“The risk that the death penalty will be imposed in spite of factors which may call for a less severe penalty ... is enhanced, not only by the possibility of false confessions, but also by the lesser ability of mentally retarded defendants to make a persuasive showing of mitigation in the face of prosecutorial evidence of one or more aggravating factors. ... Mentally retarded defendants in the aggregate face a special risk of wrongful execution.”

—U.S. Supreme Court in Atkins v. Virginia (2004)