The decision clarifies a question left open in a 1977 case, Coker v. Georgia, in which the high court invalidated a death sentence for a man who had raped a 16-year-old married woman. The justices said capital punishment was disproportionate to the crime.
Many legal analysts said the court had drawn a bright line forbidding the death penalty in all cases that did not result in the death of the victim.
But other analysts suggested that the 1977 case held only that the circumstances of most rapes of adult women do not rise to a level justifying capital punishment. In contrast, the aggravated rape of a young child can be as bad as murder, they said.
On Wednesday, the majority justices disagreed. "The incongruity between the crime of child rape and the harshness of the death penalty poses risks of overpunishment and counsels against a constitutional ruling that the death penalty can be expanded to include this offense," Kennedy wrote.
In a dissent, Justice Samuel Alito said the majority justices were usurping the work of state lawmakers. "The harm that is caused to the victims and to society at large by the worst child rapists is grave. It is the judgment of the Louisiana lawmakers and those in an increasing number of other states that these harms justify the death penalty," he wrote.
"The court provides no cogent explanation why this legislative judgment should be overridden," Justice Alito wrote. "Conclusory references to 'decency,' 'moderation,' 'restraint,' 'full progress,' and 'moral judgment' are not enough."