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Home > Law > Law glossary > Law glossary
M'Naghten Rules
Last modified: Thu Feb 23 16:37:37 2006
The standard rules applied to the determination of whether
a person who offers an InsanityDefence was
insane at the time of the commission of the offence.
The Rules derive from a debate in the
HouseOfLords concerning the outcome of
MNaghtensCase1843. The relevant passage is
``...it must be clearly proved that, at the time of
committing the act, the party accused was labouring under such a
defect of reason, from disease of the mind, as not to know
the nature and quality of the act he was doing, or, if he
did know it, that he did not know he was doing what was
wrong.''
Analyzed in detail, this statement says that the accused
must be suffering from a `disease of the mind', and the
disease gave rise to a `defect of reason', and
he was not aware of the nature of the
act did not know he was doing wrong.
Disease of the mind
A `disease of the
mind' need not be a disease of the brain, or even a
psychiatric condition: arteriosclerosis, epilepsy and
diabetes have all be held to be forms of insanity. The
rationale is that all have an effect on the `mind', and all
result from internal factors. On the contrary, a blow to
the head resulting in mental derangement will probably lead
to acts classifiable as Automatism rather than
insanity. Although there is no logical or medical basis for
this, it may have the practical benefit that `internal'
factors are liable to recur, and therefore require
detention for treatment.
Defect of reason
The disease of the mind must interfere with the process of
reasoning or understanding. The defence does not exist for
a person who simply fails to think about his actions,
perhaps as a result of depression.
Nature of the act
The accused must show that he was unaware of the
nature of the act, not the moral or
legal nature. A person who believes he was not doing wrong
may, however, escape liability under the other limb of
the M'Naghten test (as
described below). A person who kills someone under the
delusion that he is on a battlefield may escape liability
in this way. A person who is so deluded that he stabs
someone thinking he is cutting down a tree may also
escape, but he also lacks the MensRea of
murder, so may not be liable anyway.
Whether the accused knew he was doing `wrong'
If the accused knew that the act was illegal, then he knew
he was `doing wrong' for the purpose of this test. In other
words, legal wrongness, not moral wrongness, is important.
Note that an irresistable impulse to commit a crime cannot
found a defence of insanity, however strong.
CriminalLaw
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