By Kevin Anderson
BBC News website, Washington
Legal experts say that the US Supreme Court's decision to prevent youths from being put to death for crimes they commit before they turn 18 is part of a trend towards limiting the use of the death penalty in the US.
Since 1976, 22 juvenile offenders have been executed in America
It is the second time in three years that the court has banned the use of the death penalty for a certain class of criminals.
Three years ago, the court banned the execution of the mentally retarded.
However, despite these incremental limits, legal experts also say that the ruling should not be read as growing support for the complete abolition of the death penalty in the US.
"The Supreme Court continued a march at whittling away at the edges of the death penalty," said Adam Liptak, the New York Times' national legal correspondent.
Liptak said the decision was notable for taking into account international law.
In the majority opinion, Justice Anthony Kennedy wrote: "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."
But University of Richmond law professor Corinna Barrett Lain said public opinion was also turning away from the execution of juvenile offenders.
Despite public support for stiff penalties for youth offenders such as allowing them to be tried as adults, support for the execution of juveniles is waning.
Only 19 of 50 states allowed the execution of juvenile criminals, and of those 19, 13 had never carried out such an execution.
Only 22 juvenile offenders have been executed since the US reinstated the death penalty in 1976.
And Ms Lain said only Virginia and Texas were actively exercising the option to put juvenile criminals to death.
This is important, because in determining whether juvenile executions constituted a violation of the 8th Amendment to the US Constitution's ban on cruel and unusual punishment, the court took into account "evolving standards of decency".
'It's all in the brain'
Part of what is driving this "evolving standard of decency" is science.
19 states allow juvenile execution, but only six have carried it out
227 juveniles sentenced to death since 1976
22 of them executed - 13 in Texas
More than 3,400 people on death row in total
Between 1976 and 2001, 88 people on death row were found innocent using DNA evidence, Ms Lain said.
That and the awareness of the lack of proper funding for public defenders in death penalty cases have cast doubt in the system, leading governors in Maryland and Illinois to impose a moratorium on executions.
And science played an important role in the Supreme Court decision in this case.
Joseph McLaughlin, a managing partner in the New York law firm of Heller Ehrman White & McAuliffe, co-wrote a key brief examining the science of adolescent brain development.
In 1988, the death penalty was ruled unconstitutional for defendants who committed a crime under the age of 16 in a case called Thompson v Oklahoma.
Mr McLaughlin also wrote a brief in that case quoting psychological evidence that the brains of adolescents lacked development, and could lead to a lack of judgment.
But he said the evidence was somewhat subjective.
However, MRI scans of adolescent brains have subsequently shown that development key to impulse control, decision-making and the ability to make moral judgements does not occur until the early 20s.
Proper development of grey matter, the development of the pre-frontal cortex and a process called myelination, which increases the efficiency of nerve cells, doesn't occur until much later than thought previously.
"The research showed adolescent brains are more active in regions related to aggression, anger and fear, and less active in regions related to impulse control, risk assessment and moral reasoning than adult brains," Mr McLaughlin wrote in the brief.
And Justice Kennedy picked up the language of the brief when he wrote: "The adolescent's brain works differently from ours. Parents know it."
The court may continue to curtail the use of the death penalty as public opinion changes in the US, Mr McLaughlin said.
Many are looking for other classes of criminals, possibly those with substance abuse problems or the mentally ill, to be spared the death penalty in the future, he and other legal experts say.
But Robert Blecker, a leading advocate for the death penalty as a form of retributive justice, said that the ruling should not be taken that the Americans "had lost our stomach" for the death penalty.
"We are becoming more morally discriminating," he said.
US courts have ruled the death penalty is constitutional and justified for two reasons: retribution and deterrence.
"I spent more than 2000 hours over 12 years probing the minds of killers," Mr Blecker said.
And he found that the death penalty does not deter juvenile criminals. "They think they will live forever," he said.
He agrees that the death penalty should be rare for youth offenders who commit capital offences under the age of 18, but he still believes it should be an option for the worst of the worst.
"It's about dignity, humanity. It's about loving and celebrating goodness and punishing evil," he said.
And in arguing for the death penalty as a form of retribution, he draws a distinction between retribution and revenge.
"Revenge is limitless. Retribution is limited and proportionate to the crime," he said.
He worries that death penalty opponents will next try to exempt the mentally ill from the death penalty.
"If you sign onto that, most evil will become exempt," he said.
But Mr McLaughlin said that while new cases could add additional limits to the use of the death penalty and while he personally opposed capital punishment, "as a matter of law, saying that the death penalty is unconstitutional in all cases is not a winning argument".