Such executions violate “the evolving standards of decency that mark the progress of a maturing society both nationally and in the state,” Justice Riley Anderson wrote in the majority opinion released Tuesday.
The ruling came on an appeal by Heck Van Tran, who was sentenced to death for killing three people during a 1987 Memphis restaurant robbery.
Van Tran´s lawyers argued that his execution would violate state law because his IQ is lower than 70. The law prohibits the execution of the retarded, defined as someone with an IQ of 70 or below, and says signs of mental retardation must have been shown before age 18.
The court said the law does not apply to defendants like Van Tran, 35, because he was sentenced before 1990, when the law took effect.
However, the court said the Eighth Amendment and the Tennessee Constitution prohibit executing mentally retarded individuals. It sent the case back to a lower court for a hearing to determine whether Van Tran demonstrated signs of mental retardation before age 18.
The U.S. Supreme Court is scheduled to hear a similar case this term involving a Virginia inmate.