For the Supreme Court itself, perhaps the most significant effect of today's decision is to reaffirm the relevance of international law to its interpretation of the U.S. Constitution.

The European Union, human right lawyers from the United Kingdom and a group of former Nobel Peace Prize winners had urged the court in friend-of-the-court briefs to strike down the juvenile death penalty.

In saying that this strong expression of international sentiment "provide[s] respected and significant confirmation for our own conclusions," Kennedy lengthened the recent string of decisions in which the court has incorporated foreign views -- and decisively rejected the arguments of those on the court, led by Justice Antonin Scalia, who say the court should consider U.S. law exclusively.

There were actually six votes in Kennedy's favor on that point today, because in her dissenting opinion, O'Connor said that she agreed with Kennedy that international trends should be considered when determining the meaning of "cruel and unusual punishment" in modern times.

O'Connor's opinion suggested that she came fairly close to joining the majority entirely. If she were a legislator, O'Connor wrote, "I, too, would be inclined to support legislation setting a minimum age of 18 in this context."

But, O'Connor wrote, too few states had recently enacted such laws to convince her that the country generally had "set its face" against the juvenile death penalty.

Scalia, in a separate dissent that was joined by Rehnquist and Thomas, took the majority to task for "proclaim[ing] itself sole arbiter of our Nation's moral standards -- and in the course of discharging that awesome responsibility purport[ing] to take guidance from the views of foreign courts and legislatures."

Noting that the vast majority of countries in the world have more restrictive abortion laws than the United States does, Scalia accused the court of "invok[ing] alien law when it agrees with one's own thinking, and ignor[ing] it otherwise." He read his opinion from the bench, a sign of especially strong disapproval for the court's decision.

Scalia also pointed out that the 18 death-penalty states that limit capital punishment to offenders 18 and over amount to only 47 percent of the 38 death penalty states.

"Words have no meaning if the views of less than 50 percent of death penalty States can constitute a national consensus," he wrote.