Opinion
Petitioner Anderson Hawthorne, Jr., is under a judgment of death. He challenges his sentence as cruel and unusual punishment based on allegations he is mentally retarded. Under the authority of Penry v. Lynaugh (1989) 492 U.S. 302 [106 L.Ed.2d 256, 109 S.Ct. 2934], we denied three previous petitions for writ of habeas corpus raising this same claim. Subsequently, however, the United States Supreme Court overruled Penry and held that execution of the mentally retarded violates the Eighth Amendment. *44(Atkins v. Virginia (2002) 536 U.S. 304, 321 [153 L.Ed.2d 335, 122 S.Ct. 2242] (Atkins).) Thereafter, petitioner filed this fourth petition asserting a single claim for relief under Atkins.
While the matter was pending, the California Legislature enacted Penal Code section 1376 (section 1376), which sets forth the standards and procedures for determining whether a defendant against whom the prosecution seeks the death penalty is mentally retarded within the meaning of Atkins. (Stats. 2003, ch. 700, § 1.) By its terms, section 1376 applies only to preconviction proceedings. We issued an order to show cause to determine how to resolve postconviction claims of mental retardation. (Cf. In re Steele (2004) 32 Cal.4th 682 [10 Cal.Rptr.3d 536, 85 P.3d 444].) For the reasons discussed below, we conclude that such claims should be adjudicated in substantial conformance with the statutory model. Since petitioner has met the threshold showing of mental retardation, the matter will be transferred to the superior court for an evidentiary hearing on that question in accordance with the definitional standards set forth in section 1376.
Discussion
Although, as a constitutional principle, execution of the mentally retarded violates the Eighth Amendment, the United States Supreme Court “ ‘le[ft] to the State[s] the task of developing appropriate ways to enforce the constitutional restriction upon [their] execution of sentences.’ [Citation.]” (Atkins, supra, 536 U.S. at p. 317.) The California Legislature responded by enacting section 1376, applicable in “any case in which the prosecution seeks the death penalty.” (§ 1376, subd. (b)(1).) The statute defines “ ‘mentally retarded’ ” as “the condition of significantly subaverage general intellectual functioning existing concurrently with deficits in adaptive behavior and manifested before the age of 18.” (Id., subd. (a); see Atkins, at p. 309, fn. 3; see also id. at p. 309, fn. 5.) “Upon the submission of a declaration by a qualified expert stating his or her opinion that the defendant is mentally retarded, the court shall order a hearing to determine whether the defendant is mentally retarded.” (§ 1376, subd. (b)(1).)1 The defendant may request either *45that the court hear the claim prior to trial or that the jury decide the question following a guilty verdict and special circumstance finding. (Ibid.) The trial court may order the defendant examined by one or more qualified experts. (Id., subd. (b)(2).)2 The defendant must also submit to an examination by a prosecution expert. (Centeno v. Superior Court (2004) 117 Cal.App.4th 30, 39-41 [11 Cal.Rptr.3d 533]; cf. People v. Carpenter (1997) 15 Cal.4th 312, 412 [63 Cal.Rptr.2d 1, 935 P.2d 708] [tendering issue of mental condition waives Fifth and Sixth Amendment rights at penalty phase].) At the hearing, the defendant bears the burden of proof by a preponderance of the evidence, and any jury verdict must be unanimous. (§ 1376, subd. (b)(3).)3
The new legislation makes no provision for cases in which the death penalty has already been imposed. The task thus falls to this court to formulate appropriate procedures for resolving postconviction claims.
