Suppose that we simplify the court's Eighth Amendment jurisprudence greatly and pretend that a "national consensus" against a certain type of punishment exists when 10 states or fewer authorize that punishment and not otherwise; and that when a national consensus against a punishment comes into existence, the courts will strike down that punishment in any remaining state that continues to use it or any state that introduces it.
This rule acts as a ratchet. When a punishment falls to the 10-state threshold, it ceases to be permissible. If people in the various states change their minds and come to believe that the punishment is justified, legislatures will not be able to enact the punishment without violating the Constitution. It seems likely that they will therefore not bother, and so a new consensus in the other direction cannot get started. Perhaps, in the rare instances when a national consensus will develop quickly, dozens of states will enact the law even though it violates the Constitution, and courts will recognize a change in the consensus. But this is likely to be rare, and it loads the dice against national consensuses developing in favor of harsher punishments.
If the Eighth Amendment is just about national consensus or some such thing, why can't a consensus emerge in favor of a punishment that previously had been barred? The dissent in Kennedy v. Louisiana makes this argument, which is acknowledged but rejected (without any attempt at justification, as far as I can tell) by the majority.
Is there any justification in political, constitutional, or moral theory for such a ratchet? I don't see one. There is an old, simple-minded Whig view that human history reflects progressive moral development, and perhaps the idea is that courts can prevent temporary backsliding caused by public overreaction to ephemeral events—and such rhetoric about society "maturing" can be found in Justice Kennedy's majority opinion. But this view took a hit in the 1930s and has never recovered. And even if it were correct, a society might "mature" by introducing new harsh punishments against behavior—such as spousal and child abuse, or, say, honor killings of daughters—that earlier generations found unobjectionable. Current Eighth Amendment jurisprudence, or at least the logic behind it, would block such moral evolution.
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