Not Guilty by reason of a mental illness defense.  All state in the U.S. have a statutory provision that allow a jury to find an accused person not guilty because of a mental illness insanity defense—even if the accused did the crime.

When a guilty person “gets off” like this it makes many of us upset or even mad.

What does this really mean and how does it work in a trial?

In Minnesota, the law allows a not guilty verdict by reason of a mental illness defense if: at the time of committing the crime the defendant was laboring under such a degree of mental illness that he was unable to know the nature of the act he was committing or he did not know his act was wrong.

Interestingly, this standard comes from the 1800s in England when Daniel M’Naghton killed someone he mistakenly thought was the English Prime Minister.  Even today, we call the Minnesota law the “M’naghton Rule.”http://en.wikipedia.org/wiki/Daniel_M’Naghten

The mental illness defense mechanism works like this in a trial.  In the first part, the prosecutor presents evidence of factual guilty which the jury decides.  In the second part, experts such a psychologists or psychiatrists testify about the defendant’s mental health.  The jury must make two decisions.  If they fin the defendant not guilty, the trial is over.  If they find the defendant guilty, the jury must go further and decide if the defendant’s mental illness defense fits within the M’Naghton Rule and is, therefore, not guilty.

Does this mean the accused walks away from any responsibility?

No.  The defendant would be moved into mental health court where another judge (no jury here) decides if the accused will be committed to a hospital for an indeterminate time but not prison.  Some legal scholars are looking at the laws in light of modern day knowledge of mental health.  A few states have experimented with new, broader statutes than the M’Naghton Rule.  The new rules tend to allow more people to be found not guilty by reason of a mental illness defense.

On the frontier of expert thinking is something called, the amygdala hijack.  It refers to people who suffer from post-traumatic stress disorder and a “primitive” part of the brain called the amygdala.  For most of us, when faced with a traumatic situation, our “higher” or thinking part of the brain over-rides the amygdala and makes a rational choice.  But for others, the amygdala “hijacks” the rational part and causes them to do violent things they normally wouldn’t do.

Should this become a mental illness defense?

I’m working on a new book in which the amygdala hijack plays a big part in the story.  I’ll write more about it in my next blog and explain what it is.