On Tuesday I discussed the Supreme Court’s decision to strike down laws that allow juveniles to be sentenced to life without parole (LWOP) for non-homicide crimes. What concerns me here isn’t so much the morality or policy wisdom in applying such sentences — though Chief Justice Roberts makes some good policy points in his concurrence — or even the interpretation of what constitutes a “cruel and unusual punishment” — which I think Justice Kennedy mishandles in a confusing discussion of national consensuses.

No, the most troubling part of that case was the unfortunate reference to foreign authorities to support the Court’s interpretation of the Eighth Amendment. Justice Kennedy notes that juvenile LWOP has been “rejected the world over.” “The judgment of the world’s nations that a particular sentencing practice is inconsistent with basic principles of decency,” he writes, “demonstrates that the court’s rationale has respected reasoning to support it.”

Justice Thomas, in his dissent, disputes Justice Kennedy’s math, noting that 11 countries allow the punishment. More importantly, “foreign laws and sentencing practices” are “irrelevant to the meaning of our Constitution.” He adds that most democracies around the world remain free to adopt the punishment should they wish to. “Starting today,” Thomas concludes, “ours can count itself among the few in which judicial decree prevents voters from making that choice.”

And that’s the crux of the matter: citing foreign law, using it to support a given reading of domestic law undermines democratic self-governance. The interpretation of the U.S. Constitution should depend on that document’s text, structure, and history, what it means in the context of the American polity. Even if a judge cares about “evolving standards of decency” or invokes the “living Constitution,” it should be the updated standards in America that matter, or the opinions and values of modern Americans.

That is, federal judges derive their powers from the Constitution, which is a wholly American document. To the extent they use foreign extrinsic evidence to interpret this document, they are engaging in something — comparative law? social science? — that is not judging. It’s not a matter of being closed-minded or provincial — I actually enjoy reading comparative political research, and think our legislators and constitutional draftsmen engage in malpractice if they don’t use it — but, as Justice Thomas describes in Graham, the judicial role is different than the legislative or academic one.

Now, in practice U.S. courts actually rarely cite foreign law, and most of the time when they do it’s not controversial. For example, it’s relevant to see how all the contracting parties interpret a treaty, because you want a treaty (a contract among nations) to be understood the same way everywhere. Similarly, foreign court pronouncements are relevant to interpreting customary international law — the law of nations as the Framers understood it — to the limited extent it applies to a given case (crime on the high seas and the like). Next we have the coordination of litigation, with international companies suing each other based on contracts that specify that “X” provision is subject to British law whereas “Y” deals with Hong Kong law, and that the arbitration forum is supposed to be Switzerland: here the citation of foreign law is absolutely appropriate. Another appropriate use is in conflict of laws analysis: figuring out which law applies and sometimes even applying foreign law as binding in a dispute.

But using foreign law to interpret domestic law, and especially the Constitution, is problematic — but the Supreme Court does it more than lower courts, particularly in high profile cases: those involving the culture wars, moral issues like the death penalty and abortion, and other charged cases like affirmative action and sex discrimination. Libertarians should not welcome this trend because it signals judging based on something other than the principled reading of our own laws — in short, judicial usurpation of the policy-making function.

Hans Bader of CEI provides a longer write-up of Graham, and here again is Cato’s brief. For a pithy critique of the improper use of foreign law by U.S. courts, see Richard Posner’s now-famous article in Legal Affairs. And for an in-depth and entertaining exploration of these issues, read or watch a debate Justices Scalia and Breyer had in 2005.

Coincidentally, the same day the Court issued both Graham and Comstock (which I discuss here), it also decided an important case, Abbott v. Abbott, that uses foreign law to interpret an international treaty on child abduction. (While I haven’t yet gone through the Abbott decision, both the majority and dissent are correct to use foreign law to help them reach their conclusions.)