How should a jury consider other criminal activity at the penalty phase of a criminal case?

25 The problems revealed by the record in this case suggest that in many cases it may be advisable for the trial court to conduct a preliminary inquiry before the penalty phase to determine whether there is substantial evidence to prove each element of the other criminal activity. This determination, which can be routinely made based on the pretrial notice by the prosecution of the evidence it intends to introduce in aggravation ( 190.3), should be made out of the presence and hearing of the jury. (Evid.Code, 402.) Once the trial court has determined what evidence is properly admissible as other criminal activity (id., 310), "the prosecution should request an instruction enumerating the particular other crimes which the jury may consider as aggravating circumstances in determining penalty.... [T]he jury should be instructed not to consider any additional other crimes in fixing the penalty." (People v. Robertson, supra, 33 Cal.3d at p. 55, fn. 19, 188 Cal.Rptr. 77, 655 P.2d 279.) Whether such other criminal activity has been proven beyond a reasonable doubt is then a question of fact for the jury. (Evid.Code, 312.) If the jury finds that the evidence does not establish such criminal activity beyond a reasonable doubt, it could still consider such evidence, if appropriate, under any of the other listed factors enumerated in former section 190.3.