The Supreme Court reaffirmed yesterday that states have the authority to make their own rules for determining whether a capital defendant is mentally retarded and thus ineligible for the death penalty.

The court's 2002 decision abolishing the death penalty for mentally retarded offenders left to the states "the task of developing appropriate ways to enforce the constitutional restriction."

The question of how to decide who is actually retarded, through evidence from IQ tests, medical exams, school reports or other sources, is a crucial issue in determining how widely the court's ban on capital punishment for the retarded will ultimately extend.

Last year, the San Francisco-based U.S. Court of Appeals for the 9th Circuit ordered Arizona to give death row inmate Robert Douglas Smith a jury trial on his claim of mental retardation -- even though Arizona law provides for a proceeding before a judge.

Overruling the 9th Circuit in a 11/2-page unsigned opinion that included no dissents, the court said that the 9th Circuit "erred" and "exceeded its narrow authority."

"Arizona had not even had a chance to apply its chosen procedures when the 9th Circuit preemptively imposed its jury trial condition," the court's opinion said.

Arizona's appeal was supported by 15 states. Among states that have the death penalty, 18 permit a judge to determine whether a defendant is mentally retarded, according to the states' friend-of-the-court brief.

In nine states, including Maryland and Virginia, the defendant gets a jury trial on his claim, but the burden of proof is on him, not the prosecution.

A Virginia jury recently found Daryl Atkins, the death row inmate whose claim prompted the Supreme Court's 2002 decision, not retarded and eligible for execution.