People who glide easily from test scores to a conclusion of "mental retardation" have a faith which passeth all understanding. There are kids whose IQs have varied by 40 points from one test to another, even without the incentives of welfare or the death penalty, because of the circumstances of the child, the conditions surrounding the test or whatever. One of the kids in a group that I have been dealing with over the years went from a "mentally retarded" IQ range to an above average range inside of a year. Nobody gets that much smarter that fast.
Psychology and psychiatry are not sciences, though some courts have been pretending that they are, ever since a landmark case in 1954 expanded the insanity defense. This was "merely one way of welcoming the psychiatrist into the courtroom," in the words of Chief Justice David Bazelon, who presided over that case. This opened the floodgates to unverifiable speculations.
Even when the insanity defense did not succeed, it took up precious time in trial courts and appellate courts, while a backlog of cases left career criminals walking the streets awaiting trial. Now that it was no longer enough to simply prove whodunit, more prosecutors' time and resources had to be spent rebutting psychological speculations, instead of moving on to getting other criminals off the streets.
That is also likely to be the consequence of adding IQ scores to the long list of things to be litigated endlessly on appeal. Even if not a single murderer escapes the executioner, there will still be heavy costs to a legal system that already takes far too long to resolve issues.
No one wants to see executions of individuals who are genuinely incapable of understanding what they were doing. That exemption has long been part of traditional Anglo-American law. What happened in 1954 was an expansion of that exemption on the basis of psychological speculations, as a prelude to a general increase in the leniency of criminal law, which preceded a skyrocketing increase in crime rates in the 1960s.
We have been down that road before. We don't need to go further down that road