The decision was the third in the last six years to place a categorical limitation on capital punishment. In 2002, the court barred the execution of mentally retarded defendants. In 2005, it ruled that the Constitution bars the death penalty for crimes committed before the age of 18.

Nonetheless, despite this trend toward narrowing the application of the death penalty, there was no suggestion from the majority that the court was moving toward the abolition of capital punishment, which Justice John Paul Stevens called for in an opinion two months ago that no other justice joined.

Justice Kennedy said Wednesday that while the court’s death penalty jurisprudence “remains sound,” it should not be expanded to cover a crime for which no one has been executed in the United States for the past 44 years.

The case, Kennedy v. Louisiana, No. 07-343, was an appeal by one of the two Louisiana inmates, Patrick Kennedy. He was convicted and sentenced to death in 2003 for raping his 8-year-old stepdaughter, whose injuries were severe enough to require emergency surgery. The Louisiana Supreme Court upheld Mr. Kennedy’s conviction and rejected his challenge to the constitutionality of his sentence.

The United States Supreme Court prohibited capital punishment for rape in a 1977 case, Coker v. Georgia, in which the victim, while only 16 years old, was married and had the legal status of an adult. It was not clear at the time whether that decision was limited to the rape of an adult woman, or whether it barred the death penalty for any rape. The court on Wednesday treated the issue of capital punishment for child rape as a fresh question, not governed by any existing precedent. As a matter of constitutional analysis, the question in the case was whether the death penalty was so disproportionate to the offense as to amount to cruel and unusual punishment, in violation of the Eighth Amendment. The court’s modern precedents interpret the Eighth Amendment according to “the evolving standards of decency that mark the progress of a maturing society.”