WASHINGTON – When the justices of the Supreme Court debate the constitutionality of the juvenile death penalty tomorrow, the governments of 48 countries will be among those asking them to strike it down.

America stands “virtually alone” in carrying out the executions of murderers who were 17 and 18 years old at the time they committed the crime, states a brief to the court from the European Union and other countries including Canada, Mexico, and Iceland.

But the extent to which international opinion is relevant to the interpretation of the American Constitution is itself a hotly debated issue before the court.

The question is part of a broader battle over America’s acceptance of international laws, treaties, and institutions that was highlighted in the runup to the Iraq war, in the treatment of prisoners in the fight against terrorism, and in the visions of the nation’s place in the world outlined by the presidential candidates.

Tomorrow the court will grapple with the Eighth Amendment, which bars “cruel and unusual punishments” but does not provide criteria for identifying such practices. The Supreme Court has said the list includes punishments considered cruel and unusual in 1789 when the Bill of Rights was adopted, and also draws its meaning “from the evolving standards of decency that mark the progress of a maturing society.”

The question is, which society?

Lawyers for convicted murderer Christopher Simmons, who was 17 when he killed a woman he had robbed, argue that “a world-wide revulsion against the execution of juveniles” confirms what they say is a national consensus that the death penalty for 17- and 18-year-olds is “cruel and unusual.”

Lawyers for the state of Missouri, where Simmons had been on death row, argue that the court must consider only whether the punishment serves a purpose, whether it is proportional to the gravity of the crime, and whether state legislatures and American juries continue to apply it.

“In ascertaining the standard of decency of American society, courts should look to the political attitudes of our society, not of societies from around the globe,” their brief states.

In 1993, then 17-year-old Simmons, who is now 28, broke into the home of Shirley Crook and robbed her with the help of two teenage accomplices. When the victim recognized him from an earlier car accident, the youths gagged and bound her and drowned her in a river. Simmons later confessed to the murder and was convicted for first-degree murder and sentenced to death. In 2003, the Missouri Supreme Court set aside his death sentence and re-sentenced him to life in prison without the possibility of parole. State prosecutors appealed the court’s decision.

Human rights advocates see the Simmons case as a chance to open more fully the door to international norms that the court nudged open in 2002, when it reversed its own precedent and struck down the death penalty for people with mental retardation in a case known as Atkins. The court noted that the practice was banned in other countries. The Missouri court relied on that reversal to strike down Simmons’s death sentence, finding both a national and international consensus against the practice.

Dozens of groups have filed passionate briefs on both sides of the debate, ranging from Nobel laureates to psychiatric organizations and religious figures like the Dalai Lama.

Simmons’s lawyers claim the court has historically allowed foreign opinions and practices to inform the definition of “cruel and unusual,” a phrase that was taken directly from the English Declaration of Rights of 1688 and represented a principle that can be traced back to the Magna Carta.

“The basic concept underlying the Eighth Amendment is nothing less than the dignity of man,” states a 1958 ruling holding unconstitutional the stripping of army deserters of their citizenship.

In 1977, the court also said the climate of international opinion is “not irrelevant” to whether a punishment was cruel and unusual.

However, foreign practices have generally been relegated to footnotes in the court’s decisions. American reluctance to give weight to international opinion is something human-rights lawyers would like to see changed.

“It is a broader issue about how the courts look to sources outside the United States and the executive branch looks to and honors treaty obligations, and the evolving views in the world community, and our respect for them,” said the author of the E.U.’s brief and a professor of law at the American University’s Washington College of Law, Richard Wilson.

There has been “incredible damage to the U.S. image in the world because of our unwillingness to look to international standards and to say, we know best and we will decide what is appropriate,” he said, citing the prison abus es at Abu Ghraib, and the coalition building in the war in Iraq.

But advocates of a historically grounded interpretation of the Constitution say the practices of other nations should “not be legally relevant” to the understanding of the Eight Amendment.

“That is not a plausible way to interpret the Constitution. …I see it as a huge threat to interpreting the Constitution according to its original meaning,” said a professor of law at Northwestern University School of Law, John McGinnis.

Other countries have different legal systems and historical experiences, may have weaker due process protections, or may ban the death penalty altogether, he said. And while Europeans may embrace more liberal interpretations of some rights, they take a narrower view of the rights of property and free speech, he said.

To date, America has followed its own path on the juvenile death penalty. In 1989, the United Nations adopted the Convention on the Rights of the Child, which bars capital punishment for those under 18. Only the United States and Somalia did not ratify it.

Of the other countries believed to have executed young people in the past decade, only in America is the practice still legal. China, Pakistan, Nigeria, Yemen, and the Democratic Republic of the Congo have either banned the practice or commuted sentences. Saudi Arabia has denied reports that it executed a juvenile in 1992.

In January, Iran executed a man for a crime he committed as a 17-year-old. Iran has legislation pending that would ban the practice, but it is uncertain whether it will be enacted.

International arguments aside, the lawyers for Simmons argue they can win the case on domestic law alone. As part of their argument, they contend there is an evolving consensus in this country against the juvenile death penalty. Among the 38 states that have the death penalty, 18 prohibit the execution of juveniles.

They also point to a recent study by a Columbia University law professor, Jeffrey Fagan, and researched by Valerie West, which found a statistically significant decline in the use of the death penalty by judges and juries in the states where it is still legal.

“We see a decline in the use of juvenile death sentences virtually to zero, and that decline exceeds any other possible explanatory factor – greater than decline in homicide arrests and the decline in the murder rate,” Mr. Fagan said.

Missouri disputes the results. Mr. Nixon argues the trend is not in a consistent direction, because various states have passed legislation to officially make juvenile death sentences legal, with some setting legal age thresholds lower than state precedents.

They also note that Mr. Fagan’s study does not account for choices by prosecutors not to seek the death penalty. But Mr. Fagan said that could reflect “a judgment that judges and juries won’t convict” if death is the penalty, or because they themselves oppose the use of the death penalty.