SINCE THE U.S. SUPREME Court ruled last summer that states may no longer execute the mentally retarded, prosecutors, defense attorneys, judges and legislators have been grappling with how South Carolina should determine who is and who isn't retarded.

Last week, defense attorneys asked the state Supreme Court to write the rules itself. They said there was no assurance that the Legislature would write new rules and noted that they would challenge any such rules anyway. One attorney went so far as to say prior to the hearing that he didn't understand why the Legislature would even consider legislation before the court had decided whether to let lawmakers act.

Such a statement -- like the request for court action itself -- completely misunderstands the proper roles of the branches of government. So let's make it clear: It is the job of the courts to apply and interpret the laws. It is the job of the Legislature to write the laws.

Certainly, if the Legislature were to refuse to do its job, the day could come when the court would have no choice but to act -- when the court had a death-penalty case before it involving a claim of mental retardation.

Fortunately, the Legislature seems intent on doing its job. In fact, the day after defense attorneys were in court asking the justices to write law, a House subcommittee approved a bill that was the result of congenial negotiations between the two sides.

The bill, which will be debated next week in the Judiciary Committee, reasonably answers the key questions: What does "mental retardation" mean? Who decides that, and when? And what will the state do about people already on death row who claim retardation?

The bill requires a judge to determine, before a capital case goes to trial, whether the accused meets the legal definition of mental retardation: "significant limitations both in intellectual functioning and in adaptive behavior as expressed in conceptual, social, and practical adaptive skills" and first detected before age 18.

This approach, which uses the standard medical definition of mental retardation, acknowledges that mentally retardation is a question of fact, not of opinion and emotion, and therefore should be decided by a judge; and it gets the question out of the way up front, before the prosecution and defense spend inordinate amounts of money on a death-penalty case that might turn out later to be ineligible for the death penalty.

If the judge determined that the accused was mentally retarded and the accused was found guilty at trial, he would automatically be sentenced to life in prison without parole. If the judge found the accused was not mentally retarded, and if a jury found him guilty, the death penalty would be an option only if all 12 jurors found he was not mentally retarded. This provision might strike some as too soft, but there is good reason to believe the U.S. Supreme Court would demand it.

Killers already on death row would have 120 days to appeal their sentences to a judge, who would be required to hold a "prompt" hearing. If the judge determined the person was retarded, the sentence would be commuted to life without parole; if not, the killer would remain on death row. This procedure wisely uses the same definitions that will be used in new cases.

Whether you believe the mentally retarded should be executed or not -- and we believe they should not be -- the fact is that the nation's high court has declared they may not be. It is the job of our Legislature to amend our death penalty law to reflect that fact. This bill does a good job of that. It should be passed.