Can a psychiatrist give evidence in a criminal trial where they have opinions beyond a reasonable doubt?

[14 Cal.3d 331] No special exception from this principle is justified for psychiatrists. It is true we have held that medical witnesses, like any other experts, need not limit their testimony in criminal trials to matters on which they have opinions 'beyond a reasonable doubt.' (People v. Phillips (1966) 64 Cal.2d 574, 579, fn. 2, 51 Cal.Rptr. 225, 414 P.2d 353.) But it does not follow that the

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Other examples of this joint operation of two different standards about. Perhaps the most relevant is the defense of diminished capacity in murder trials. The assertion of such a defense typically results in psychiatrists' testifying both for and against the defendant on the crucial issue of whether at the time of the killing he lacked the necessary mental capacity to be guilty of murder in the first degree. On that issue the witnesses may properly speak with no more than 'the degree of assurance with which reputable psychiatrists express themselves.' But when the case is submitted to the jury that body is nevertheless required to determine the defendant's guilt of first degree murder beyond a reasonable doubt, even though the determination may turn on tentative or conflicting opinions of the medical experts. (See, e.g., People v. Bassett (1968) 69 Cal.2d 122, 139--140, 70 Cal.Rptr. 193, 443 P.2d 777, and cases cited.) The same rule applies in the case at bar. ²¹

[14 Cal.3d 332]