In State v. King (1988) 158 Ariz. 419 [763 P.2d 239], the Arizona Supreme Court considered an issue similar to the one confronting us. The defendant was charged with murder and claimed insanity as a defense. Under Arizona law, he was required to prove insanity by clear and convincing evidence.
On appeal from his conviction, he contended that the jury instruction defining "clear and convincing evidence" established an unduly strict standard of proof. The instruction stated: " 'To be clear and convincing, evidence should be clear in the sense that it is certain, plain to the understanding, unambiguous, and convincing in the sense that it is so reasonable and persuasive as to cause you to believe it.' " (Id. at p. 241.)
In a unanimous decision, the Arizona Supreme Court held that the instruction was erroneous. (State v. King, supra, 763 P.2d at pp. 243-244.)
The court concluded: "The correct instruction would have apprised the jury that the clear and convincing standard is an intermediate standard, between proof beyond a reasonable doubt and proof by a preponderance of the evidence, and that clear and convincing evidence is evidence that makes the existence of the issue propounded 'highly probable.'" (Id. at p. 246.)
The court rejected the argument that the "highly probable" standard of civil cases should not be applicable in criminal actions: "We see no good and much harm coming from adopting differing definitions of 'clear and convincing evidence' for use in civil cases . . . and cases involving criminal law. Nor do we find any authority for the proposition that the 'clear and convincing' standard is given a different definition in criminal cases than that followed in civil cases." (Id. at p. 242.)