Does a court have a duty to instruct on the defense of others as a legal defense to assault?

In People v. Cain, supra, 10 Cal.4th 1, 72-73, 40 Cal.Rptr.2d 481, 892 P.2d 1224, we considered whether a trial court at the penalty phase had a duty to instruct, on its own motion, on a defense to a crime presented under factor (b). The factor (b) evidence there involved an assault, and the defendant argued that the trial court had a duty to instruct on the defense of others as a legal defense to assault. ( Cain, supra, at p. 72, 40 Cal.Rptr.2d 481, 892 P.2d 1224.) In rejecting the claim, we concluded that, on the facts presented, even without an instruction on the legal defense to assault the jury had before it evidence and argument from which it could rationally assess the degree of culpability [the] defendant bore in the prior incident.