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Criminal | Defences

Automatism: Criteria

Study Note | A Level

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Automatism is a defence which can be argued in relation to all offences. If a defendant does so successfully he will be found not liable and acquitted.


Automatism has two requirements: that the act is involuntary and is caused by an external factor.

  • Involuntary act

    The defendant must have acted involuntarily. Involuntary is defined as meaning that his mind is not controlling his limbs in a purposeful way.

    Bratty (1963)

    Defendant strangled the female victim in his car. Medical evidence showed he suffered from a psychomotor epileptic seizure at the time of the killing. Defendant claimed that.. a blackness came over him...

    Lord Denning: ..No act is punishable if it is done involuntarily: and an involuntary act in this context… means an act which is done by the muscles without any control by the mind such as a spasm, a reflex action or a convulsion; or an act done by a person who is not conscious of what he is doing such as an act done whilst suffering from concussion or whilst sleepwalking….

    Attorney General’s Reference (No. 2 of 1992)

    Defendant, a heavy goods vehicle driver, killed two victims after driving on to a motorway hard shoulder. Defendant’s defence was driving without awareness, a trance-like state induced by repetitive stimuli experienced on long journeys. The trial judge left the defence of automatism to the jury and the defendant was acquitted.

    The court decided as a matter of law if the defendant is still partly in control that automatism could not be considered a defence.

  • External factor

    Automatism must be caused by an external factor rather than an internal one, such as a disease.

    Quick (1973)

    Defendant, a nurse, assaulted the victim, a paraplegic patient. Victim sustained several injuries including black eyes, broken nose and bruising and medical evidence confirmed these injuries could not be self inflicted. Defendant was a diabetic who had taken his insulin but not eaten enough. Defendant also admitted he had drunk alcohol, although his defence was not intoxication. Defendant was suffering from hypoglycaemia, caused by too much insulin, which leads to mental instability.

    Court of Appeal found that hypoglycaemia did not come within the definition of insanity as it is caused by the drug insulin. Defendant could rely on the defence of automatism and was entitled to be acquitted.

    The general principle is that internal factors lead to insanity and external factors lead to automatism.

    Hennessey (1989)

    Defendant was driving a stolen car while disqualified. Defendant was a diabetic who had not taken his insulin for four days. He argued he was suffering from hyperglycaemia, as he had not taken the insulin, which can lead to unconscious actions.

    The court found that the hyperglycaemia was caused by an internal factor, diabetes and so automatism was not available as a defence. However, insanity was available to the defendant.

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