We are not alone in confronting this gap in the law. Following Penry v. Lynaugh, supra, 492 U.S. 302, the Georgia Legislature enacted the first statutory ban on execution of the mentally retarded. Like section 1376, it applies only preconviction. With respect to postconviction claims, the Georgia Supreme Court determined in Fleming v. Zant (1989) 259 Ga. 687 [386 S.E.2d 339], that “[w]hen a defendant who was tried before the effective date of [the operative statute] alleges in a petition for habeas corpus that he or she is mentally retarded, the habeas corpus court must first determine whether the petitioner has presented sufficient credible evidence, which must include at *46least one expert diagnosis of mental retardation, to create a genuine issue regarding petitioner’s retardation. The court, in its discretion, may hold a hearing on the issue, or may make the determination based on affidavits, depositions, documents, etc. If, after examining the evidence, the habeas corpus court finds that there is a genuine issue, a writ shall be granted for the limited purpose of conducting a trial on the issue of retardation only. This trial shall be held in the court in which the original trial was conducted. Petitioner shall be entitled to a full evidentiary hearing on the issue of retardation. The determination shall be made by a jury using the definition of retardation enunciated in the statute. [Citation.] The petitioner will bear the burden of proving retardation by a preponderance of the evidence. The jury shall not be bound by the opinion testimony of expert witnesses or by test results, but may weigh and consider all evidence bearing on the issue of mental retardation.” (Id. at pp. 342-343, fn. omitted; see Zant v. Beck (1989) 259 Ga. 756 [386 S.E.2d 349, 351]; Ga. Code Ann. § 17-7-131.)
Oklahoma’s statute likewise operates prospectively only. In Lambert v. State (2003) 2003 OKCR 11 [71 P.3d 30], the Oklahoma Supreme Court addressed a claim of mental retardation in a case that predated the legislation. Finding that the defendant had “raised sufficient evidence to create a question of fact on the issue of mental retardation” (id. at p. 31), the court remanded the question to the trial court for further proceedings with the following directions: “The hearing [—solely on the question of Lambert’s mental retardation—] shall be conducted after complete discovery is afforded both parties under the Oklahoma Criminal Discovery Code. The District Court shall empanel a jury of twelve persons, granting each party nine peremptory challenges. As Lambert has the burden of proof, he shall open his case first, present evidence first, and have the opportunity to present the first and last closing arguments. Each party may have Lambert examined by an expert, and may present that expert testimony in support of the claim that Lambert is or is not mentally retarded by a preponderance of the evidence. The jury shall be instructed using a modified version of the jury instruction provided in Murphy [v. State (2002) 2002 OKCR 32 [54 P.3d 556, 567-568, 570] (defining mental retardation in terms substantially similar to § 1376)]. If the jury finds Lambert has shown he is mentally retarded by a preponderance of the evidence, it shall indicate that finding on a verdict form.” (Lambert, at pp. 31-32, fns. omitted; see Murphy, at p. 569.)
Neither Ohio nor Louisiana has a statutory bar to executing the mentally retarded. The holding in Atkins thus left to the state supreme courts the responsibility of devising appropriate standards and procedures. (See State v. Williams (La. 2002) 831 So.2d 835; State v. Lott (2002) 97 Ohio St.3d 303 [2002 Ohio 6625, 779 N.E.2d 1011]; see also Wiley v. State (Miss. 2004) 890 So.2d 892.) Both courts required a threshold showing of mental retardation. (Williams, at p. 861 [trial court must have *47“ ‘reasonable grounds’ to believe a defendant is mentally retarded”]; Lott, at p. 1014 [based on IQ tests and affidavits of family and friends, defendant’s mental retardation was “a disputed factual issue”].) The Louisiana court applied the statutory definition of mental retardation utilized “for the purpose of determining those individuals who qualify for mental retardation and developmental disabilities services” (Williams, at p. 853), which is essentially the same as California’s definition, but with manifestation required by age 22 rather than 18. (Id. at pp. 853-854.) The Ohio court adopted the clinical definitions referenced in Atkins, which likewise conform to section 1376. (Lott, at p. 1014.) Both courts provided for an evidentiary hearing and allocated the burden of proof to the defendant by a preponderance of the evidence. However, they reserved the question of mental retardation to the trial court only. (Williams, at pp. 854, 859-860; Lott, at pp. 1015-1016.)
We conclude a similar approach—tracking section 1376 as closely as logic and practicality permit—is warranted here, both to maintain consistency with our own legislation and the judicial frameworks adopted in other jurisdictions and to avoid due process and equal protection implications.
