Definition – Known as the M’Naghten rules

 Key Notes:  insanityshortnotes.doc insanityshortnotes.doc

Life in a Victorian mental institution - watch the slide show here.

The three key rules the D has to establish in order to successfully argue insanity are on a balance of probabilities:


  1. Defect of reason.
  2. Caused by disease of the mind.
  3. So that the defendant does not know the nature and quality of his act or as not to know that what he was doing was wrong.


M’Naghten (1843). This case involved the attempted assassination of the then Prime Minister, Sir Robert Peel. The defendant missed, but killed Peel’s secretary, Edward Drummond. Medical opinion suggested he was mentally ill and this resulted in the House of Lords setting out rules for use in such cases.


Raising the defence and the definition


Insanity as a defence is relevant only at the time the offence was committed and is defined by common law. Insanity after that is only of relevance if the defendant stands trial or not. Insanity can be used for any offence, which has MR.


All defendants are presumed sane. The D, the P or the judge can raise insanity as a defence at anytime during the trial. If the judge or the P raises the defence then the P must prove the defence beyond reasonable doubt.


The defence has been rarely used since the introduction of Diminished responsibility and the abolition of the death penalty in the late 1950’s.


Special verdict and disposals


If the defence is successfully proved the D is given a special verdict of “not guilty by reason of insanity”. This is because even though the D is technically not guilty of the offence criminal law recognizes that he is still potentially a danger to the public and as a consequence may need special medical care and attention, called a disposal.

Under the Criminal Procedure (Insanity and Unfitness to Plead) Act 1991 the court has a range of options available. For the offence of murder, the court must make a hospital order restricting the defendant’s discharge indefinitely. For any other offence, the court may make one of the following orders:


  1. A hospital order and an order restricting discharge either for a limited or unlimited period of time.
  2. A guardianship order.
  3. A supervision and treatment order.
  4. An order for absolute discharge.






Defect of reason


Definition:  Is based on an inability by the defendant to use powers of reason, rather than his failing to use his powers of reason. This means a complete loss of the power of reasoning, not mere confusion or absentmindedness.


Clarke (1972): the defendant was charged with stealing from a supermarket. She transferred some items from the supermarket’s basket to her own bag and left the shop without paying for these items. She claimed that she must have put the items in her bag in a moment of absent-mindedness and had no intent to steal. She suffered from diabetes, a number of domestic problems and clinical depression, which could cause her to be absent-minded. At her trial the judge said this was a defence of insanity, so she quickly withdrew the defence (in 1972 the only disposal was indefinite detention in a hospital for the criminally insane). Held, a person who is confused or absent minded is not insane and, indeed, may have a defence to a crime through lacking the mens rea.


Caused by disease of the mind


The term ‘disease of the mind’ is a legal term and not a medical term. This is because the law is concerned with whether the defendant should be liable the criminal offence.


It is anything IN the body that affects the mind, hence the creation of a disease INTERNALLY caused. The disease of the mind must be a physical disease, rather than one brought about by external factors such as drugs. It can be a permanent state or a temporary state – hence the fact that where the defence is successful, the defendant remains in the control of the court (for example, at a secure hospital) for an indeterminate period.


Sullivan (1983): D kicked neighbour in the head and body while having epileptic fit. Held, The House of Lords defined disease of the mind as:


‘Any condition which impairs the mental faculties of reason, memory and understanding’


Charlson (1955): D, for no apparent reason, hit his ten-year-old son on the head with a hammer and threw him into a river.  There was evidence that D, an otherwise good father suffered from a cerebral tumour which could have caused the sudden violence. Held a disease of the mind is any mental disorder, which manifests itself in violence and is prone to reoccur. A disease can be intermittent or transient.


Examples include:


Epilepsy:                                Sullivan 1983

Diabetes:                               Quick 1973

Sleepwalking:                        Burgess 1991

Arteriosclerosis:                     Kemp 1957

Paranoid schizophrenia:        Johnson 2007


Any disease of the mind that is not internally caused cannot successfully use the defence of insanity. The courts have classed such diseases as EXTERNAL causes, which are not a disease of the mind but can be used for the related defence of automatism.


Internal causes


Kemp (1957): The defendant suffered from arteriosclerosis, which affected the flow of blood to his brain. This sometimes caused a temporary lack of consciousness. During one such episode he attacked his wife with a hammer and killed her. Held that this was a case of insanity established the fact that a disease of the mind could be temporary and the actual medical condition only needed to affect the brain, rather than be of the brain.


Bratty (1963): D strangled a girl with her stocking.  He claimed that at the time he was suffering from psychomotor epilepsy. Held that any mental disorder, which was demonstrated by violence that was prone to occur was a disease of the mind. This should be seen as an additional pointer rather than a limiting factor on what amounts to a disease of the mind.


