The common law defence of insanity was set out definitively in the English case R v M’Naghten in 1843. The accused was labouring under the belief that he was being persecuted by the Tory party and hence had to kill the British Prime Minister.At his trial, the judges set down what became known as the M‟Naghten Rules.
Firstly, it must be clearly shown that, at the time of committing the act, the defendant was labouring under a defect of reason caused by a disease of the mind and, secondly, that the defect of reason must mean that either the defendant was not aware of what he was doing or he was not aware that what he was doing was wrong.
The first question proposed by your Lordships is this: ” What is the law respecting alleged crimes committed by persons afflicted with insane delusion in respect of one or more particular subjects or persons: as, for instance, where at the time of the commission of the alleged crime the accused knew he was acting contrary to law, but did the act complained of with a view, under the influence of insane delusion, of redressing or revenging some supposed grievance or injury, or of producing some supposed public benefit?”
In answer to which question, assuming that your Lordships’ inquiries are confined to those persons who, labour under such partial delusions only, and are not in other respects insane, we are of opinion that, notwithstanding the party accused did the act complained of with a view, under the influence of insane delusion, of redressing or revenging some supposed grievance or injury, or of producing some public benefit, he is nevertheless punishable according to the nature of the crime committed, if he knew at the time of committing such crime that he was acting contrary to law; by which expression we understand your Lordships to mean the law of the land.
Your Lordships are pleased to. inquire of us, secondly, ” What are the proper questions to be submitted to the jury, where’ a person alleged to be afflicted with insane delusion respecting one or more particular subjects or persons, is charged with the commission of a crime (murder, for example), and insanity is set up as a defence? ” And, thirdly, “In what terms ought the question to be left to the jury as to the prisoner’s state of mind at the time when  the act was committed ”
And as these two questions appear to us to be more conveniently answered together, we have to submit our opinion to be, that the jurors ought to be told in all cases that every man is to be presumed to be sane, and to possess a sufficient degree of reason to be responsible for his crimes,until the contrary be proved to their satisfaction; and that to establish a defence on the ground of insanity, it must be clearly proved that, at the time of the committing of 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.
The mode of putting the latter part of the question to the jury on these occasions has generally been, whether the accused at the time of doing the act knew the difference between, right and wrong: which mode, though rarely; if ever, leading to any mistake with the jury, is not, as we conceive, so accurate when put. generally and in the abstract, as when put with reference to the party’s knowledge of right and wrong in respect to the very act with which he is charged.
If the question were to be put as to the knowledge of the accused solely and exclusively with reference to the law of the land, it might tend to confound the jury, by inducing them to believe that an actual knowledge of the law of the land was essential in order to lead to a conviction; whereas the law is administered upon the principle that every one must be taken conclusively to know it, without proof that he does know it. If the accused was conscious that the act was one which he ought not to do, and if that act was at the same time contrary to the law of the land, he is punishable; and the usual course therefore has been to leave the question to the jury, whether the party accused had a sufficient degree of reason to know that he was doing an act that was wrong: and this course we think is correct, accompanied with such observations and explanations as the circumstances of each particular case may require.
The fourth question which your Lordships have proposed to us is this:-“If a person under an insane delusion as to existing facts, commits an offence in consequence thereof, is he thereby excused?” To which question the answer must of course depend on the nature of the delusion: but, making the same assumption as we did before, namely, that he labours under such partial delusion only, and is not in other respects insane, we think he must be considered in the same situation as to responsibility as if the facts with respect to which the delusion exists were real. For example, if under the influence of his delusion he supposes another man to be in the act of attempting to take away his life, and he kills that man, as he supposes, in self-defence, he would be exempt from punishment. If his delusion was that the deceased had inflicted a serious injury to his character and fortune, and he killed him in revenge for such supposed injury, he would be liable to punishment.
The question lastly proposed by your Lordships is:-” Can a medical man conversant with the disease of insanity, who never saw the prisoner previously to the trial, but who was present during the whole trial and the examination of all the witnesses, be asked his opinion as to the state of the prisoner’s mind at the time of the commission of the alleged crime, or his opinion whether the prisoner was conscious at the time of doing the act that he was acting contrary to. law, or whether he was labouring under any and what delusion at the time?” In answer thereto, we state to your Lordships, that we think the medical man, under the circumstances supposed, cannot in strictness be asked his opinion in the terms above stated, because each of those questions involves the determination of the truth of the facts deposed to, which it is for the jury to decide, and the questions are not mere questions upon a matter of science, in which case such. evidence is admissible. But where the facts are admitted or not disputed, and the question becomes substantially one of science only, it may be convenient to allow the question to be put in that general form, though the same cannot be insisted on as a matter of right.
