The Insanity Defense in Criminal Law Mental Illness, Legal Standards,
and Criminal Responsibility
Practical Material: The Insanity Defense: Mental Illness and Criminal Responsibility
Introduction
The insanity defense is a legal doctrine that allows a defendant to avoid criminal
responsibility due to a severe mental illness that impaired their ability to understand the
nature or wrongfulness of their actions. This PM explores the elements, tests, and
implications of the insanity defense in modern criminal law.
1. What Is the Insanity Defense?
A legal defense used when a person commits a crime but lacks mental capacity at the time
due to a mental disorder.
It does not claim the crime did not occur, but that the defendant should not be held legally
responsible.
2. Legal Tests for Insanity
There are several standards used across jurisdictions to determine insanity:
a) M'Naghten Rule (Most Common)
A defendant is not guilty if, at the time of the crime, they did not know:
What they were doing, or
That what they were doing was wrong.
b) Irresistible Impulse Test
The defendant knew the act was wrong but was unable to control their behavior due to mental
illness.
c) Durham Rule (Less common)
A person is not criminally responsible if the act was the product of mental illness.
d) Model Penal Code Test
A person is not responsible if, due to mental disease, they lack substantial capacity to:
Appreciate the criminality of their conduct, or
Conform their conduct to the law.
3. Procedure and Burden of Proof
The defense must typically present evidence of the mental disorder.
In most cases, the burden of proof is on the defense to prove insanity by clear and convincing
evidence.
Courts often require psychiatric evaluations and expert testimony.
4. Outcome of a Successful Insanity Defense
The defendant is found “Not Guilty by Reason of Insanity” (NGRI).
This does not result in release; they are usually committed to a mental health facility.
Release is only considered once the individual is no longer a threat to themselves or others.
5. Public Perception vs. Legal Reality
Public often believes the insanity defense is frequent and abused.
In reality:
It is used in less than 1% of cases.
Only a small percentage of those are successful.
6. Famous Case Example
John Hinckley Jr. was found not guilty by reason of insanity after attempting to assassinate
President Ronald Reagan. He was committed to a mental institution for decades.
7. Summary
The insanity defense recognizes that mental illness can impair legal culpability.
Multiple tests are used to assess insanity across jurisdictions.
Successful use does not lead to freedom but to mental health treatment under court
supervision.
8. Practice Questions
What is the purpose of the insanity defense in criminal law?
What is the main principle behind the M'Naghten Rule?
What happens to defendants who are found NGRI?
Why is the insanity defense rarely successful?
What role do psychiatric experts play in these cases?
