I join in the dissenting opinion of Justice Arguelles, but write to add some thoughts of my own.
Although defendant was charged under the 1977 death penalty law, the trial court mistakenly instructed the jury under the 1978 law. The majority find no reversible error because, they say, the sentencing provisions of the 1977 and 1978 death penalty laws are essentially identical. Under the 1977 law, the jury could spare defendant’s life even if it believed aggravation outweighed mitigation. (See maj. opn., ante at p. 1026.) Indeed a 1977-law jury could reach a verdict without determining the aggravation-mitigation balance. The question then arises whether a 1978-law jury also can spare defendant’s life even if it believes aggravation outweighs mitigation.
It is a simple question, which could be answered “yes” or “no.” The majority equivocate, for it places them in an impossible dilemma.1 If they answer “no,” that a 1978-law jury is bound by its view of the aggravation mitigation balance, then their answer discloses a major difference between the two laws—a difference they do not discuss, and which might require a reversal of the penalty judgment before us. If they answer “yes,” their answer calls into question our affirmance of the death penalty in any case in which judge and prosecutor told the jury that if aggravating circumstances outweigh mitigating circumstances, the law requires a death verdict.
The majority cannot escape this dilemma by the “explanation” that under the 1978 law the “ ‘weighing’ of the pertinent factors is the process by which the sentencer performs the guided, but normative and subjective, task of deciding for itself what penalty is ‘appropriate.’ ” (Ante, p. 1028, fn. 12.)2 *1039The weighing of the factors is not the process by which a 1977-law jury determines which penalty is appropriate, so there would still remain a substantial difference between the two laws.3 This court should not hold that the 1977 and 1978 laws are essentially identical to affirm the death penalty for David Murtishaw under the 1977 law, then insist that 1978-law juries use a significantly different method of determining penalty when it affirms death penalties under the 1978 law.
The majority opinion, moreover, incorrectly analyzes the question of prejudicial error. As the majority note, it is not error to instruct a 1978-law jury in the terms of that statute, but because the instruction is susceptible of misleading analysis we look to the whole record to determine whether the jury was misled. (See People v. Brown, supra, 40 Cal.3d 512, 544, fn. 17.) It is, however, plain error to instruct a 1977-law jury under the language of the 1978 law. (People v. Easley (1983) 34 Cal.3d 858, 883 [196 Cal.Rptr. 309, 671 P.2d 813].) Thus, contrary to the majority view, we should not undertake the Brown analysis in the present case, but move directly to the question whether there is a reasonable possibility that the error affected the verdict. (See People v. Brown (1988) 46 Cal.3d 432, 447-449 [250 Cal.Rptr. 604, 758 P.2d 1135].)
Mitigating factors in the present case include the absence of any prior criminal activity, defendant’s family history and present marriage, his voluntary surrender before he was a suspect, and the testimony that his acts were the result of prior PCP usage or organic brain damage. A jury could still reasonably find, as this jury presumably did, that the substantial mitigating evidence is outweighed by the single aggravating factor, the circumstances of the crime. But if that same jury were properly instructed under the 1977 law that it could return a life verdict despite the relative weight of the aggravating and mitigating factors—indeed that it could return a verdict without weighing or comparing factors at all—I think it reasonably possible that it would have decided that defendant should be punished by a sentence of life imprisonment without possibility of parole.
My answer to this question is clear. A 1978-law jury must take into account the balance of aggravation and mitigation, but is not required to return a death verdict simply because it finds that aggravation marginally outweighs mitigation. That matter was settled in People v. Brown (1985) 40 Cal. 3d 512, 541-542, footnote 13 [220 Cal.Rptr. 637, 709 P.2d 440], where we said that the jury must decide whether aggravation is “so substantial in comparison” with mitigation as to warrant death—language which necessarily implies that a jury could find a preponderance of aggravation insufficient to warrant death. (See also p. 545, fn. 19.) This method of determining penalty is still significantly different from the 1977 law, which did not require the jury even to determine the aggravation-mitigation balance. This difference, and others discussed in the opinion of Justice Arguelles, establish that instructing the present jury under the wrong law was reversible error.
The process of determining penalty under the 1978 law involves three steps: (a) weighing the statutory factors; (b) comparing the weight of the aggravating and mitigating factors; and (c) determining whether aggravation is so substantial in comparison to mitigation that death *1039is the appropriate penalty. (See fn. 1, ante, p. 1038.) The “weighing process” is the method by which the jury decides whether death is the appropriate penalty only if by that process we refer inclusively to all three steps. Regrettably, in cases tried before People v. Brown, supra, 40 Cal.3d 512, the instructions and argument often omit the final step, and mistakenly assume that the slightest preponderance of aggravation is sufficient to require a death penalty. No such mistake is possible under the 1977 law unless, as in this case, the court instructs under the wrong law.
The majority state that “[t]hough a 1977-law sentencer may have more latitude with respect to aggravating evidence . . . , its responsibility for deciding the proper punishment is essentially the same [as a 1978-law sentencer]." It is true that both have the same responsibility: to determine whether death is the appropriate penalty. But the methods by which they discharge this responsibility are different. A crucial requirement for the 1978-law jury is determining the relative balance of aggravation and mitigation. The 1977 law omits this requirement. The majority’s language obscures this difference between the two laws.