A habeas court cannot apply the U.S. Supreme Court’s 2017 opinion in Moore v. Texas to claims of intellectual disability which arose before Moore, the Court held January 7.
That’s because Moore was not “clearly established” law before Moore was decided.
Danny Hill was convicted of murder and sentenced to death in 1986.  In 2002 and again in 2010, Hill filed federal habeas petitions claiming that his death sentence violated Atkins v. Virginia, 536 U.S. 304 (2002), which held that the Eighth Amendment bars execution of persons with intellectual disability. 
After state and district courts denied relief, the Sixth Circuit ultimately granted relief.  The Sixth Circuit, relying heavily on the Supreme Court’s 2017 opinion in Moore, held that state courts had unreasonably applied clearly established federal law in denying Hill’s claim.  Moore held that lower courts had given too much emphasis to a petitioner’s adaptive strengths and improved behavior in prison. 
But a unanimous Court reversed the Sixth Circuit, in a per curiam opinion.
Moore was decided “years after the decisions” of the state courts here, the Court said.
 “Atkins gave no comprehensive definition of ‘mental retardation’ for Eighth Amendment purposes,” the Court said.
 “Although the Court of Appeals asserted that the holding in Moore was ‘merely an application of what was clearly established by Atkins,’ … the court did not explain how the rule it applied can be teased out of the Atkins’ Court’s brief comments about the meaning of what it termed ‘mental retardation.’” 
The Court remanded the case to the Sixth Circuit to apply the law that was “clearly established” at the “relevant time.”