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insanity definition – insanity defined
insanity n. (in criminal law) A defect of reason, arising from mental disease, that is
severe enough to prevent a defendant from knowing what he did (or what he did
was wrong). A person accused of a crime is presumed sane and therefore responsible
for his acts, but he can rebut this presumption and escape a conviction if he can
prove (see BURDEN OF PROOF) that at the time of committing the crime he was insane.
For purposes of this defence, insanity is defined by the McNaghten Rules. These
were formulated by judges after the trial of Daniel McNaghten (1843), who killed the
Prime Minister’s secretary by mistake for the Prime Minister, under the delusion
that the government was persecuting him, and was acquitted on the grounds of
insanity. According to the rules, the defendant must show that he is suffering from
a defect of reason arising out of “a disease of the mind”. This would usually include
most psychoses, paranoia, and schizophrenic diseases, but psychopaths and those
suffering from neuroses or subnormality would not normally fall within the terms
of the rules. The defendant must also show that, as a result of the defect of reason,
he either did not know the “nature and quality” of his acts, i.e. he did not know
what he was doing (for example, if he put a child on a fire, thinking it was a log of
wood) or he did not know that his acts were wrong, even if he knew their nature
and quality (for example, if he knew he was murdering, but did not know that this
was wrong). If the defendant is suffering from an insane delusion, he is treated as
though the delusion was true and will have a defence if there would normally be
one on those facts (for example, if he kills someone under the insane delusion that
he is acting in self-defence, since self-defence is a defence). Medical evidence may be
brought, but the jury are entitled to form their opinion on the facts. If found to be
insane the defendant is given a special verdict of “not guilty by reason of insanity”
and may be admitted to hospital. In cases of homicide, the accused must be sent to
hospital (usually a *special hospital, such as Broadmoor). Because of the consequences
of successfully pleading it, in practice insanity was usually only pleaded to avoid the
death penalty. However, a defendant who puts his mental state in issue (e.g. by
raising a defence of *diminished responsibility on a murder charge) might have to
change his plea to guilty to avoid being treated as pleading insanity (though he is
entitled to appeal against an insanity verdict).
Magistrates’ courts are not empowered to return a special verdict. They will either
grant a complete acquittal, if the defendant’s evidence of mental abnormality
amounts to a denial that he had any necessary *mens rea for the crime, or they may
make a *hospital order, if the crime with which he is charged is one for which they
could usually imprison him.
If someone in custody for trial is suffering from mental illness or severe
subnormality, he may be detained in hospital and not brought to trial until he is fit.
A person who is insane at the time of his trial, in the sense that he does not
understand the charge and cannot properly instruct his lawyers, may be found
*unfit to plead.
See also GENERAL DEFENCES; IRRESISTIBLE IMPULSE.