An assistant attorney general countered that there's no proof to back up the claim.
"The evidence is there to support the fact that he is not mentally retarded," said Assistant Attorney General Marvin L. White Jr.
Chase was convicted and sentenced to death in 1990 for the Aug. 14, 1989, killing of 70-year-old vegetable salesman Elmer Hart in Copiah County. Authorities said Hart apparently arrived at home as his wife was being robbed by Chase and another man. Hart's wife had been doused with an ammonia-like substance and tied up and gagged. She watched helplessly as her husband was shot, authorities said.
Hart's body was found on his bedroom floor beside his wife.
The Mississippi Supreme Court upheld Chase's sentence in 1994.
Chase has argued that a pretrial mental examination showed he was mentally incapacitated. Chase said he was examined by two doctors. But court records show one doctor did not find him mentally disabled.
Chase's attorney, Jim Craig, told justices Monday that one of the doctors who examined Chase gave "a seat-of-the-pants judgment about the intellectual functioning of the defendant," which Craig argued was wrong.
Craig said people had testified that Chase, as an adult, lacked the mental capacity to perform simple tasks like choosing his own clothes or making macaroni and cheese.
In 2010, a Copiah County judge ruled against Chase in his request to throw out the death sentence.