From pages 25-29 of the decision:
"Our determination that the death penalty is disproportionate punishment for offenders under 18 finds confirmation in the stark reality that the United States is the only country in the world that continues to give official sanction to the juvenile death penalty. This reality does not become controlling, for the task of interpreting the EighthAmendment remains our responsibility. Yet at least from the time of the Court’s decision in Trop, the Court has referred to the laws of other countries and to international authorities as instructive for its interpretation of the Eighth Amendment’s prohibition of “cruel and unusual punishments.” 356 U. S., at 102–103 (plurality opinion) (“The civilized nations of the world are in virtual unanimity that statelessness is not to be imposed as punishment for crime”); see also Atkins, supra, at 317, n. 21 (recognizing that “within the world community, the imposition of the death penalty for crimes committed by mentally retarded offenders is overwhelmingly disapproved”); Thomp22 ROPER v. SIMMONS Opinion of the Court son, supra, at 830–831, and n. 31 (plurality opinion) (noting the abolition of the juvenile death penalty “by other nations that share our Anglo-American heritage, and by the leading members of the Western European community,” and observing that “[w]e have previously recognized the relevance of the views of the international community in determining whether a punishment is cruel and unusual”); Enmund, supra, at 796–797, n. 22 (observing that “the doctrine of felony murder has been abolished in England and India, severely restricted in Canada and a number of other Commonwealth countries, and is unknown in continental Europe”); Coker, supra, at 596, n. 10 (plurality opinion) (“It is . . . not irrelevant here that out of 60 major nations in the world surveyed in 1965, only 3 retained the death penalty for rape where death did not ensue”). As respondent and a number of amici emphasize, Article37 of the United Nations Convention on the Rights of the Child, which every country in the world has ratified save for the United States and Somalia, contains an express prohibition on capital punishment for crimes committed by juveniles under 18. United Nations Convention on the Rights of the Child, Art. 37, Nov. 20, 1989, 1577 U. N. T. S. 3, 28 I. L. M. 1448, 1468–1470 (entered into force Sept. 2, 1990); Brief for Respondent 48; Brief for European Union et al. as Amici Curiae 12–13; Brief for President James Earl Carter, Jr., et al. as Amici Curiae 9; Brief for Former U. S. Diplomats Morton Abramowitz et al. as Amici Curiae 7; Brief for Human Rights Committee of the Bar of England and Wales et al. as Amici Curiae 13–14. No ratifying country has entered a reservation to the provision prohibiting the execution of juvenile offenders. Parallel prohibitions are contained in other significant international covenants. See ICCPR, Art. 6(5), 999U. N. T. S., at 175 (prohibiting capital punishment for anyone under 18 at the time of offense) (signed and ratified by the United States subject to a reservation regard23 Cite as: 543 U. S. ____ (2005) Opinion of the Court ing Article 6(5), as noted, supra, at 13); American Convention on Human Rights: Pact of San José, Costa Rica, Art. 4(5), Nov. 22, 1969, 1144 U. N. T. S. 146 (entered into force July 19, 1978) (same); African Charter on the Rights and Welfare of the Child, Art. 5(3), OAU Doc. CAB/LEG/ 24.9/49 (1990) (entered into force Nov. 29, 1999) (same).Respondent and his amici have submitted, and petitioner does not contest, that only seven countries other than the United States have executed juvenile offenders since 1990: Iran, Pakistan, Saudi Arabia, Yemen, Nigeria,the Democratic Republic of Congo, and China. Since then each of these countries has either abolished capital punishment for juveniles or made public disavowal of the practice. Brief for Respondent 49–50. In sum, it is fair to say that the United States now stands alone in a world that has turned its face against the juvenile death penalty.Though the international covenants prohibiting the juvenile death penalty are of more recent date, it is instructive to note that the United Kingdom abolished the juvenile death penalty before these covenants came into being. The United Kingdom’s experience bears particular relevance here in light of the historic ties between our countries and in light of the Eighth Amendment’s own origins. The Amendment was modeled on a parallel provision in the English Declaration of Rights of 1689, whichprovided: “[E]xcessive Bail ought not to be required nor excessive Fines imposed; nor cruel and unusuall Punishments inflicted.” 1 W. & M., ch. 2, §10, in 3 Eng. Stat. at Large 441 (1770); see also Trop, supra, at 100 (plurality opinion). As of now, the United Kingdom has abolished the death penalty in its entirety; but, decades before it took this step, it recognized the disproportionate nature of the juvenile death penalty; and it abolished that penalty as a separate matter. In 1930 an official committee recommended that the minimum age for execution be raised to 21. House of Commons Report from the Select Commit24 ROPER v. SIMMONS Opinion of the Court tee on Capital Punishment (1930), 193, p. 44. Parliament then enacted the Children and Young Person’s Act of 1933, 23 Geo. 5, ch. 12, which prevented execution of those aged 18 at the date of the sentence. And in 1948, Parliament enacted the Criminal Justice Act, 11 & 12 Geo. 6, ch. 58, prohibiting the execution of any person under 18 at thetime of the offense. In the 56 years that have passed since the United Kingdom abolished the juvenile death penalty, the weight of authority against it there, and in the international community, has become well established. It is proper that we acknowledge the overwhelming weight of international opinion against the juvenile deathpenalty, 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 worldcommunity, while not controlling our outcome, does provide respected and significant confirmation for our ownconclusions. 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 25 Cite as: 543 U. S. ____ (2005) Opinion of the Court 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."
When discussing a novel issue of law, courts can look at arguments from pretty much anywhere. These arguments are not controlling legal authority, but are what is known as persuasive legal authority.
I actually don't see a problem with this passage. This is just a wide-ranging discussion of international trends and SCOTUS's history of ruling on the 8th Amendment.