What is the burden of proving a defendant in a criminal trial for a specific intent crime?

Where the commission of a crime requires a specific mental state that state must be proved like any other fact. (People v. Wells (1949) 33 Cal.2d 330, 350, 202 P.2d 53.) Therefore, "it is fundamental that ... evidence ... which tends to show that a defendant, at the time he committed the overt act, either possessed or did not possess the specific essential mental state (as of malice aforethought, deliberate intent, etc.) is admissible." (Id., at p. 347, 202 P.2d 53.) This fundamental rule is codified in Penal Code section 28, subdivision (a) which provides in relevant part, "Evidence of mental disease, mental defect, or mental disorder is admissible solely on the issue of whether or not the accused actually formed a required specific intent, premeditated, deliberated or harbored malice aforethought, when a specific intent crime is charged." Penal Code, section 22, subdivision (b) contains a similar provision

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In the case before us, Markarian was convicted of crimes requiring a particular mental state--malice aforethought. In addition, first degree murder, of which Markarian was convicted, is willful, deliberate and premeditated. (Pen.Code, 187, 189.) "In a criminal trial, the burden is upon the prosecution to prove beyond any reasonable doubt every essential element of the crime of which a defendant is to be convicted." (People v. Borchers (1958) 50 Cal.2d 321, 328, 325 P.2d 97.) Thus, the burden was on the People to prove beyond a reasonable doubt that Markarian possessed the required state of mind when he committed the overt acts.