Insanity And Diminished Responsibility Defences
These two defences differ significantly in origin, application, legal test, and consequences.
PART A — INSANITY (M’Naghten Rules)
1. Nature of the Defence
Insanity is a complete defence.
If proven, the defendant is found “not guilty by reason of insanity.”
2. Legal Basis – The M’Naghten Rules (1843)
To succeed, the defendant must prove:
Defect of reason
Caused by disease of the mind
So that the defendant did not know:
The nature and quality of the act, OR
That the act was wrong
3. Key Characteristics
Focus is on the mental state at the time of the act.
It is a legal, not medical, test.
“Disease of the mind” refers to a condition affecting the functioning of the brain, not necessarily a psychiatric illness (e.g., epilepsy can qualify).
Burden of proof is on the defendant, on the balance of probabilities.
IMPORTANT CASES ON INSANITY
CASE 1 — M’Naghten (1843)
Principle: Established the legal test for insanity.
Facts:
Daniel M’Naghten, suffering from delusions, believed he was being persecuted. He shot a government official believing him to be a political enemy.
Held:
Because of his mental condition, he did not understand the nature and wrongfulness of his act. This led to formulation of the M’Naghten Rules, still governing insanity today.
CASE 2 — R v Sullivan (1984)
Principle: “Disease of the mind” includes epilepsy.
Facts:
Sullivan kicked a friend during an epileptic seizure.
Held:
Epilepsy was classified as a “disease of the mind” because it impaired mental faculties, even though it was temporary.
CASE 3 — R v Kemp (1957)
Principle: Physical disorders affecting the mind count.
Facts:
Kemp suffered from arteriosclerosis (hardening of arteries) leading to blackouts. During one episode, he attacked his wife.
Held:
A purely physical condition could be a "disease of the mind" if it affected mental functioning.
CASE 4 — R v Hennessy (1989)
Principle: Hyperglycaemia (high blood sugar) = insanity.
Facts:
A diabetic who had not taken insulin was found driving a stolen car while in a state of confusion.
Held:
Since the condition was caused by internal factors (diabetes), it fell under insanity.
CASE 5 — R v Burgess (1991)
Principle: Sleepwalking may amount to insanity.
Facts:
Burgess attacked a woman while sleepwalking.
Held:
Sleepwalking caused by internal brain malfunction is a disease of the mind.
PART B — DIMINISHED RESPONSIBILITY (Homicide Act 1957, as amended)
1. Nature of Defence
This is a partial defence to murder.
If successful, the charge is reduced from murder to manslaughter.
2. Legal Requirements
To use diminished responsibility, the defendant must show:
They were suffering from an abnormality of mental functioning
Arising from a recognised medical condition
Which substantially impaired their ability to:
Understand nature of conduct
Form rational judgment
Exercise self-control
The abnormality must explain the killing.
IMPORTANT CASES ON DIMINISHED RESPONSIBILITY
CASE 6 — R v Byrne (1960)
Principle: Leading definition of “abnormality of mind.”
Facts:
A sexual psychopath strangled a woman and mutilated her body. Medical evidence showed he had uncontrollable violent sexual impulses.
Held:
He suffered from an abnormality of mind that substantially impaired his ability to exercise self-control. Conviction reduced to manslaughter.
CASE 7 — R v Dietschmann (2003)
Principle: Intoxication + mental disorder = DR still possible.
Facts:
Dietschmann killed a man while drunk but also suffered from depression due to bereavement.
Held:
Even though he had been drinking, the mental disorder alone was enough to impair responsibility. DR applied.
CASE 8 — R v Golds (2016)
Principle: Clarified “substantial impairment.”
Facts:
Golds killed his partner. He claimed diminished responsibility due to mental disorder.
Held:
“Substantial” means more than minimal but not necessarily total. Jury must decide seriousness. Clarified the threshold.
CASE 9 — R v Lloyd (1967)
Principle: “Substantial impairment” need not be total.
Facts:
Lloyd strangled his wife. Evidence showed mild mental abnormality.
Held:
Impairment need not be total, but it must be more than trivial or minimal.
CASE 10 — R v Tandy (1989)
Principle: Alcoholism and DR.
Facts:
Tandy, an alcoholic, strangled her daughter. She argued that alcoholism made her incapable of controlling her drinking.
Held:
DR only applies if the drinking was involuntary due to addiction. Since she consumed alcohol voluntarily that day, DR failed.
🌟 DISTINCTION BETWEEN INSANITY AND DIMINISHED RESPONSIBILITY
| Feature | Insanity | Diminished Responsibility |
|---|---|---|
| Type of defence | Complete | Partial (reduces murder to manslaughter) |
| Legal foundation | M’Naghten rules | Homicide Act 1957 |
| Applicable to offence | All offences | Only murder |
| Cause | Disease of the mind (legal) | Recognised medical condition (medical) |
| Outcome | Not guilty by reason of insanity | Conviction reduced to manslaughter |
| Burden | On defendant | On defendant |
| Internal vs external | Must be internal | Can be any medical condition |
✅ CONCLUSION
The insanity defence focuses on legal wrongfulness and complete absence of understanding due to a disease of the mind.
Diminished responsibility focuses on significant impairment, but not removal, of mental capacity and only applies to murder.
You now have 10 detailed cases supporting both defences.
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