David Johnston, who was convicted of killing an Orlando woman in 1983, is scheduled to die on Tuesday.

On Thursday, his attorney will appear before the Florida Supreme Court to argue Johnston should not be put to death because he's mentally retarded.

Florida law and a 2002 U.S. Supreme Court ruling prohibit executing retarded people.

To be considered legally retarded, a defendant must have an IQ of 70 or lower and cannot perform "adaptive functions." Those include everyday living skills such as going to work, preparing a meal and getting dressed. Both conditions must have existed before the person turned 18.

As a kid, Johnston was diagnosed as retarded, and he was sent to the Leesville State School for the Retarded. He wasn't allowed to cook, fearing he would start a stove fire. He failed to earn a driver's license, wrecking the car too many times.

When he was 7, he scored a 57 on an IQ test.

"It seems like a slam dunk," his attorney, Todd Doss, said.

It's not.

Columbia University psychiatry professor Dr. Paul S. Appelbaum said several factors can affect a person's IQ score, including how a person feels that day, the type of test given and whether the person has any mental illness.

After the 2002 U.S. Supreme Court decision, Johnston appealed his death sentence, pointing to his retardation. A hearing was held, and his IQ tested at 84.