Automatism, Insanity and Diminished Responsibility

A defence in criminal law arises when conditions exist to negate specific elements of the crime: the actus reus when actions are involuntary, the mens rea when the defendant is unaware of the significance of their conduct, or both. These defences will mitigate or eliminate liability from a criminal offence. Insanity, automatism and diminished responsibility are examples of said defences. They each share characteristics but can be distinguished in their scope and application.

Insanity, automatism and diminished responsibility all play a significant role in cases where the defendant’s mind is abnormal while committing a crime. The definition of abnormal will be reviewed in relationship to each defence. In order to identify how these three defences compare and contrast, it is first important to understand their definition and application. The appropriate defence will be used once the facts of the cases have been distinguished and they meet the legal tests. The legal test of insanity is set out in M’Naghten’s Case: “to establish a defence…of insanity 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.” To be specific, the defect of reason arises when the defendant is incapable of exercising normal reasoning. The defect of reason requires instability in reasoning rather than a failure to exercise it at a time when exercise of reason is possible. In the case of R v Clarke, the defendant was clinically depressed and in a moment of absent-mindedness, stole items from a supermarket...

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...emains on the prosecution to disprove automatism beyond reasonable doubt.

It is apparent that insanity, automatism and diminished responsibility share similarities and differences in their range of application and in definition. Insanity and automatism are most similar in that they both are full defences (with different outcomes) which exist when a defendant does not have the necessary actus reus or mens rea, whereas diminished responsibility is a partial defence which only applies to murder. The source of the defendant’s mental abnormality is the greatest point of distinction between all of the defences. Whether the abnormality is internal, external or a diagnosed medical condition will play a significant role in which defence can be used. As defences they are all used for a similar reason, and that is to eliminate or reduce liability for criminal offences.