The Supreme Court, citing "evolving standards of decency that mark the progress of a maturing society," ruled 5-4 Tuesday that executions of those who killed before they were 18 is unconstitutional.
Moderate Justice Anthony Kennedy led the court's four liberals in the majority.
Before the ruling the United States was the only country in the world that still officially executed those who committed their crimes before age 18, though others are believed to do so unofficially.
Until now the issue of such executions was the subject of robust debate in the United States, where juvenile murderers were sometimes sentenced to death because of the particular heinousness of their crimes.
Other countries that have executed such criminals since 1990 include the Democratic Republic of Congo, Iran, Nigeria, Pakistan, Saudi Arabia, China and Yemen, according to Amnesty International. China has since abolished such death penalties, but Amnesty International said such executions still occur in the countryside.
Iran also is considering legislation to ban the juvenile death penalty.
Thirty-eight people have been executed worldwide since 1990 for crimes committed before they were 18. Of those, 19 were executed in the United States.
In Tuesday's majority opinion Kennedy said the execution of someone who committed murder before age 18 violated the Eighth Amendment's ban on cruel and unusual punishments. The Eighth Amendment's ban is applicable to the states through the 14th Amendment.
He added that a national consensus against the juvenile death penalty had developed since a 1989 Supreme Court decision upholding the executions of those 16 and 17. That evolving consensus was a guide to interpreting the Eighth Amendment.
But he also cited international covenants and British law in his opinion, adding, "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 heritage of freedom."
In dissent, Justice Sandra Day O'Connor said the majority had banned executions for those who kill before age 18 "no matter how deliberate, wanton or cruel the offense. Neither the objective evidence of contemporary societal values," nor the high court's analysis on the proportionality of punishment, "nor the two in tandem suffice to justify this ruling."
Justice Antonin Scalia, joined by fellow conservatives Chief Justice William Rehnquist and Justice Clarence Thomas, also dissented Tuesday, but separately from O'Connor.
Scalia's language was blistering.
He noted that Alexander Hamilton had assured New York citizens in the Federalist Papers that federal judges, appointed for life, would be "bound down by strict rules and precedents."
"Bound down, indeed," Scalia said. "What a mockery today's opinion makes of Hamilton's expectation, announcing the court's conclusion that the meaning of our Constitution has changed over the past 15 years -- not, mind you, that this court's decision 15 years ago (supporting such executions) was wrong, but that the Constitution has changed. ...
"The court thus proclaims itself the sole arbiter of our nation's moral standards -- and in the course of discharging that awesome responsibility purports to take guidance from the views of foreign courts and legislatures," Scalia added. "Because I do not believe that the meaning of our Eighth Amendment, any more than the meaning of our Constitution, should be determined by the subjective views of five members of this court and like-minded foreigners, I dissent."
The landmark ruling affects those 19 states that previously reserved the death penalty for adult and juvenile offenders: Alabama, Arizona, Arkansas, Delaware, Florida, Georgia, Idaho, Kentucky, Louisiana, Mississippi, Nevada, New Hampshire, North Carolina, Oklahoma, Pennsylvania, South Carolina, Texas, Utah and Virginia.
Amnesty International said 12 states have death-row inmates who committed their crimes before age 18: Texas, 29 inmates; Alabama, 14; Mississippi, 5; North Carolina, 4; Arizona, 4; Louisiana, 4; Florida, 3; South Carolina, 3; Georgia, 2; Pennsylvania, 2; Virginia, 1; and Nevada, 1.
All are affected by Tuesday's decision.
In the case that brought Tuesday's ruling, Christopher Simmons was 17 and a high school junior when he and a 15-year-old friend broke into the home of Shirley Crook near Fenton, Mo.
Kennedy said Simmons earlier had bragged that he wanted to kill someone. "In chilling, callous terms, he talked about his plan, discussing it for the most part with two friends," ages 15 and 16, Kennedy said. "Simmons proposed to commit burglary and murder by breaking and entering, tying up a victim and throwing the victim off a bridge. Simmons assured his friends they could 'get away with it' because they were minors."
One of the friends backed out of the plan and would later testify against the others. Simmons and the other teenager broke into Crook's home. When the victim recognized him, Kennedy said, it confirmed Simmons' plan to kill.
The two teenagers bound Crook's hands and feet with electrical wire and wrapped her whole face in duct tape. They then drove her in a van to a railroad trestle spanning the Meramec River. They threw her, still very much alive, into the river.
Her body was recovered the next day by fishermen.
Simmons drew attention to himself by bragging to friends that he had killed a woman "because the bitch seen my face," Kennedy said.
He was later convicted of the murder and, since Missouri was one of the states with a juvenile death penalty, was sentenced to death. However, his execution eventually was blocked by the Missouri Supreme Court, which said the process was unconstitutional.
The limits of the death penalty have been changing over the last two decades.
In 1989 the Supreme Court ruled 5-4 in Stanford vs. Kentucky that executions of those who committed their crimes before age 16 was unconstitutional. That left in place executions of those who committed their murders at age 16 and 17.
But Kennedy said in Tuesday's majority opinion, citing a 1958 precedent, "To implement this (constitutional) framework, we have established the propriety and affirmed the necessity of referring to the 'evolving standards of decency that mark the progress of a maturing society' to determine which punishments are so disproportionate as to be cruel and unusual."
On the same day in 1989, Kennedy said, the Supreme Court ruled that the execution of the mentally retarded was constitutional. However, in 2002 a 6-3 Supreme Court majority ruled in Atkins vs. Virginia that the state may not execute the mentally retarded without violating the Eight Amendment's ban.
Kennedy said the national consensus against the death penalty for juveniles "is similar, and in some respects parallel, to the evidence Atkins held sufficient to demonstrate a national consensus against the death penalty for the mentally retarded."
The U.S. Supreme Court opinion affirms the decision of the Missouri Supreme Court.
(No. 03-633, Superintendent. Roper vs. Simmons)
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