California Supreme Court opinion implementing Atkins

I recently noted here that states have developed, and struggled with, a wide array of "ways to enforce the constitutional restriction" on executing mentally retarded offenders as required by the Supreme Court's 2002 Atkins decision.  The Supreme Court of California take on this issue today in People v. Super. Ct. (Vidal), No. S134901 (Cal. Apr. 12, 2007) (available here).  The ruling is intriguing for both substantive and procedural reasons, and persons interested in the use of scientific evidence in court will want to be sure to check this case out.  Here's one passage that spotlights just some of the interesting issues that can arise in these sorts of cases:

The Court of Appeal majority erred in thus purporting to resolve a factual question ― the best scientific measure of intellectual functioning ― as a matter of law. In finding the facts of a particular case, courts and juries untrained in science are sometimes called upon to resolve contested scientific issues, but such factual findings do not establish generally applicable rules of law. The superior court here, for example, found on the basis of Couture's and Widaman's testimony that in Vidal's case his Full Scale IQ scores in the low average to average range did not preclude a finding of mental retardation. In a given case an appellate court might, within its proper role, hold that such a finding was not supported by substantial evidence in the hearing record. But an appellate court cannot convert a disputed factual assertion into a rule of law simply by labeling it a "legal standard," as the Court of Appeal purported to do here....

The Legislature has mandated that trial courts, in determining mental retardation for Atkins purposes (Atkins, supra, 536 U.S. 304), find whether the individual's "general intellectual functioning" is significantly impaired (§ 1376, subd. (a)), but has not defined that phrase or mandated primacy for any particular measure of intellectual functioning. The question of how best to measure intellectual functioning in a given case is thus one of fact to be resolved in each case on the evidence, not by appellate promulgation of a new legal rule.