Question Hayley Law

What is the defence of insanity in criminal law? How is it established?

In criminal law, what is the defence of Insanity and how is it established?

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The defence of insanity operates where an individual has committed a criminal offence, but due to some internal bodily malfunction, cannot be held fully responsible (Child and Ormerod, 2015). The result is a ‘special acquittal’, where there is no criminal liability, but can result in institutionalisation amongst other alternatives (S5 Criminal Procedure Act 1991).

The three-part test for insanity was established in R v M’Naghten. First, the defendant must suffer from a disease of the mind, defined as any disease producing malfunctioning of the mind (R v Kemp), of which examples include Schizophrenia and Hyperglycaemia (Mackay et al, 2006; R v Quick). Secondly, the disease must cause a defect of reason; absent-mindedness is insufficient (R v Clarke).

Finally, this must cause the defendant to not know the nature or quality of the act, which requires that the defendant is unaware of their actions, through unconsciousness or insane delusions, as opposed to inadvertence (Mackay, 2009; R v Sullivan).

Alternatively, this limb is satisfied if the defendant does not know that their act is legally wrong and R v Windle exemplifies this; the defendant’s statement ‘I suppose they will hang me for this’ demonstrated awareness of the illegality of his action, therefore preventing the insanity defence.


Child, J and Ormerod, D., (2015) Smith and Hogan’s Essentials of Criminal Law. Oxford University Press
Mackay, R, Mitchell, B and Howe, L., (2006) Yet More Facts About The Insanity Defence. Crim LR 399
Mackay, R., (2009) ‘Righting the wrong? Some observations on the second limb of the M Naghten Rules. Crim LR, 80
Criminal Procedure (Insanity and Unfitness to Plead) Act 1991
R v Clarke [1972] 1 All ER 219
R v Quick [1973] 3 WLR 26
R v M’Naghten [1843] UKHL J16
R v Kemp [1957] 1 QB 399
R v Sullivan [1983] 2 All ER 673
R v Windle [1952] 2QB 826