polls. In declaring the death penalty for mentally retarded inmates to be cruel and unusual punishment, the high court rooted its decision in a recent trend among states to spare the retarded from execution. It was not the first time the court had considered this issue. Just 13 years earlier, the court reached exactly the opposite outcome on precisely the same question, holding that executing the mentally retarded didn't violate the Eighth Amendment. Whether...