MBE Rules · Criminal Law
Insanity — M'Naghten
M'Naghten
The rule
A defendant is not criminally responsible if, at the time of the act, due to a defect of reason from a disease of the mind, the defendant either (1) did not know the nature and quality of the act, OR (2) did not know that the act was wrong. Most common test (majority).
In plain English
A person isn't guilty if, because of a mental illness, they didn't understand what they were doing or didn't realize it was wrong.
Worked example
The defendant, due to a severe mental disorder, genuinely believed they were watering plants, not setting fire to a neighbor's yard. They may be found not guilty due to insanity.
Memory hook
M'Naghten: Mind Malfunction Means No Malice. If a mental defect blinds nature or wrongness, no criminal liability.
The trap
Students think: Insanity means any mental illness. Wrong, because it requires inability to understand nature or wrongness. The actual test is M'Naghten's cognitive incapacity.
How examiners test it
The MBE loves: defendant with diagnosed mental illness commits a crime. Trap: assuming diagnosis alone suffices. Focus on whether defendant knew nature or wrongness of the act.
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