(a) As used in this section, “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.
(b) (1) In any case in which the prosecution seeks the death penalty, the defendant may, at a reasonable time prior to the commencement of trial, apply for an order directing that a mental retardation hearing be conducted. 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. At the request of the defendant, the court shall conduct the hearing without a jury prior to the commencement of the trial. The defendant’s request for a court hearing prior to trial shall constitute a waiver of a jury hearing on the issue of mental retardation. If the defendant does not request a court hearing, the court shall order a jury hearing to determine if the defendant is mentally retarded. The jury hearing on mental retardation shall occur at the conclusion of the phase of the trial in which the jury has found the defendant guilty with a finding that one or more of the special circumstances enumerated in Section 190.2 are true. Except as provided in paragraph (3), the same jury shall make a finding that the defendant is mentally retarded, or that the defendant is not mentally retarded.
(2) For the purposes of the procedures set forth in this section, the court or jury shall decide only the question of the defendant’s mental retardation. The defendant shall present evidence in support of the claim that he or she is mentally retarded. The prosecution shall present its case regarding the issue of whether the defendant is mentally retarded. Each party may offer rebuttal evidence. The court, for good cause in furtherance of justice, may permit either party to reopen its case to present evidence in support of or opposition to the claim of retardation. Nothing in this section shall prohibit the court from making orders reasonably necessary to ensure the production of evidence sufficient to determine whether or not the defendant is mentally retarded, including, but not limited to, the appointment of, and examination of the defendant by, qualified experts. No statement made by the defendant during an examination ordered by the court shall be admissible in the trial on the defendant’s guilt.
(3) At the close of evidence, the prosecution shall make its final argument, and the defendant shall conclude with his or her final argument. The burden of proof shall be on the defense to prove by a preponderance of the evidence that the defendant is mentally retarded. The jury shall return a verdict that either the defendant is mentally retarded or the defendant is not mentally retarded. The verdict of the jury shall be unanimous. In any case in which the jury has been unable to reach a unanimous verdict that the defendant is mentally retarded, and does not reach a unanimous verdict that the defendant is not mentally retarded, the court shall dismiss the jury and order a new jury impaneled to try the issue of mental retardation. The issue of guilt shall not be tried by the new jury.
(c) In the event the hearing is conducted before the court prior to the commencement of the trial, the following shall apply:
(1) If the court finds that the defendant is mentally retarded, the court shall preclude the death penalty and the criminal trial thereafter shall proceed as in any other case in which a sentence of death is not sought by the prosecution. If the defendant is found guilty of murder in the first degree, with a finding that one or more of the special circumstances enumerated in Section 190.2 are true, the court shall sentence the defendant to confinement in the state prison for life without the possibility of parole. The jury shall not be informed of the prior proceedings or the findings concerning the defendant’s claim of mental retardation.
(2) If the court finds that the defendant is not mentally retarded, the trial court shall proceed as in any other case in which a sentence of death is sought by the prosecution. The jury shall not be informed of the prior proceedings or the findings concerning the defendant’s claim of mental retardation.
(d) In the event the hearing is conducted before the jury after the defendant is found guilty with a finding that one or more of the special circumstances enumerated in Section 190.2 are true, the following shall apply:
(1) If the jury finds that the defendant is mentally retarded, the court shall preclude the death penalty and shall sentence the defendant to confinement in the state prison for life without the possibility of parole.
(2) If the jury finds that the defendant is not mentally retarded, the trial shall proceed as in any other case in which a sentence of death is sought by the prosecution.
(e) In any case in which the defendant has not requested a court hearing as provided in subdivision (b), and has entered a plea of not guilty by reason of insanity under Sections 190.4 and 1026, the hearing on mental retardation shall occur at the conclusion of the sanity trial if the defendant is found sane.