A common misconception that gets thrown around is that if someone is "crazy" or "insane" they can commit any type of crime they want, claim "insanity," and walk out the courtroom as if nothing happened. Not only is this inaccurate, but the defense of "insanity" is also rare and complex. Let's break it down below.
First things first, let's be clear about what "insanity" actually means. Insanity is a legal term and NOT a medical term. What this means is that "insanity" is not a type of mental illness or other formal diagnosis given by a mental health professional. (Furthermore, rather than referring to it as "insanity," it is now more commonly referred to as a defense of "mental illness or cognitive impairment.")
Rather, the issue in Minnesota is "whether, because of mental illness or cognitive impairment, the defendant, at the time of committing the alleged criminal act, was laboring under such a defect of reason as not to know the nature of the act or that it was wrong." Minn. R. Crim. P. 20.02 Subd. 4 (b). So, the issue isn't simply whether or not a person has, or had, a mental illness or cognitive impairment. It has to be specific to what was happening to the person at the time of the offense AND needs to have caused a "defect of reason" as just previously noted.
An extreme example could be that a person knows that they killed someone, but because of a mental illness or cognitive impairment, they believed the person they were killing was Darth Vader and that it needed to be done to save the Galaxy.
It's important to note that this type of defense is an affirmative defense -- that means the burden is on the defense to raise the issue and present some evidence that it is applicable. And, it involves lots of procedure.
When the defense is raised, the Court (aka the Judge) will order the defendant to undergo an assessment by a professional to determine whether or not they were suffering from a mental illness or cognitive impairment such that they did not know what they were doing, that it was wrong, or both, at the time of the incident. Remember, the issue is not whether the person is presently suffering from a mental health issue (that would fall more under the category of "competency" which is a separate issue). It's whether, at the time of the incident, the mental health issues were sufficiently present to satisfy this defense.
Several things may happen depending on the results of the assessment, specifically whether the defendant or government agrees or disagrees with the report. Either Party can utilize a separate expert to challenge the opinion, which can then turn into a "battle of the experts" at trial. Assuming the assessment comes back indicating the defendant DOES qualify for the defense, and the case continues to trial, the case typically unfolds in two phases. First, the jury determines the issue of "guilty vs not-guilty" without hearing any evidence related to the mental health defense. If the jury finds the defendant "not-guilty" based on the specific facts of the case alone, the defendant is acquitted, the case is over, and the issue of mental health is never reached. However, if the jury finds the defendant "guilty" in the first phase, then the case proceeds to the second phase where they will determine whether or not the mental health defense applies. The jury will then hear evidence on the defendant's mental health at the time of the incident, and decide either not guilty by reason of mental illness; not guilty by reason of cognitive impairment; or, guilty. Minn. R. Crim. P. 20.02 Subd. 7.
While the prosecution must always prove every element of any case beyond a reasonable doubt (the highest standard of proof in our legal system), when making a mental health defense, the defendant has the burden of proving it by a preponderance of the evidence (imagine the equally balanced scales of justice, and just one grain of sand is placed on the left scale vs the right scale, tipping it just slightly -- that is all that is needed to prevail). Minn. R. Crim. P. 20.02 Subd. 7.
So, the defendant is found not guilty by reason of either mental illness or cognitive impairment -- what happens next? The reason this defense exists is because under our system of law, people are not supposed to be punished for things that they are not responsible for. While a person may have broken the law, if they only did so because of a mental illness or cognitive impairment that prevented them from knowing what they were doing, or that it was wrong, we don't convict them of crimes, and we don't imprison them.
But, are they simply free to leave and go on their way? The answer is NO.
If the defendant prevails on a mental health defense and is already under civil commitment, then the Court must order that the civil commitment continue. If the defendant is not under civil commitment, then the County Attorney must file a petition for civil commitment in the county in which the acquittal took place. Pertinently, the Court "must order the defendant to be detained in a state hospital or other facility pending completion of the proceedings." Minn. R. Crim. P. 20.02 Subd. 8 (1). This means the person will likely be placed in a mental health facility and receive treatment as deemed necessary to address any mental health issues.
So, in sum, an argument of not-guilty based on mental health is not something anyone can walk into a courtroom and easily claim (just like Michael Scott yelling that he declares bankruptcy doesn't actually get him into bankruptcy). It's complex, nuanced, and although it may excuse otherwise criminal conduct, the person will likely still face extensive and ongoing state intervention.