The insanity defence concerns the defendant’s mental state at the time of the alleged offence. Where it succeeds, the verdict is not guilty by reason of insanity. It is a different question from fitness to plead, which concerns the defendant’s condition at the time of trial.
The M’Naghten Rules
The test comes from the answers given by the judges to the House of Lords in 1843, following the acquittal of Daniel M’Naghten, who had shot and killed the Prime Minister’s secretary under a delusion. Every defendant is presumed sane. To establish insanity, it must be shown that at the time of the act the defendant was labouring under such a defect of reason, from disease of the mind, as not to know the nature and quality of the act they were doing – or, if they did know it, that they did not know it was wrong.
The elements
Defect of reason. The defendant’s powers of reasoning must have been impaired, not merely left unused. Absent-mindedness or confusion in someone whose reasoning is intact is not enough (R v Clarke [1972] 1 All ER 219).
Disease of the mind. This is a legal concept, not a psychiatric one. The courts have treated any internal cause that impairs the functioning of the mind as a disease of the mind, including physical conditions: arteriosclerosis (R v Kemp [1957] 1 QB 399), epilepsy (R v Sullivan [1984] AC 156), hyperglycaemia in diabetes (R v Hennessy [1989] 1 WLR 287) and sleepwalking (R v Burgess [1991] 2 QB 92). An external cause, such as a blow to the head or the effect of a drug, does not qualify; that is the territory of non-insane automatism.
Nature and quality of the act. This refers to the physical character of what the defendant was doing – for example, not appreciating that they were strangling someone.
Not knowing it was wrong. “Wrong” means legally wrong. A defendant who knew the act was against the law is not insane within the Rules, even if they believed it to be morally justified (R v Windle [1952] 2 QB 826, confirmed in R v Johnson [2007] EWCA Crim 1978).
Proof and medical evidence
The burden of proving insanity lies on the defence, on the balance of probabilities. A jury may not return the special verdict except on the written or oral evidence of two or more registered medical practitioners, at least one approved under section 12 of the Mental Health Act 1983 (Criminal Procedure (Insanity and Unfitness to Plead) Act 1991, section 1).
The special verdict and disposal
In the Crown Court the verdict is not guilty by reason of insanity. Under section 5 of the Criminal Procedure (Insanity) Act 1964 the court may then make a hospital order (with or without a restriction order), a supervision order, or an absolute discharge. Where the charge is murder and the conditions for a hospital order are met, the order must be made with restrictions. In the magistrates’ court a successful defence results in acquittal, although the court has power under section 37(3) of the Mental Health Act 1983 to make a hospital order without convicting.
Criticism and reform
The Rules date from 1843 and are widely criticised: they focus on knowledge rather than on a defendant’s capacity to control their actions, they label conditions such as diabetes and epilepsy as diseases of the mind, and the narrow meaning of “wrong” excludes defendants whose delusions led them to believe they were morally justified. The Law Commission examined the area in a 2013 discussion paper, but the law has not changed.
What the psychiatric report addresses
An insanity report has to reconstruct the defendant’s mental state at the material time – often months earlier – from medical records, witness statements, the police interview and the defendant’s own account. It addresses the diagnosis, how the condition affected the defendant’s reasoning at the time, each element of the Rules, the role of any intoxication, and alternative issues such as diminished responsibility or capacity to form intent. Where the defence may succeed, it also addresses the appropriate disposal and whether a hospital bed is available.