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Criminal | Defences

Insanity: Criteria

Model Answer | A Level

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Discuss the defence of insanity

Defendant must prove, on the balance of probabilities, he was insane at the time of the offence. Defence of insanity is available for all offences which require mens rea.

If successfully argued a special verdict of not guilty by reason of insanity is given. Judge will then make an appropriate order under the Criminal Procedure (Insanity and Unfitness to Plead) Act 1991.

The M’Naghten rules require three elements to be proved: there must be a defect of reason, as a result of a disease of the mind, which causes the defendant not to know the nature and quality of his act or not to know what he was doing was wrong.

Defect of reason

Defect of reasoning means the defendant’s powers of reasoning must be impaired, based on an inability to use powers of reasoning not merely a failure to do so.

Temporary confusion or absent mindedness does not amount to a defect of reason (Clarke (1972)).

Disease of the mind

The term disease of the mind is a legal not a medical one. The law is concerned whether the defendant can be held liable for his act, not his medical condition. A disease of mind must be a physical one, not brought on by external factors, eg drugs. No express definition, case law has developed meaning.

Temporary insanity may be sufficient (Kemp (1957)).

Source of the disease is irrelevant (Sullivan (1984)).

Reoccurring violent tendencies could amount to insanity (Bratty (1963)).

Sleep disorders may amount to insanity (Burgess (1991)).

Defendant’s knowledge

Defendant’s disease of mind must cause him not to know the nature and quality of his act or not to know what he was doing was wrong .

Nature and quality

The term nature and quality refers to physical character of the act.

Defendant can argue he did not know nature and quality of his act if he can prove that he did not know what he was doing, or appreciate the consequences or circumstances in which he was acting.

If defendant can prove he did not know nature and quality of his act, he lacks sufficient mens rea.


Defendant may know nature and quality of his act, but not realise it is wrong. So may have the necessary mens rea, but because of insanity may not realise it was wrong. In these cases defendant can use the defence of insanity.

If defendant knew it was unlawful to kill then defence is unavailable (Codere (1916)). It has been found that the word wrong means contrary to the law (Windle (1952)).

The difference between legally and morally wrong is not recognised. In Johnson (2007) , the Court of Appeal noted an Australian decision which found a defendant could be acquitted even if he knew his act was legally wrong, but believed his act to be morally right. Stating that it highlighted the .. difficulties and internal inconsistencies which can arise from the application of the M’Naghten rules if the decision in Windle is correct.... However the court was bound to follow Windle (1952).

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