Christopher Simmons was 17 on Sept. 8, 1993, when he and a 16-year-old accomplice broke into Shirley Crook's house, kidnapped her, drove her to a railroad trestle and threw her, bound and gagged, into the Meramec River. Crook's body surfaced the next day.

To the state of Missouri, this was not only a brutal crime but also a coldly calculated one, amply deserving of the death sentence a jury meted out.

But Simmons argues that he was a troubled youth at the time, emotionally unformed, impulsive and influenced by an older criminal mentor. To execute him, or anyone else, for a murder he committed while under the age of 18 would itself be an outrage -- a violation of the constitutional prohibition on "cruel and unusual" punishment.

Today, lawyers for both sides will argue Simmons's case before the Supreme Court, as the justices take up an issue that has divided them and touched a nerve at home and abroad.

There are 72 juvenile offenders on death row, including 42 in Texas and Alabama. There have been 22 executions of juvenile offenders since 1976, 18 of them in Texas, Virginia and Oklahoma.

With the United States under fire in international human rights forums because it is the only democracy that still permits the death penalty for offenders younger than 18, the court's ruling will have both national and worldwide significance.

The court is being heavily lobbied by international organizations and dignitaries in this case, with the European Union, Nobel Peace laureates headed by former president Jimmy Carter and a group of former U.S. ambassadors urging an end to what they consider U.S. isolation on the issue.

Such considerations are relevant in the Simmons case, because the court weighs death penalty laws according to what a 1958 ruling called the "evolving standards of decency that mark the progress of a maturing society." The court looks to state legislation and jury verdicts to decide whether a "national consensus" has developed against a practice that was previously accepted -- but it has recently opened the door to world opinion as a measure of moral consensus.

In 2002, the court voted 6 to 3 to strike down the death penalty for the moderately mentally retarded, which it had upheld 5 to 4 in 1989. In the 2002 case, Atkins v. Virginia, the court noted that the number of death penalty states that ban the practice had grown from two in 1989 to 13 in 2002, while none had gone the other way.

The ruling also took account of the fact that within the "world community," capital punishment for the retarded was "overwhelmingly disapproved."

The court struck down capital punishment for offenders age 15 and younger in 1988, but the following year it upheld the death penalty for 16- and 17-year-old murderers by 5 to 4. At the time, only 12 of the 37 death penalty states banned the practice -- insufficient evidence, the court ruled, of a "national consensus" against it.

Simmons and his supporters argue that, as in Atkins, times have changed. Since 1989, the number of death penalty states that do not permit the death penalty for juvenile offenders has grown to 19, with no states lowering the age. Juries have imposed capital punishment less frequently.

Perhaps the most recent case to focus the debate was that of Lee Boyd Malvo, 17, one of two D.C.-area snipers. Despite prosecutors' pleas, a Virginia jury chose to sentence Malvo, who had had a troubled childhood and fallen under the strong influence of his adult accomplice, John Allen Muhammad, to life imprisonment without parole.

But Missouri's brief notes that the trend toward abolition involves only a few states, so the legislative head count is "not appreciably different" from what it was in 1989.

And the states of Alabama, Delaware, Oklahoma, Texas, Utah and Virginia argue in a friend-of-the-court brief supporting Missouri that the Malvo case demonstrates not that society has rejected the death penalty for minors, but that jurors can distinguish when it is merited and when it is not.

Opponents of the juvenile death penalty cite recent progress in brain-scan techniques that has permitted scientists to see that juveniles lack full development of brain areas responsible for impulse control and risk assessment.

Health organizations led by the American Medical Association and the American Psychiatric Association told the justices in a friend-of-the-court brief that to execute adolescents is to "hold them accountable not just for their acts, but also for the immaturity of their neural anatomy and psychological development."

But Missouri responds that "The variation in the maturation process minimizes the value of age, standing alone, as a basis for determining when capital punishment serves or fails to serve societal interest."

The Supreme Court agreed to address the juvenile issue only after years of evident reluctance.

As recently as April last year, while Malvo was still awaiting trial, the court granted Oklahoma's request to reinstate the death sentence of a 17-year-old offender after it had been blocked by a federal appeals court.

In 2002, the court refused to hear two appeals from under-18 offenders asking it to reconsider their penalties in light of Atkins.

Two of these three cases came to the court under special rules that would have required five votes rather than the usual four to hear them. But four justices who oppose the death penalty for minors could not muster a fifth vote, which they protested publicly.

"We should put an end to this shameful practice," Justice John Paul Stevens wrote in a 2002 dissent joined by Justices David H. Souter, Ruth Bader Ginsburg and Stephen G. Breyer.

The justices had little choice but to hear the Simmons case, however, because it comes to them as the state's appeal from a Missouri Supreme Court ruling that explicitly refused to follow the 1989 precedent and decided that the logic of Atkins dictates the abolition of the juvenile death penalty. They could not ignore such a challenge to their authority by a state court.

Given that Chief Justice William H. Rehnquist and Justices Antonin Scalia and Clarence Thomas opposed Atkins, the case is likely to be decided by the courts two swing voters, Justice Sandra Day O'Connor and Anthony M. Kennedy.

They voted to uphold the death penalty for 16- and 17-year-olds in 1989, along with the death penalty for the mentally retarded; but they voted in the majority in Atkins.

Today's case is Roper v. Simmons, No. 03-633.

Christopher Simmons was 17 when he helped kidnap and murder a woman in Missouri. He received the death penalty.