Source: Common Law
Definition: A defence where the individual who’s charged with a crime admits that they committed a crime, but claim
that they aren’t responsible for the crime due to mental illness
The disease of the mind has an INTERNAL CAUSE
McNaughten Rules (1843)
Presumption that the defendant is sane (sensible)
At the material time D was
Labouring under a defect of reason
Caused by a disease of the mind
Not to know the nature or quality of their act, or if they do know the nature or quality they do not know that
it is wrong
Judges who created the rules did not elaborate on their meaning nor define them fully -> case law did this in the
following years
ELEMENT 1: DEFECT OF REASON
The defendant’s powers of reasoning must be impaired
If the defendant is capable of reasoning but has failed to use those powers, then this is not a defect of reason
This was decided in the case of Clark – woman with depression forgot to pay for items including jar of mincemeat
It was held defect of reason must be more than absent mindedness or confusion
ELEMENT 2: DISEASE OF THE MIND
Legal term, not a medical one – must be supported by medical evidence
The disease can be a mental disease or a physical disease which affects the mind e.g R v Kemp (internal factor)
In R v Sullivan the House of Lords was asked to decide whether epilepsy came within the rules of insanity
Broad definition covering organic or functional, permanent or transient or intermittent issues
- Organic insanity is when the brain has been damaged by a physical cause such as epilepsy or a degenerative
disease like Alzheimer’s
- Functional insanity is when there is no organic reason for the damage to the brain
In R v Hennessy, high blood sugar levels of diabetes was classed as insanity because the sugar levels affected his mind
ELEMENT 3
a) Not knowing the nature or quality of the act
The physical nature and consequences, not the moral quality of the act – defendant is delusional
May be due to a state of unconsciousness or impaired unconsciousness, or lack of understanding or awareness due
to a mental condition while conscious
Example case: Codere – woman who though she was cutting bread and was in fact cutting her husband’s neck
b) Knowing the nature and quality, but not knowing it was wrong
Defence will fail if the defendant understood their actions were legally wrong, even if they had a mental illness at the
time of the act
Definition: A defence where the individual who’s charged with a crime admits that they committed a crime, but claim
that they aren’t responsible for the crime due to mental illness
The disease of the mind has an INTERNAL CAUSE
McNaughten Rules (1843)
Presumption that the defendant is sane (sensible)
At the material time D was
Labouring under a defect of reason
Caused by a disease of the mind
Not to know the nature or quality of their act, or if they do know the nature or quality they do not know that
it is wrong
Judges who created the rules did not elaborate on their meaning nor define them fully -> case law did this in the
following years
ELEMENT 1: DEFECT OF REASON
The defendant’s powers of reasoning must be impaired
If the defendant is capable of reasoning but has failed to use those powers, then this is not a defect of reason
This was decided in the case of Clark – woman with depression forgot to pay for items including jar of mincemeat
It was held defect of reason must be more than absent mindedness or confusion
ELEMENT 2: DISEASE OF THE MIND
Legal term, not a medical one – must be supported by medical evidence
The disease can be a mental disease or a physical disease which affects the mind e.g R v Kemp (internal factor)
In R v Sullivan the House of Lords was asked to decide whether epilepsy came within the rules of insanity
Broad definition covering organic or functional, permanent or transient or intermittent issues
- Organic insanity is when the brain has been damaged by a physical cause such as epilepsy or a degenerative
disease like Alzheimer’s
- Functional insanity is when there is no organic reason for the damage to the brain
In R v Hennessy, high blood sugar levels of diabetes was classed as insanity because the sugar levels affected his mind
ELEMENT 3
a) Not knowing the nature or quality of the act
The physical nature and consequences, not the moral quality of the act – defendant is delusional
May be due to a state of unconsciousness or impaired unconsciousness, or lack of understanding or awareness due
to a mental condition while conscious
Example case: Codere – woman who though she was cutting bread and was in fact cutting her husband’s neck
b) Knowing the nature and quality, but not knowing it was wrong
Defence will fail if the defendant understood their actions were legally wrong, even if they had a mental illness at the
time of the act
