AMERICANS consistently arrive late to enlightened notions about fairness and the death penalty, which helps explain why this country so often finds itself in league with barbarous nations such as Iran and Rwanda on this issue.

But in scattered increments over the past few years, national policy on executions has finally seemed to be moving toward a more cautious and humane approach.

The Supreme Court last week took a huge step in the right direction when it barred executions of the mentally retarded, reversing the precedent the justices set 13 years ago when they allowed the execution of an impaired Texas inmate.

The justices noted that since their 1989 decision, 16 states have enacted their own bans on killing inmates of limited mental capacity. (Maryland and Georgia already had similar restrictions in place when the 1989 case was heard.) The justices said a national consensus has coalesced around the idea that mentally retarded offenders are "categorically less culpable than the average criminal" because they have limited abilities to either control their impulses or understand the implications or consequences of their actions. They said executing retarded prisoners now constitutes cruel and unusual punishment, a violation of the Constitution's Eighth Amendment.

It's a common-sense ruling that brings the nation closer to sensible probity in this, the most serious of judicial matters.

Further, the court's reasoning in this decision can easily be applied to other classes of death row inmates whose executions raise serious questions.

Juveniles, for example, are deemed less culpable than adults in criminal matters. And yet, in 17 states it's still permissible to put someone as young as 14 to death. (Maryland does not permit the execution of minors.) At the very least, it's morally incongruous for the court to ban the execution of retarded prisoners but to continue to sanction the death penalty for minors.

The same logic could be applied to the plight of mentally ill death row inmates, whom the Supreme Court has not deemed privy to Eighth Amendment protections. Again, how can it be cruel and unusual to execute the mentally retarded but acceptable to put the mentally ill to death?

Maryland has its own shameful anecdote on this particular issue. Francis Zito, an Eastern Shore man with a long and well-documented history of mental illness, was sentenced to death last month for killing two state troopers. His execution, should it take place, would seem the very definition of cruel and unusual.

The Supreme Court, in addition to getting this country out of the sordid business of executing the mentally retarded, opened the door last week for broader discussions about who is fit to face the ultimate judgment, and who is not. The results of those debates will define nothing less than this nation's moral character.