Washington ? A divided Supreme Court reversed course Thursday and ruled that executing mentally retarded people is unconstitutionally cruel, giving scores of inmates on death row the possibility of a reprieve.

The most immediate effect of the ruling will be in the 20 states that allow execution of retarded people. Dozens or perhaps hundreds of inmates in those states will likely now argue that they are retarded, and that their sentences should be converted to life in prison.

The 6-3 ruling is confined to mentally retarded defendants convicted of murder and does not address the constitutionality of capital punishment in general. The court is expected to rule next week on whether judges, not juries, can impose a death sentence. That ruling could affect 800 inmates in nine states.

The majority view in Thursday’s case reflects changes in public attitudes on the issue since the court declared such executions acceptable in 1989. Then, only two states that used capital punishment outlawed the practice for retarded defendants. Now, 18 states prohibit it.

“It is not so much the number of these states that is significant, but the consistency of the direction of the change,” Justice John Paul Stevens wrote for the majority.

“The practice … has become unusual, and it is fair to say that a national consensus has developed against it,” Stevens wrote for himself and Justices Sandra Day O’Connor, Anthony M. Kennedy, David H. Souter, Ruth Bader Ginsburg and Stephen Breyer.

“This consensus unquestionably reflects widespread judgment about the relative culpability of mentally retarded offenders, and the relationship between mental retardation and the penological purposes served by the death penalty,” Stevens wrote.

In the future, the ruling will mean that people arrested for a killing will not face a potential death sentence if they can show they are retarded, generally defined as having an IQ of 70 or lower.

The court left it to states to develop their own systems to ensure that mentally retarded people are not executed.

Chief Justice William H. Rehnquist and Justices Antonin Scalia and Clarence Thomas dissented. The three, the court’s most conservative members, telegraphed their views earlier this month, when they complained bitterly about reprieves the court majority had granted to Texas inmates who claim they are retarded.

The court ruled in favor of a Virginia inmate, Daryl Renard Atkins, who was convicted of shooting an Air Force enlisted man for beer money in 1996. Atkins’ lawyers say he has an IQ of 59 and has never lived on his own or held a job.

The court used state laws as a barometer, but also went beyond them to look at why mentally retarded killers are different than killers of normal intelligence, and whether any wider social purpose is served by executing them.

Executing mentally retarded people neither appropriately punishes the criminal nor serves as a deterrent to future crimes, the majority found.

Many mentally retarded defendants know right from wrong, but they are more likely to act on impulse or to be swayed by others in a group, Stevens wrote.

“Their deficiencies do not warrant an exemption from criminal sanctions, but they do diminish their personal culpability.”

Retarded people make bad witnesses, and may come off in court as unrepentant, Stevens wrote.

“Mentally retarded defendants in the aggregate face a special risk of wrongful execution.”

In a rare dissent read from the bench, Scalia said the majority hung a constitutionally suspect ruling on the slim reed of recent state lawmaking.

Thursday’s ruling is not grounded either in the Constitution nor in current social attitudes about the death penalty, Scalia wrote for himself and the other two dissenters.

“Seldom has an opinion of this court rested so obviously upon nothing but the personal views of its members.”

In a separate dissent, Rehnquist said the majority went too far in looking at factors beyond the state laws. The majority puts too much stock in opinion polls and the views of national and international observers, Rehnquist wrote.

“Believing this view to be seriously mistaken, I dissent,” Rehnquist said. Rehnquist omitted the customary word “respectfully” before “dissent.”

The Justice Project, a group that monitors the criminal justice system for any inequities, applauded the ruling, saying it “reflects a growing national concern that the administration of the death penalty is unfair. This decision comes at a time when there is growing agreement among death penalty proponents and opponents alike that the capital punishment system is broken.”

The case turned on the Eighth Amendment’s protection against “cruel and unusual punishments,” and how to define those terms today.

Times change, and with them public sentiment about what is appropriate punishment for various crimes, the court has observed in the past. For example, at various times in the country’s history it was considered acceptable to flog people in public, or to execute those convicted of rape.

Counting the 12 states that do not allow capital punishment at all, 30 states prohibit execution of retarded people.

The number of states that banned the practice increased ninefold between the court’s last ruling on the issue and the time it heard arguments in Atkins’ case. The court was forced to toss out a North Carolina case it originally selected to reconsider the retardation question last year, because that state banned the practice before the court could hear the case.

Virginia authorities argued that Atkins planned his crime and understood afterward what he had done. The state claimed he was no less culpable for the crime than a person of normal intelligence.

Atkins had 20 previous felonies on his record at the time of the killing, the state argued. Atkins gave a detailed confession to police when he was arrested, describing how he and an accomplice kidnapped the victim, forced him to withdraw cash from a bank teller machine and then drove him to a deserted field and shot him eight times.

O’Connor wrote the 5-4 decision in 1989 upholding execution of the retarded.

There was “insufficient evidence of a national consensus” against the executions to determine that they were unconstitutionally cruel and unusual, she wrote then.

President Bush has said he opposes executing the mentally retarded although two such inmates were executed while he was governor of Texas. Bush’s successor as governor vetoed a ban on the practice.

White House spokesman Ari Fleischer reiterated Bush’s support for the death penalty “for those people who commit violent, heinous crimes.”

“Under Texas law, juries are required to consider such mitigating factors during both the trial phase and the sentencing phase that deal with issues involving mental abilities,” Fleischer said.

The case is Atkins v. Virginia, 01-8452.