I just love the non-sequitur of the dissent written by Chief Justice John Roberts: 

"Clinicians, not judges, should determine clinical standards,"..."And judges, not clinicians, should determine the content of the Eighth Amendment."

Sounds profound until you unpack it.

As Roberts demands, in Atkins v. Virgina, SCOTUS did determine the content of the Eighth Amendment (as state in this article, "executing the intellectually disabled violated the Eighth Amendment's ban on cruel and unusual punishment"), and that judicial determination created the need for psychologists to establish clinical standards to measure "intellectual disability."

The majority ruling, it seems to me, asserts that the lower courts did not do that, but rather substituted the judgement of lawmakers rather than for the latest judgement of clinicians as Roberts et al. would, on my analysis, have it.