In my career I have noticed a great deal of confusion between what it means to be legally "insane" and what it means to be "mentally ill". Most of the confusion seems to stem from how pop culture and the media portrays insanity and mental illness. When considering that different states have somewhat different definitions and that there is rarely a bright line between insanity on one hand and mental illness on the other, it is no wonder even attorneys become confused. As my previous article deals with the definition of insanity, we'll turn first to the definition of mental illness here.
Illinois law defines mental illness as "a substantial disorder of thought, mood, or behavior which afflicted a person at the time of the commission of the offense and which impaired that person's judgment, but not to the extent that he is unable to appreciate the wrongfulness of his behavior." 720 ILCS 5/6-2 (d). Some classic example of mental illness are mental retardation or post-traumatic stress disorder, or even anti-social personality disorder.
The key to remember about mental illness is that it IS NOT a legal defense like insanity is. If a person is found to be insane, they are NOT GUILTY of the offense. If a person is mentally ill they ARE GUILTY of the offense BUT will almost certainly receive a lesser punishment at sentencing. The burden is on the defendant to plead insanity as a defense and prove insanity. In order to find a defendant guilty but mentally ill, the defendant must present some form of evidence as to mental health, the court cannot make a finding in a vacuum. As a practical matter, the defendant has usually put on an insanity defense but has failed to prove it up.
It seems unfair to some that a court can find that a person is mentally ill and yet still find them guilty of an offense. Complicating matters are the facts that many times both the defense and prosecution have experts who disagree on sanity and even insane people carry on many activities you and I do on a daily basis (get dressed, live in a home of some sort, collect things of importance). It is my experience that many people with psychological problems self-medicate with some form of drugs or alcohol causing another layer of facts for the court to wade through. i.e. was it the brain tumor that caused the man to attack the woman on the street or was it that he was hurting for his usual fix of crack cocaine?
These cases are almost always incredibly fact driven and complex to present to the court. It is important to consult with an attorney and cooperate with mental health professionals to assist in gathering evidence for this type of case.