Washington ? The Supreme Court overturned the death sentence laws of five states Monday, affecting more than 160 death row inmates, by ruling that juries and not judges must make life-or-death determinations about the fate of convicted killers.

The 7-2 ruling means that executions ordered for 168 people will be reconsidered, although it is not clear how the affected states will respond.

The decision concerned instances in which juries determined defendants’ guilt or innocence and judges alone decided their punishment. The court held that such sentences violate defendants’ constitutional right to trial by jury, rejecting the argument that judges can be more evenhanded.

“The Sixth Amendment jury trial right … does not turn on the relative rationality, fairness or efficiency of potential fact-finders,” Justice Ruth Bader Ginsburg wrote for a majority that included an unusual alliance of conservative and liberal-leaning justices.

In some states juries determine guilt or innocence, but a judge then bases a death sentence on aggravating factors such as the heinous nature of a murder or whether it was committed for monetary gain.

Monday’s was the second major Supreme Court ruling in less than a week affecting the ways that states sentence people to death. Last week, the justices voted 6-3 to exempt mentally retarded people from execution.

Neither of the cases attacked the basic constitutionality of capital punishment for the general population.

Nationwide, about 3,700 people await execution for crimes committed in the 38 states that allow the death penalty.

Monday’s ruling turned on the Constitution’s guarantee of a jury of one’s peers and a Supreme Court ruling two years ago that struck down another kind of sentence determined by a judge instead of a jury.

Ginsburg said the court’s 2000 ruling in a case called Apprendi v. New Jersey cannot be reconciled with the death penalty sentencing laws in Arizona and four other states in which one or more judges impose the sentence.

The Apprendi case concerned a judge’s ability to lengthen a sentence by two years if a crime was determined to be a hate crime. The high court struck down that sentencing law.

“We hold that the Sixth Amendment secures to capital defendants, no less than to noncapital defendants, the right to a jury determination of any fact on which the legislature conditions an increase in their maximum punishment,” Ginsburg said in announcing Monday’s decision from the bench.

“We hold that the Sixth Amendment applies to both” cases, Ginsburg wrote.

She was joined by Justices John Paul Stevens, Antonin Scalia, Anthony M. Kennedy, David H. Souter and Clarence Thomas. Justice Stephen Breyer wrote separately to agree with the outcome.

The case concerned an Arizona inmate, and the ruling will immediately apply in that state and in Idaho and Montana, where a single judge decides the sentence. It will also apply immediately in Colorado and Nebraska, where a panel of judges makes the sentencing decision.

It was not immediately clear what will happen to inmates in those states. Some lawyers have said death row inmates’ sentences could be commuted to life in prison, as was done when the Supreme Court put a temporary halt to the death penalty in the 1970s. Or the inmates could be resentenced, with some receiving death sentences all over again.

Also unclear was whether the ruling will have a spillover effect in four other states in which juries only recommend whether a convicted murderer should receive the death penalty or life in prison: Florida, Alabama, Indiana, and Delaware.

A judge makes the final call in those states. Indiana, however, recently passed a law that will require judges to follow a jury’s sentencing recommendations.

In dissent, Justice Sandra Day O’Connor predicted that many inmates in the additional four states will challenge their sentences now.

The earlier Apprendi ruling “had a severely destabilizing effect on our criminal justice system,” O’Connor wrote in a dissent joined by Chief Justice William H. Rehnquist. “The decision today is only going to add to these already serious effects.”

Arizona has 129 people on death row, Idaho 21 and Montana six. Colorado has five, and Nebraska seven. Florida has 383, Alabama 187, Indiana 39 and Delaware 20.

Timothy Stuart Ring, the Arizona death row inmate at the heart of Monday’s case, was convicted of killing an armored car driver during a 1994 robbery in Phoenix.

Ring challenged his sentence and Arizona’s law on grounds that his constitutional right to a jury was violated when a judge held a separate hearing after the jury that convicted Ring was dismissed.

At the sentencing hearing, an accomplice testified that Ring planned the robbery and murdered the guard. The judge then determined that the aggravating factors warranted death.

“I was essentially given two trials,” Ring said in an Associated Press interview earlier this year. “One before a jury and then one before a judge.”

The Arizona Supreme Court rejected Ring’s constitutional challenge last year.

Ring’s case put the court in an awkward position. The high court had already upheld the constitutionality of Arizona’s law in 1990, but that was before its ruling in Apprendi v. New Jersey.

Finding the two rulings irreconcilable, the high court took the rare step of overturning one of its own fairly recent decisions. The first decision was written by O’Connor, who defended it in her dissent Monday.

The case is Ring v. Arizona, 01-488.