Specialists in comparative law acknowledge that there have been occasions when young murderers who would have served life terms in the United States were released from prison in Europe and went on to kill again. But comparing legal systems is difficult, in part because the United States is a more violent society and in part because many other nations imprison relatively few people and often only for repeat violent offenses.

“I know of no systematic studies of comparative recidivism rates,” said James Q. Whitman, who teaches comparative criminal law at Yale. “I believe there are recidivism problems in countries like Germany and France, since those are countries that ordinarily incarcerate only dangerous offenders, but at some point they let them out and bad things can happen.”

The differences in the two approaches, legal experts said, are rooted in politics and culture. The European systems emphasize rehabilitation, while the American one stresses individual responsibility and punishment.

Corrections professionals and criminologists here and abroad tend to agree that violent crime is usually a young person’s activity, suggesting that eventual parole could be considered in most cases. But the American legal system is more responsive to popular concerns about crime and attitudes about punishment, while justice systems abroad tend to be administered by career civil servants rather than elected legislators, prosecutors and judges.

In its sentencing of juveniles, as in many other areas, the legal system in the United States goes it alone. American law is, by international standards, a series of innovations and exceptions. From the central role played by juries in civil cases to the election of judges to punitive damages to the disproportionate number of people in prison, the United States is an island in the sea of international law.

And the very issue of whether American judges should ever take account of foreign law is hotly disputed. At the hearings on their Supreme Court nominations, both John G. Roberts Jr. and Samuel A. Alito Jr. said they thought it a mistake to consider foreign law in constitutional cases.