Sullivan (1984): The defendant kicked and injured his friendly elderly neighbour during a minor epileptic fit. Held, the fact that this was only temporary did not preclude it from being a defence of insanity.


Burgess (1991): D fell asleep watching tv and then attacked girlfriend. Dr said the sleepwalking caused the attack was as a result of a sleep disorder. Held, as the sleep disorder was affected the brain and was likely to reoccur it was an internal cause and a disease of the mind.




This is a disorder of the pancreas, which results in insufficient insulin being produced. Injections of insulin are needed to control the body’s blood sugar level. If the blood sugar level gets too high or too low then a seizure may result. There are two types of diabetic seizure, which causes the sufferer to become uncontrollably aggressive and violent. The two seizure types are:

Hypoglycaemic (low blood sugar level), caused by taking too much insulin and failing to eat afterwards – classed by the courts as an external cause and not a disease of the mind.


Quick (1957): D, a nurse, assaulted a patient.  He was a diabetic, had taken insulin and not eaten sufficient food.  He drank whisky and rum and he could not remember the assault.  He pleaded automatism. Held, that as his state was caused by external factors taking too much insulin, not eating and drinking alcohol this was not an internal cause of a disease of the mind and not insanity.


Hyperglycaemic (high blood sugar level) caused by failing to take any or enough insulin, which is classed as an internal cause and a disease of the mind.


Hennessy (1989): D took a motorcar without consent whilst disqualified.  He argued that hyperglycaemia was caused by failure to take insulin, which in turn was caused by stress and depression. Held, as his diabetes was an internal cause that affected his mind this was insanity.


So that the defendant does not know the nature and quality of his act

or as not to know that what he was doing was wrong


The expression ‘nature and quality ‘merely refers to the physical quality of the act. This means that the defendant must prove any of the following:


  1. That he did not know what he was doing.
  2. That he did not appreciate the consequences of his act.
  3. That he did not appreciate the circumstances in which he was acting.


In these situations D lacks mens rea but because this is a result of his insanity, the special verdict is given rather than being acquitted.


Where the D is arguing the disease and arising defect of mind mean he didn’t know what he was doing was wrong the courts have held wrong means legally wrong, rather than morally.


Windle (1952) the defendant killed his insane wife. He pleaded his own insanity, but had originally given himself up to the police saying, ‘I suppose I’ll hang for this!’.

Held, As this showed he knew the nature and quality of his act and that what he was doing was wrong, he was convicted of murder, a hanging offence at the time.


Johnson (2007): D suffered paranoid schizophrenia and medical evidence said he knew what he was doing was wrong though 1 doctor said the D didn’t know what he was doing was morally wrong.

Held that Windle was still good law and D did know the nature and quality of his acts.


Byrne (1963) the D would have been unable to successfully plea insanity even though he was suffering from a disease of the mind that forced him to sexually assault and kill women. Because he had “irresistible impulses” to do the acts he still knew what he was doing was legally wrong (Byrne successfully argued Diminished responsibility).



Evaluation of the defence of Insanity & Automatism


Insanity and automatism have been combined because the substantive law on these defences is so intertwined.


There are a number of points to consider about the defence. They are:


  1. The shift of the burden of proof
  2. The definition of insanity the ineffectiveness of the verdict
  3. The scope of the defence.
  4. Based on legal rather than medical rules
  5. The extent of involuntariness required for the defence of Automatism



The shift of the burden of proof


There is a different standard of proof depending on whether the defence or the prosecution raise the issue of insanity. This is likely to confuse a jury, or at least be deemed an irrelevant distinction by many. This also conflicts with the Woolmington v DPP (1935) principle that the burden is always on the prosecution to prove the offence, not to prove a defence.


The definition of insanity


The definition of insanity is said to be ‘medically irrelevant’. The legal definition of insanity has not advanced significantly since 1843; in 1953 evidence was given to the Royal Commission on Capital Punishment that doctors even then regarded the legal definition to be obsolete and misleading.


The distinction between internal and external factors relating to diabetes and hyperglycaemia and hypoglycaemia is particularly arcane. A disease of the mind is a legal rather than a medically defined term, which is not suitable for a modern society where medical discoveries are much more advanced, compared to medical knowledge in the Victorian era, when the rules of insanity were established.


In any event, no one would recognise that a diabetic person was insane purely as a result of their condition! It is also unacceptable that a social stigma of being classed as “insane” is attached to Ds for conditions that are widely accepted by the public as being sane, e.g. diabetic, epileptic, sleepwalking, depression.


The jury will also be confused by technical and conflicting medical evidence as to what is a “disease of the  mind” and its application to the archaic law. 


