The use of "international opinion" in today's death penalty decision.

Justice Kennedy, writing for the majority in Roper v. Simmons, justifies the use of "international opinion" in constitutional decisionmaking:

It is proper that we acknowledge the overwhelming weight of international opinion against the juvenile death penalty, resting in large part on the understanding that the instability and emotional imbalance of young people may often be a factor in the crime. See Brief for Human Rights Committee of the Bar of England and Wales et al. as Amici Curiae 10—11. The opinion of the world community, while not controlling our outcome, does provide respected and significant confirmation for our own conclusions.

Over time, from one generation to the next, the Constitution has come to earn the high respect and even, as Madison dared to hope, the veneration of the American people. See The Federalist No. 49, p. 314 (C. Rossiter ed. 1961). The document sets forth, and rests upon, innovative principles original to the American experience, such as federalism; a proven balance in political mechanisms through separation of powers; specific guarantees for the accused in criminal cases; and broad provisions to secure individual freedom and preserve human dignity. These doctrines and guarantees are central to the American experience and remain essential to our present-day self-definition and national identity. Not the least of the reasons we honor the Constitution, then, is because we know it to be our own. It does not lessen our fidelity to the Constitution or our pride in its origins to acknowledge that the express affirmation of certain fundamental rights by other nations and peoples simply underscores the centrality of those same rights within our own heritage of freedom.

Justice Scalia responds:

I do not believe that approval by “other nations and peoples” should buttress our commitment to American principles any more than (what should logically follow) disapproval by “other nations and peoples” should weaken that commitment. More importantly, however, the Court’s statement flatly misdescribes what is going on here. Foreign sources are cited today, not to underscore our “fidelity” to the Constitution, our “pride in its origins,” and “our own [American] heritage.” To the contrary, they are cited to set aside the centuries-old American practice–a practice still engaged in by a large majority of the relevant States–of letting a jury of 12 citizens decide whether, in the particular case, youth should be the basis for withholding the death penalty. What these foreign sources “affirm,” rather than repudiate, is the Justices’ own notion of how the world ought to be, and their diktat that it shall be so henceforth in America. The Court’s parting attempt to downplay the significance of its extensive discussion of foreign law is unconvincing. “Acknowledgment” of foreign approval has no place in the legal opinion of this Court unless it is part of the basis for the Court’s judgment–which is surely what it parades as today.

Justice O'Connor, who also dissents, looks at it differently:

Over the course of nearly half a century, the Court has consistently referred to foreign and international law as relevant to its assessment of evolving standards of decency. See Atkins, 536 U.S., at 317, n. 21; Thompson, 487 U.S., at 830—831, and n. 31 (plurality opinion); Enmund, 458 U.S., at 796—797, n. 22; Coker, 433 U.S., at 596, n. 10 (plurality opinion); Trop, 356 U.S., at 102—103 (plurality opinion). This inquiry reflects the special character of the Eighth Amendment, which, as the Court has long held, draws its meaning directly from the maturing values of civilized society. Obviously, American law is distinctive in many respects, not least where the specific provisions of our Constitution and the history of its exposition so dictate. ... But this Nation’s evolving understanding of human dignity certainly is neither wholly isolated from, nor inherently at odds with, the values prevailing in other countries. On the contrary, we should not be surprised to find congruence between domestic and international values, especially where the international community has reached clear agreement–expressed in international law or in the domestic laws of individual countries–that a particular form of punishment is inconsistent with fundamental human rights. At least, the existence of an international consensus of this nature can serve to confirm the reasonableness of a consonant and genuine American consensus. The instant case presents no such domestic consensus, however, and the recent emergence of an otherwise global consensus does not alter that basic fact.

UPDATE: Here's my account of a recent debate between Justices Scalia and Breyer about the use of international opinion in American constitutional law decisions.