Penal Code sections 25, 28, and 29 do not preclude expert testimony that allows the jury to infer that a defendant actually did not premeditate, deliberate, or act with malice. All the statutes prohibit is expert testimony on the ultimate issue of whether or not the defendant actually acted with the requisite mental state. The court reversed a first degree murder conviction because the trial court prejudicially erred by unduly restricting the testimony of the defense psychiatric expert. Appellant stabbed and killed a person at a party who tried to get him and his friends to leave. Appellant wanted to introduce testimony by an expert that at the time of the stabbing he exhibited behavior consistent with dissociation and post-traumatic stress disorder. But the defense expert was only allowed to testify in general terms about the condition of a dissociative state, and was prohibited from testifying about appellant’s mental condition at all. The trial court reasoned a dissociative state was akin to a diminished capacity defense which Penal Code sections 25, 28, and 29 aimed to abolish. Appellant complained on appeal this infringed on his right to present a defense. The appellate court reviewed and discussed all the relevant cases on point. It concluded the statutes do not preclude presenting a defense via an expert that there is absence of a mental state that is an element of the charged offense. All they preclude is expert opinion that the mental state was in fact absent. The trial court erred in restricting the testimony. Applying the Watson standard, reversal was required. The jury could have convicted of voluntary manslaughter instead of murder since the killing occurred during a fight. The prosecutor took advantage of the excluded evidence, arguing there was no other explanation besides premeditation given the 13 stab wounds to the victim, and that the expert’s testimony only gave lots of general information.