The defence of insanity is also too narrow and excludes irresistible impulses as the D knows the nature and quality of their act, they just can’t resist killing as in Byrne.


Some research suggests that juries are confused when trying to consider if a D is insane and in highly emotive cases such as Peter Sutcliffe (Yorkshire Ripper) may base their opinions on the emotional impact of a case rather than evidence of a mental disorder, Sutcliffe for example was not allowed to raise the defence of paranoid Schizophrenia by the judge but if had been would the wide spread anger of public opinion have coloured the juries view of whether or not he was insane?


The final element of the defence of insanity does not protect those D’s who clearly have mental problems.  If the D knows that their actions were legally wrong (Windle and Johnson) then regardless of the fact they may have a disease of the mind that causes them to commit an offence (Windle killed his wife due to depression) the D cannot successfully plea insanity. The CA in Johnson has stuck to this legal principle as recently as 2007.


The rules currently do not distinguish between defendants who represent a public danger (the claimed main purpose of the special verdict) and those who do not. Illnesses such as diabetes and epilepsy can be controlled by medication.


The ineffectiveness of the verdict


A finding of insanity can be an indefinite place in a secure hospital, whereas a conviction for murder or grievous bodily harm with intent may well result in a life sentence that does not mean life and be given a tariff of between ten and fifteen years. In these circumstances, most defendants would prefer the conviction and sentence. Given the numbers of prisoners with severe mental problems (if not insanity), it appears that few consider raising the defence of insanity.


Article 5 of the European Convention on Human Rights provides that a person of unsound mind may only be detained where proper account of objective medical expertise has been taken. It is not always the case that a plea of insanity can be said to have done so. It is to be noted that there is likely to be a human rights challenge on all indeterminate sentences.


For automatism, which results in a full acquittal this may mean the courts have to release a D who is potentially a threat to society and in need of medical care, such as in the case of those with post traumatic stress (R v T 1990). This is clearly unsatisfactory from the point of view of one of the aims of criminal law, which is protection of the public.


The defence of Insanity is based on legal rather than a medical rules and the fiction of an internal/external cause


If the court decides it is an external cause and automatism then the result is an acquittal. However if the court decide it is an internal cause then the D can find himself not guilty by reason of insanity and be given a hospital order e.g. Sleepwalking in Burgess compared to R v T and post traumatic stress. Clearly the public would be confused as to why they are not classed as the same defence.


Extent of Involuntariness required for Automatism


A large number of offences are driving related. The courts have insisted that the D can only use the defence where he cannot perform the AR voluntarily. This seems to be a very harsh test as any D who retains even the smallest amount of control faces conviction. In Broome v Perkins 1987 D was charged with driving without due care and attention after he had been observed driving erratically for some miles. He pleaded loss of consciousness. The CA found that he was only intermittently an automaton: although he was not in full control there was evidence that his mind was controlling his limbs enough to allow him to avoid crashing and veering into other traffic or braking violently. 


The Law Commission commentary on the draft criminal code stated “finding it necessary to choose between the authorities (case law), we propose a formula under which we expect that a person in the condition of Broome v Perkins would be acquitted (subject to the question of prior fault)”.




The scope of the defence and suggested reform


The Butler Committee recommended that proof of severe mental disorder should be sufficient to negate responsibility. This would create a presumption of no criminal responsibility where there is proof of a severe mental disorder. However, this assumes a lack of criminal responsibility simply because there is evidence of some sort of mental dysfunction, rather than establishing a standard of criminal responsibility. This may be seen to be an interesting view of fault based liability. This then questions the application of the defence to all crimes rather than those involving mens rea only.


More recently the Law Commission has instigated a project on Unfitness to plea (2011) and the insanity defence. All recent reforms have clearly recognised the weaknesses of the current law with the Law Commission steering a course for a defence based on mental disorders and clear medical evidence rather than the archaic principles of the current defence. Current summary of problems with the law are here.


In terms of automatism the The Law Commission also suggested an expansion of the defence to allow sleepwalking (Burgess) and epileptic seizures, such as Sullivan, to be automatism.


However, the judge can ensure those that commit dangerous offences and end up being found not guilty by reason of insanity can get the medical treatment they need.


For a successful plea of automatism the acquittal could possibly leave a dangerous person to do the same act again, even though they may not intend to do this.


The Law Commission is due to start a project looking into the reform of insanity and automatism defences in 2011 with some of its aims being to establish a more modern framework, closer links to medical opinions on conditions and making sure that the defence does not conflict with the Human Rights Act 1998.


Finally, it can be argued that the defence of insanity as largely become an anachronism of the justice system as it is rarely used with no more than 10 cases per year. It is likely that the government’s response to the Law Commission’s report is going to be one of in action.

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