Updated

Georgia was the first state to ban executing mentally disabled death row inmates, but the case of an inmate who is to be put to death next week has highlighted the state's strictest-in-the-nation standard for proving mental disability.

Former President Jimmy Carter is among those who have said the state pardons board should commute Warren Lee Hill's death sentence to life in prison without parole. However, the state argues defense attorneys have failed to meet their burden of proving beyond a reasonable doubt that Hill is mentally disabled. Hill was convicted of the 1991 murder of a fellow inmate.

Most states that impose the death penalty have a lower threshold for defendants to prove they are mentally disabled, while some states don't set standards at all. Hill's lawyer Brian Kammer said the high standard for proving mental disability is problematic because psychiatric diagnoses are subject to a degree of uncertainty that is virtually impossible to overcome.

Prosecutors have presented expert testimony and evidence that Hill is not disabled, while his attorneys have presented their own evidence to prove he is disabled. That can make it difficult to determine anything beyond a reasonable doubt, said Kay Levine, an associate professor of law at Emory University.

"Beyond a reasonable doubt can never be met if you're simply not sure which side is unequivocally telling the truth and which side is not," said Levine, who has no connection to the Hill case. "The issue with Georgia setting its mental health standard as high as it's set is that it requires such a high level of certainty that even scientists will rarely reach."

Nonetheless, Georgia's strict standard has repeatedly been upheld by state and federal courts.

Last year, the 11th U.S. Circuit Court of Appeals ruled in an appeal of Hill's case that it couldn't strike down Georgia's law because the U.S. Supreme Court allows states to create their own definitions for mentally disabled. The decision, written by Judge Frank Hull, noted the justices were careful not to set their own rigid guidelines for such a definition.

Even if Georgia "somehow inappropriately struck the balance" when it adopted its standard, Hull wrote, only the U.S. Supreme Court can overturn the state's law.

The Supreme Court last month declined to hear Hill's case, but his lawyer has already submitted a new request to the high court.

Diagnostic guidelines from the American Psychiatric Association for "mild mental retardation" — which is what Hill's defense claims — include an IQ of approximately 50-70 and onset before age 18. The guidelines also include simultaneous deficits or impairments in the person's effectiveness in meeting the standards expected for the person's age or cultural group in at least two of the following areas: communication, self-care, home living, social/interpersonal skills, use of community resources, self-direction, functional academic skills, work, leisure, health and safety.

Georgia's law reflects those guidelines, defining "mentally retarded" — which is the term used by the law — as "having significantly subaverage general intellectual functioning resulting in or associated with impairments in adaptive behavior which manifested during the developmental period."

"Difficulties in diagnosis arise whenever these criteria are not clearly delineated, for example an IQ score of 70," said University of Georgia psychology professor L. Stephen Miller. In such cases, he said, arguments can be made in either direction because the accuracy of the test is likely plus or minus five points. Another factor that can cause conflicting diagnoses are inconsistent reports on whether someone is having trouble functioning in the real world, Miller said.

Most people who are "mildly mentally retarded" are able to live relatively successfully in the community, he said.

The state has cited expert testimony and IQ tests that concluded Hill is not mentally disabled. Before trial, Hill's family members described him as "the leader of the family" and "a father figure," the state notes. He was not in special education classes and served in the Navy, where he received promotions, the state has said.

The defense has referenced a state court judge's assessment that Hill was mentally disabled and a test that shows his IQ to be about 70. The defense has also cited expert testimony that it is not unusual for someone who is mildly mentally disabled to be able to function at a satisfactory level in an environment as structured as the military. Attorneys also presented a letter from some of Hill's teachers saying that he could never read or write at the proper grade level, and that he was promoted to the next grade only so he would continue to be with children his own age.

The Georgia Board of Pardons and Paroles is set to hear Hill's case Friday. His lawyer has asked the board to commute Hill's sentence to life in prison without parole or to grant him a 90-day delay to allow the U.S. Supreme Court to consider his case.

Included with the application to the board are letters from former President Carter and his wife, disability groups and the nephew of Joseph Handspike, the inmate Hill killed.

Richard Handspike, who says he is a representative for the family, says in the letter that they were not contacted by prosecutors but would have told authorities they did not want Hill to be executed.

"I and my family feel strongly that persons with any kind of significant mental disabilities should not be put to death," Handspike said. "I believe that if the system had evidence of such disability in Mr. Hill, it should have taken steps to treat him accordingly and prevent his execution."