What does "not guilty by reason of insanity" mean?

Claiming the "insanity defense" in fictional courts of law is common and seems pretty straightforward; in real life, it's much more rare and complex. Discover the legal definition of "insanity" -- and how it relates to mental illness -- in this episode.

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Episode Transcript

Speaker 1 (00:00):
Welcome to brain Stuff from house stuff Works dot com
where smart Happens. Hi Am Marshall Brain with today's question,
what does it mean when someone is found not guilty
by reason of insanity in a court of law? In

(00:22):
movies and on television shows, a standard legal defense for
a criminal defendant is insanity or temporary insanity. We also
hear about this from time to time in real life,
of course, but it's not an especially common legal defense
to most of us. The legal reasoning behind this defense
is fairly mysterious, even though we've probably seen it played

(00:44):
out dozens of times. On a typical lawyer show, the
defense lawyer brings in a psychologist that says that the
defendant should not be held accountable for his or her
actions because he or she has a certain mental illness
that interferes with his or her reasoning capacity. If the
jury thinks the person actually does have this mental illness,

(01:08):
it finds him or her not guilty by reason of insanity.
This raises a number of questions that most shows don't
answer very clearly. For example, why it does being mentally
ill excuse somebody from criminal guilt, or how is a
jury of regular people qualified to determine whether or not

(01:28):
someone is mentally ill or what level of mental illness
constitutes insanity, or how do you prove somebody is or
was insane when they committed a crime. The main reason
that this concept is so confusing is that lawyer shows
impressed coverage of actual cases often don't clarify the distinction

(01:51):
between insanity and mental illness. Mental illness and mental disorder
are psychiatric concepts, while insaneity is a cultural and legal concept.
In the U s Court of law, as well as
courts in some other countries, insanity and mental illness are
related conditions, but they are by no means synonymous. Mental

(02:14):
illness at the time of the offense is a prerequisite
for the not guilty by reason of insanity ruling, but
legal insanity is not simply a judgment of whether or
not a person has a mental illness. The law varies
from state to state, but in most courts that recognize
the insanity defense, someone is found to be legally insane

(02:35):
if he or she meets one of three conditions. First,
because of a mental disorder, the defendant did not understand
that what he or she was doing was illegal. Second,
because of a mental disorder, the defendant did not know
what he or she was doing. Or Third, because of
a mental disorder, the defendant was compelled to commit the

(02:57):
crime by an irresistible four So why is a person
who meets these conditions not guilty of the crime. The
reasoning is actually based on one of the most fundamental
tenets of the justice system in the United States and
many other countries. With most offenses, a person is only
guilty of a crime if he or she actually intended

(03:19):
to commit the crime. If you accidentally bump into someone
with your car, you're not guilty of assault, but you
would be guilty if you bumped into them intentionally in
exactly the same way. The action is the same, but
you have not committed the crime because you didn't intend to.
You may be guilty of a lesser crime, though e g.

(03:40):
Reckless driving. Mental illnesses can alter a person's conception of
reality so that he or she does not realize the
criminal nature of his or her actions or has no
choice but to commit the crime. When this is the case,
certain courts believe the person lacks this element of intention

(04:02):
necessary for criminal guilt. To prove legal insanity, the defense
must provide credible expert testimony that says how the defendant
is or was at the time of the crime mentally ill,
and then explain why this sort of mental illness means
that the defendant did not intend to commit a crime.

(04:23):
The jury does not decide whether the defendant is mentally ill.
It simply determines whether or not the defenses expert testimony
has demonstrated this fact, and then decides whether or not
this mental disorder meant that he or she did not
intend to commit the crime. Mental illness alone is no defense.

(04:45):
A person who suffers from deep psychosis will still be
considered guilty if he or she commits a crime intentionally
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