Postconviction claims of mental retardation should be raised by petition for writ of habeas corpus, which—consistent with our current policies—may be filed initially in this court. (See Cal. Supreme Ct., Policies Regarding Cases Arising from Judgments of Death, policy 3, std. 2-1.) To state a prima facie claim for relief, the petition must contain “a declaration by a qualified expert stating his or her opinion that the [petitioner] is mentally retarded . . . .” (§ 1376, subd. (b)(1).) Not only must the declarant be a qualified expert, i.e., an individual with appropriate education, training, and experience, the declaration must explain the basis for the assessment of mental retardation in light of the statutory standard.
“Mentally retarded” means “the condition of significantly subaverage general intellectual functioning existing concurrently with deficits in adaptive behavior and manifested before the age of 18.” (§ 1376, subd. (a).) The Legislature derived this standard from the two clinical definitions referenced by the high court in Atkins, supra, 536 U.S. at page 309, footnote 3: “The American Association on Mental Retardation (AAMR) defines mental retardation as follows: ‘Mental retardation refers to substantial limitations in present functioning. It is characterized by significantly subaverage intellectual functioning, existing concurrently with related limitations in two or more of the following applicable adaptive skill areas: communication, self-care, home living, social skills, community use, self-direction, health and safety, functional academics, leisure, and work. Mental retardation manifests before age 18.’ [Citation.] [f] The American Psychiatric Association’s definition is similar: ‘The essential feature of Mental Retardation is significantly subaverage general intellectual functioning (Criterion A) that is accompanied by *48significant limitations in adaptive functioning in at least two of the following skill areas: communication, self-care, home living, social/interpersonal skills, use of community resources, self-direction, functional academic skills, work, leisure, health, and safety (Criterion B). The onset must occur before age 18 years (Criterion C). Mental Retardation has many different etiologies and may be seen as a final common pathway of various pathological processes that affect the functioning of the central nervous system.’ [Citation.] ‘Mild’ mental retardation is typically used to describe people with an IQ level of 50-55 to approximately 70. [Citation.]” “It is estimated that between 1 and 3 percent of the population has an IQ between 70 and 75 or lower, which is typically considered the cutoff IQ score for the intellectual function prong of the mental retardation definition. [Citation.]” (Id. at p. 309, fn. 5.)
Accordingly, as with preconviction applications, the expert’s declaration must set forth a factual basis for finding the petitioner has significantly subaverage intellectual functioning and deficiencies in adaptive behavior in the categories enumerated above. The evidence must also establish that the intellectual and behavioral deficits manifested prior to the age of 18. (See generally People v. Duvall (1995) 9 Cal.4th 464, 474 [37 Cal.Rptr.2d 259, 886 P.2d 1252] [prima facie claim must “state fully and with particularity the facts on which relief is sought” and “include copies of reasonably available documentary evidence supporting the claim”].)
With respect to the intellectual prong of section 1376, respondent Attorney General urges the court to adopt an IQ of 70 as the upper limit for making a prima facie showing. We decline to do so for several reasons: First, unlike some states, the California Legislature has chosen not to include a numerical IQ score as part of the definition of “mentally retarded.” Respondent cites nothing in the language or legislative history of section 1376 to support our insertion of a standard the Legislature has omitted. Moreover, statutes referencing a numerical IQ generally provide that a defendant is presumptively mentally retarded at or below that level, rather than—as respondent impliedly argues—that a defendant is presumptively not mentally retarded above it. (See, e.g., Neb. Rev. Stat. § 28-105.01; N.M. Stat. Ann. § 31-20A-2.1.) Second, a fixed cutoff is inconsistent with established clinical definitions (see ante, at pp. 47-48) and fails to recognize that significantly subaverage intellectual functioning may be established by means other than IQ testing. Experts also agree that an IQ score below 70 may be anomalous as to an individual’s intellectual functioning and not indicative of mental impairment. (See Am. Psychiatric Assn., Diagnostic and Statistical Manual of Mental Disorders (4th ed. 2000) pp. 41-42.) Finally, IQ test scores are insufficiently *49precise to utilize a fixed cutoff in this context.4 (See Am. Psychiatric Assn., supra, at p. 41 [indicating IQ scores are considered to have a five-point measurement error]; AAMR, Mental Retardation: Definition, Classification, and Systems of Support (10th ed. 2002) p. 57; Am. Assn. on Mental Deficiency, Classification in Mental Retardation (8th ed. 1983) p. 11.)
