"The question is, is Georgia violating the Constitution by basically allowing people with mental retardation to be executed?" asks Richard Dieter, executive director of the Death Penalty Information Center, which opposes all executions.

The U.S. Supreme Court banned the execution of those who are mentally retarded, now known as intellectually disabled, in 2002. Georgia was the first state to ban executions of the mentally retarded, back in the '80s, but Dieter says forcing defendants to prove their impairment beyond a reasonable doubt is questionable — and the strictest standard in the country.

"Georgia is the only state that has such ... high criteria. Generally, if more likely than not you have mental retardation, you're exempted," Dieter says. "So the issue is, did Georgia define its law too narrowly to be keeping within due process?"