When a defendant in California (and many other states) pleads both not guilty and not guilty by reason of insanity, the sanity issue is held for a second phase, which will happen only if the defendant is found guilty in the first phase. (Lawyers say the issue is "bifurcated," but that gets Dirty Harry riled up.)
So how to explain this to the regular folks on the jury? Like this?
The way this works is in the first trial, we decide whether or not Mr. Mills is guilty of murder.... During that process, Mr. Mills is presumed or you have to accept that he is sane. That he is legally sane for the purposes of reaching that first verdict. If Mr. Mills is convicted of any crime, then we have a second trial, and the burden shifts to the defense to show by a preponderance of the evidence that Mr. Mills was legally insane at the time the crime was committed.
We will find out if this is okay tomorrow when the California Supreme Court announces its decision in People v. Mills, S191934. The briefs are here.
Update: The answer is:
We conclude that although defendant establishes no due process violation, the instruction was erroneous under state law. The question of a defendant's sanity is entirely irrelevant at the guilt phase of a bifurcated trial under section 1026. Therefore, no instruction on the subject should be given. However, the error was harmless in this case.