ATLANTA (AP) -- An attorney for a mentally disabled Athens woman convicted of beating her son to death asked the Georgia Supreme Court for another trial Monday.

Her lawyer, Russell Gabriel, said the jury should have been able to consider her motive for the beating, which was an attempt to discipline the child by spanking, her lawyer said.

"This was a case of corporal punishment -- gone awry no doubt," Gabriel said.

Vernessa Marshall was found guilty of felony murder in the 1998 death of her son Jamorio. She was 28 at the time, but had the mental capacity of a 7-year-old, according to experts at the 1999 trial.

Jamorio lived with his grandmother in Athens. But when he was suspended from school for a fourth time for allegedly stealing money, his grandmother brought him to Marshall's home and instructed her to punish him, Gabriel said.

Marshall and her boyfriend, 30-year-old Demetrius Paul, used a belt to repeatedly beat the boy on his legs, buttocks and back over a period of two hours.

A Clarke County jury found Marshall and Paul guilty of felony murder, and Superior Court Judge Steve Jones sentence both to life in prison.

Gabriel told the five-member panel in Monday's oral arguments that Marshall's trial was unfair because the jury was not allowed to consider her motive.

With that information, the jury might have found her guilty only of involuntary manslaughter, which carried a sentence of 20 years in prison, Gabriel said.

"This conduct at least began out of the motivation to punish rather than the motivation to be cruel or commit child abuse," he said.

But, District Attorney Ken Mauldin said the jury was given proper instructions.

"There was not any evidence, slight or otherwise, that what happened to this boy was justified," he said.

The court has until year's end to hand down a decision on whether to grant Marshall a new trial. The Supreme Court has already upheld Paul's conviction and life sentence.