In the guilt phase of a criminal action, any expert testifying about a defendant's mental illness, mental disorder, or mental defect shall not testify as to whether the defendant had or did not have the required mental states, which include, but are not limited to, purpose, intent, knowledge, or malice aforethought, for the crimes charged. The question as to whether the defendant had or did not have the required mental states shall be decided by the trier of fact.
Ca. Pen. Code § 29