Insanity Defences
1. Understanding the Insanity Defence
The insanity defense is a special defense in criminal law where a defendant claims that, at the time of committing the alleged offence, they lacked the mental capacity to understand the nature or wrongfulness of their actions.
A. Legal Basis
Traditionally, the insanity defense is governed by the M’Naghten Rules (1843):
The defendant must suffer from a “defect of reason” due to a “disease of the mind”.
The disease must cause the defendant to not understand the nature and quality of the act, or to not know that the act was wrong.
B. Key Features
Mental disorder at the time of the crime – not before or after.
Cognitive impairment – inability to understand or control actions.
Burden of proof – generally on the defendant to prove insanity.
2. Landmark Cases
Case 1: R v. M’Naghten (1843)
Facts: Daniel M’Naghten killed the secretary of the British Prime Minister, believing he was being persecuted.
Issue: Could he be held criminally responsible despite his delusions?
Decision: House of Lords formulated the M’Naghten Rules:
The defendant must either not understand what they were doing or not know that it was wrong due to a mental disorder.
Significance: This became the foundation of the insanity defense in common law jurisdictions.
Case 2: R v. Clarke (1972)
Facts: The defendant, suffering from depression, took items from a shop without remembering due to “absent-mindedness.”
Issue: Does mere absent-mindedness or forgetfulness qualify as insanity?
Decision: Court held that temporary confusion or absent-mindedness is insufficient for the insanity defense.
Significance: Emphasized that insanity requires a disease of the mind, not just forgetfulness or distraction.
Case 3: R v. Kemp (1957)
Facts: Defendant suffered from arteriosclerosis causing blackouts and attacked his wife.
Issue: Can a physical disease causing a mental impairment constitute insanity?
Decision: Court ruled that insanity can arise from any internal disorder affecting the mind, even if the disease is physical in origin.
Significance: Broadened the definition of “disease of the mind” to include conditions affecting reasoning and control.
Case 4: R v. Sullivan (1984)
Facts: Defendant had epilepsy and attacked a friend during a seizure.
Issue: Does a seizure caused by a medical condition qualify as insanity?
Decision: Yes, the House of Lords held that temporary mental disturbances caused by internal factors (like epilepsy) fall under the insanity defense.
Significance: Reaffirmed the principle that internal causes, not external ones, trigger the insanity defense.
Case 5: R v. Windle (1952)
Facts: Defendant killed his wife while suffering from mental distress and said, “I suppose they will hang me for this.”
Issue: Did he know the act was wrong?
Decision: Court held that knowledge that the act is legally wrong negates the insanity defense, even if the person is mentally ill.
Significance: Established that cognitive awareness of wrongfulness is key to rejecting insanity.
Case 6: R v. Burgess (1991)
Facts: Defendant attacked a friend while sleepwalking.
Issue: Can sleepwalking be considered insanity?
Decision: Court held that sleepwalking (a non-insane automatism) is not insanity, as it is caused by external factors, not a “disease of the mind.”
Significance: Distinguished insane automatism from non-insane automatism, which can lead to complete acquittal.
Case 7: R v. Hennessy (1989)
Facts: Defendant with diabetes committed theft while in a hypoglycemic state due to failing to take insulin.
Issue: Is the defense of insanity applicable?
Decision: Yes, hypoglycemia caused by internal factors qualifies as insanity, but hyperglycemia caused by external factors (like missing insulin) may be treated differently.
Significance: Further clarified internal vs external causes in insanity defenses.
3. Key Principles from Case Law
M’Naghten Rules are the foundation – defect of reason due to a disease of the mind.
Internal vs External Causes: Internal causes (disease, mental disorder) → insanity; external causes (blow to the head, sleepwalking) → automatism.
Knowledge of Wrongfulness: Even mental disorder does not excuse if the defendant knew the act was legally wrong (Windle).
Not every mental impairment qualifies: Temporary confusion, absent-mindedness, or mere emotional disturbance is insufficient (Clarke).
Extended scope: Physical illnesses affecting cognition (epilepsy, diabetes, arteriosclerosis) can fall under insanity (Kemp, Sullivan, Hennessy).
Summary Table of Cases
| Case | Facts | Principle Established |
|---|---|---|
| R v. M’Naghten | Delusional killing | Foundation of insanity rules |
| R v. Clarke | Absent-minded theft | Temporary confusion ≠ insanity |
| R v. Kemp | Arteriosclerosis → attack | Physical illness causing mental impairment counts |
| R v. Sullivan | Epilepsy → assault | Internal disorder triggers insanity |
| R v. Windle | Knew act was wrong | Knowledge of wrongfulness negates defense |
| R v. Burgess | Sleepwalking attack | External automatism ≠ insanity |
| R v. Hennessy | Diabetic theft | Internal medical condition = insanity |
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