What is the test for using mental and emotional disturbance as a mitigating circumstance in a murder trial?

The instruction on mental and emotional disturbance as a mitigating circumstance, like the instruction in Boyde v. California, supra, 494 U.S. 370, 110 S.Ct. 1190, is not concededly erroneous. Though it is claimed that the jury misinterpreted it, defendant can point to no instruction or response by the trial court as to that circumstance which could have prompted the note from the jury on the sixth day of deliberations that one juror was using "insanity or mental disturbance as a mitigating circumstance even though you have said we can't use this." Of course, the court had instructed the jury (correctly, we conclude hereafter) that the defendant was presumed sane and that they could not consider insanity as a mitigating circumstance. ⁷

The jury was instructed to consider all of the evidence received during the trial--evidence of the stab wounds and the manner of killing (testimony and photographs) had been introduced. The jury was instructed on extreme mental and emotional disturbance as a mitigating factor and was told that the evidence of mental or emotional disturbance did not have to be proved by expert testimony. It is inconceivable that the jury would have believed that it could not consider that factor. (See People v. Boyde (1988) 46 Cal.3d 212, 251, 250 Cal.Rptr. 83, 758 P.2d 25.)