Upon the submission of an appropriate declaration “by a qualified expert” (§ 1376, subd. (b)(1)), this court will—as a general rule—then issue an order to show cause returnable in the superior court in which the original trial was held, with directions to hold a hearing on the question of the petitioner’s mental retardation. (See In re Hochberg (1970) 2 Cal.3d 870, 873-874 & fn. 2 [87 Cal.Rptr. 681, 471 P.2d 1]; id. at pp. 875-876 & fn. 4; see also Griggs v. Superior Court (1976) 16 Cal.3d 341, 347 [128 Cal.Rptr. 223, 546 P.2d 727].) In addition to maintaining parity with the statutory scheme, the order for an evidentiary hearing reflects the consensus that mental retardation is a question of fact. (See Zant v. Beck, supra, 386 S.E.2d at p. 351; State v. Williams, supra, 831 So.2d at pp. 854-855; Richardson v. State (Md.Ct.Spec.App. 1992) 89 Md.App. 259 [598 A.2d 1, 3]; Murphy v. State, supra, 54 P.3d at pp. 568, 570-571.) It is not measured according to a fixed intelligence test score or a specific adaptive behavior deficiency, but rather constitutes an assessment of the individual’s overall capacity based on a consideration of all the relevant evidence. (See, e.g., Rankin v. State (1997) 329 Ark. 379 [948 S.W.2d 397, 402]; Head v. Ferrell (2001) 274 Ga. 399 [554 S.E.2d 155, 167]; Murphy, at p. 568; see also Atkins, supra, 536 U.S. at pp. 317-318; Cartwright v. State (2000) 242 Ga.App. 825 [531 S.E.2d 399, 403—404]; State v. Williams, at p. 859.)
With respect to the evidentiary hearing, section 1376 affords preconviction defendants the alternative of requesting a court proceeding or a jury trial. For several reasons, we deem it inappropriate to extend the jury trial option to postconviction claims. First, there is no statutory imperative to do so. Section 1376, subdivision (b)(1), provides that “the same jury [as rendered the guilty verdict and special circumstance finding] shall make a *50finding that the defendant is mentally retarded, or that the defendant is not mentally retarded.” Plainly, that is not possible postconviction. Although the Legislature could have provided for a comparable procedure, it chose to limit the enactment to preconviction claims. Moreover, because preconviction and postconviction defendants are not similarly situated, the Legislature could rationally distinguish between them for purposes of a jury trial.
Second, allowing for a jury trial would be inconsistent with established habeas corpus procedure. While a petitioner is entitled to “a full and fair hearing” on his prima facie claim, Penal Code section 1484 authorizes only “[tjhe Court or Judge” to hear the proceedings.
Finally, we perceive no constitutional mandate in this regard. In Atkins, supra, 536 U.S. 304, the Supreme Court expressly left to the states the responsibility of “ ‘developing appropriate ways to enforce the constitutional restriction upon [their] execution of sentences.’ [Citation.]” (Id. at p. 317.) Moreover, at least eight states cited by the court as part of the trend to bar execution of the mentally retarded provide that the question shall be determined by the court not a jury. (See Ariz. Rev. Stat. § 13-703.02, subds. (G), (H); Ky. Rev. Stat. Ann. § 532.135, subds. (1), (2); Neb. Rev. Stat. §28-105.01, subd. (4); N.M. Stat. Ann. § 31-20A-2.1, subd. (C); N.Y. Code Crim. Proc. § 400.27, subd. 12(a); S.D. Codified Laws § 23A-27A-26.3; Tenn. Code Ann. § 39-13-203, subd. (c); Wash. Rev. Code § 10.95.030, subd. (2); State v. Williams, supra, 831 So.2d at pp. 859-860; State v. Lott, supra, 779 N.E.2d at pp. 1015-1016.)
