The US bowed to international and domestic pressure yesterday, becoming the last country in the world officially to abolish the death penalty for offenders who were under 18 when they committed murder.
The supreme court ruling will spare up to 70 inmates who are on death row for committing murders while aged 16 or 17, and it removes a source of friction between the US and Europe. The EU welcomed the decision, but said it "opposes capital punishment under all circumstances".
The former American president Jimmy Carter said that with the ruling the US had joined "the community of nations".
"The supreme court decision confirms recent, compelling scientific research findings, that the capacity for curbing impulsiveness, using sound judgment, and exercising self-control is much less developed in adolescents than in adults," Mr Carter said in a statement.
The ruling, passed by a 5-4 majority, was made in the case of Christopher Simmons, who was 17 in 1993 when a woman died after he threw her off a bridge in Missouri.
The swing vote came from Justice Anthony Kennedy, who normally sides with the conservatives on the bench.
In giving his reasons, he explicitly cited the role of world opinion. "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," he wrote, adding that there was an emerging national consensus against juvenile execution.
Dissenting from the majority view, Justice Antonin Scalia argued that foreign pressure should play no role in the decision. He said the constitution should not be determined by "the subjective views of five members of this court and like-minded foreigners".
The judges ruled that juvenile execution conflicted with the eighth amendment of the constitution which outlaws "cruel and unusual punishment".
"To decide what is cruel and unusual you don't look at what was happening 200 years ago. You look at evolving standards of decency. In that specific area, what is going on in the rest of the world is relevant," said Stephen Harper, an expert on juvenile law at the University of Miami. "Clearly, international opinion had some effect on the court."
Capital punishment still has majority support in the US. However, this is the second significant judicial limit imposed in recent years. In 2002 the execution of convicts with learning difficulties was abolished.
The decision brings the US into line with the rest of the world. The execution of juveniles is explicitly banned in the UN convention on the rights of the child, which has been ratified by every country except the US and Somalia, which has no recognised government.
Of the 39 executions of child offenders recorded by Amnesty International since 1990, 19 took place in the US. The other countries include Iran, China, Congo, Saudi Arabia, Pakistan and Yemen, but the US was the last government to condone and defend the practice officially. Iran has formulated a law banning such executions, but it has not yet been put into practice.
"Until today the US was the only country that officially executed child offenders; today's ruling finally brings the US out from the cold on this issue," Kate Allen, Amnesty International's UK director, said in a statement. "The death penalty does nothing to deter crime and is a human rights violation that brings shame on those countries that use it. In addition, innocent people are always at risk of execution."
In 1988 the supreme court outlawed the execution of anyone 15 or under. At the time of yesterday's ruling, 15 states had death penalties for offenders as young as 16 while four had a minimum age of 17.
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