What is the argument that the Eighth Amendment prohibits imposition of the death penalty on a person with intellectual disabilities?

Defendant's argument relies on Atkins v. Virginia (2002) 536 U.S. 304, 122 S.Ct. 2242, 153 L.Ed.2d 335, which holds that the Eighth Amendment to

[5 Cal.5th 275]

the United States Constitution prohibits imposition of the death penalty on persons with intellectual disabilities, and Roper v. Simmons (2005) 543 U.S. 551, 125 S.Ct. 1183, 161 L.Ed.2d 1, which reached a similar conclusion

[234 Cal.Rptr.3d 694]

concerning persons who

[420 P.3d 200]