Consistent with section 1376, subdivision, (b)(2), the petitioner may be subject to examination by an expert appointed by the court or designated by the prosecution, or both. (See Centeno v. Superior Court, supra, 117 Cal.App.4th at pp. 39-41.) At the hearing, “the court . . . shall decide only the question of the [petitioner’s] mental retardation.” (§ 1376, subd. (b)(2).) Evidence relating to the underlying crimes shall be admissible only to the extent relevant on this question. (Morrison v. State (2003) 276 Ga. 829 [583 S.E.2d 873, 876]; see Lambert v. State, supra, 71 P.3d at p. 31.) The court “shall not be bound by the opinion testimony of expert witnesses or by test results, but may weigh and consider all evidence bearing on the issue of mental retardation.” (Fleming v. Zant, supra, 386 S.E.2d at p. 343; State v. Williams, supra, 831 So.2d at p. 859.) The petitioner shall have the burden of proving his mental retardation by a preponderance of the evidence. (See § 1376, subd. (b)(3).) Review of any ultimate finding shall conform to *51established appellate procedures for habeas corpus proceedings. That is, the People may challenge a finding of mental retardation by appeal to this court (Pen. Code, § 1506); the petitioner may challenge a contrary finding by further petition for writ of habeas corpus to this court.
In this case, petitioner has submitted, among other exhibits in support of his claim, the declaration of Dale G. Watson, Ph.D., a qualified clinical neuropsychologist practicing in “neuropsychological and psychodiagnostic assessment, psychotherapy, forensic psychology and in-patient hospital consultation.” According to his declaration, Watson has reviewed a substantial amount of background material relating to petitioner’s upbringing, educational performance, family environment, adaptive behavior, and mental condition. In addition to considering petitioner’s prior intelligence test results, he “conducted a comprehensive neuropsychological evaluation” in June and August of 1995, at which time he “administered a full battery of standard neuropsychological tests, conducted a mental status examination and clinical review.” As a result, Watson concluded petitioner “is one of the most profoundly impaired individuals that I have seen within a forensic population.” He further opined that “based upon [petitioner’s] obtained IQ scores [most of which show borderline retardation of 70-755] and the history of impairment in adaptive capacities . . . , [petitioner] can be legitimately classified as being mentally retarded.” That history included evidence that from early childhood petitioner was a slow learner; had trouble with basic reading, writing, and arithmetic; and had problems communicating with others.
Petitioner has also submitted a declaration of George Woods, M.D., who specializes in psychiatry and neurology. Woods interviewed petitioner but did not administer any additional intelligence tests. Based upon Woods’s “experience and education, the review of voluminous documents, [his] interview with [petitioner], the data and information compiled by other experts whose declarations [he had] reviewed and [his] review of the historical, medical, psychological and educational information,” he likewise concluded petitioner “is both mentally retarded and psychiatrically impaired.”
We find this evidentiary showing sufficient to meet the statutory threshold entitling petitioner to a hearing on the question of his mental retardation. Respondent argues that, taken at face value, the various declarations attesting to petitioner’s intellectual and adaptive deficiencies do not *52establish mental retardation within the purview of Atkins, supra, 536 U.S. 304. Rather than negate petitioner’s prima facie showing, this argument simply highlights the factual nature of the dispute between the parties—a circumstance particularly appropriate to a full evidentiary hearing on the ultimate question. (See State v. Williams, supra, 831 So.2d at pp. 854—855.)
Disposition
The matter is transferred to the Los Angeles County Superior Court with directions to hold a hearing on the issue of petitioner’s mental retardation consistent with the views expressed in this opinion.
Kennard, J., Baxter, J., Werdegar, J., Chin, J., and Moreno, J., concurred.