Thomas Sachy, a forensic psychologist in private practice from Georgia testified for the defense Thursday morning in the Stephen Stanko death penalty trial in Georgetown.
Sachy said Stanko is a psychopath and therefore cannot control his impulses when he becomes angry.
"The brakes are right here," Sachy said, pointing to an area of the forehead. "People with damage to the frontal lobe are prone to fits of anger or violence. We know damage to this part of the brain takes the emergency brakes off."
Stanko, an ex-convict and nonfiction author, is accused of rape, murder, armed robbery and assault and battery with intent to kill in the strangling death of girfriend Laura Ling, with whom he lived, and the sexual assault and beating of the teenage girl at Ling's home on Murrells Inlet Road.
Stanko also is accused of killing Conway resident Henry Lee Turner on April 8, 2005, at Turner's home.
A PET brain scan shows Stephen Stanko's brain is smaller than a normal brain and show decreased activity in the frontal lobes, Sachy said Thursday.
Sachy said this is the first time PET scan evidence has been used as part of the defense's case in a criminal trial in South Carolina.