In R. v. Windle the Court of Criminal Appeal in England held that the rules are not limited to cases in which the accused is suffering from delusions but apply in all cases of insanity, whatever may be the nature of the insanity or disease of the mind from which the accused is suffering. The Court of Criminal Appeal in Ireland in Attorney General v. O’Brien took a different view “It is to be noted that all the questions are framed in relation to crimes committed by ‘persons afflicted with insane delusion, in respect of one or more particular subjects or persons.’ The answers to the first and fourth questions are definitely limited by this qualification of the questions. It is in the answer to the second and third questions that the statement is contained to which it has been commonly sought to give a wide general application (though the questions were limited in the same way).”
“As I have mentioned, the questions submitted to the Judges for opinion were in express terms limited to crimes committed by ‘persons afflicted with insane delusion in respect of one or more particular subjects or persons.’ It follows, in our opinion, that the opinions given by the Judges must in every case be read with the like specific limitation. Nevertheless, the opinions which I have quoted from the answer to the second and third questions have been commonly read as applying to the whole field of insanity, which is, of course, of far wider area, and comprises a more extensive and varied range of cases of mental disease than those which can be conveniently summed up as affliction with insane delusion. Hence the dissatisfaction expressed by many legal and medical persons with the opinions as so read with the wide and general interpretation wrongly given to them. The scientific exploration of mental diseases has in modern times been pursued with results to knowledge not contemplated at a time not very remote from the present.”
The questions and answers were also directed toknowledge, and this matter was considered by Mr. Justice Henchy in The People (Attorney General) v. Hayes (Central Criminal Court30th November, 1967) which trial was noted in an article entitled “Not Guilty Because of Insanity” by Professor R. J. O’Hanlon: see Irish Jurist (N.S.), Vol. III, p. 61. That article was of considerable assistance to me in preparing this judgment. In that case the accused was charged with the murder of his wife. Submissions were made by counsel on behalf of the Attorney General as to the form in which the issue of insanity should be left to the jury, and in the course of his considered judgment in relation to these submissions
Henchy J. said: “In the normal case, tried in accordance with the McNaghten rules, the test is solely one of knowledge: did he know the nature and quality of his act or did he know that the act was wrong? The rules do not take into account the capacity of a man on the basis of his knowledge to act or to refrain from acting, and I believe it to be correct psychiatric science to accept that certain serious mental diseases, such as paranoia or schizophrenia, in certain cases enable a man to understand the morality or immorality of his act or the legality or illegality of it, or the nature and quality of it, but nevertheless prevent him from exercising a free volition as to whether he should or should not do that act. In the present case the medical witnesses are unanimous in saying that the accused man was, in medical terms, insane at the time of the act. However, legal insanity does not necessarily coincide with what medical men would call insanity, but if it is open to the jury to say, as say they must, on the evidence, that this man understood the nature and quality of his act, and understood its wrongfulness, morally and legally, but that nevertheless he was debarred from refraining from assaulting his wife fatally because of a defect of reason, due to his mental illness, it seems to me that it would be unjust, in the circumstances of this case, not to allow the jury to consider the case on those grounds.”
The concept of „disease of the mind‟ has been considered in a number of cases. In R v Kemp the defendant argued that he suffered from arteriosclerosis which had, on the occasion in question, caused a lack of blood to the brain, in turn causing a lack of consciousness so that he had no control over his actions. The prosecution argued that this was not a disease of the mind as there was no evidence of brain damage and in fact it was a physical condition. This was rejected by the court, however, which held that the mental faculties of reason, memory and understanding are engaged by the term „disease of the mind‟ and hence this physical condition which affected these faculties was in fact a disease of the mind. Therefore, any physical or mental condition that impacted on the working of the defendant‟s mind at the time the act was committed could be classified as a disease of the mind.
In Bratty v Attorney General for Northern Ireland epilepsy was heldto be a disease of the mind, the accused claiming that he had no knowledge of vents due to experiencing a blackout. Similarly in R v Sullivan22, a case also concerning a person having an epileptic fit, it was held that the effect on the relevant faculties can be of a temporary nature. In R v Burgess sleepwalking was held to be a disease of the mind.
This wide definition of insanity means that epilepsy and conditions caused by diabetes can be classified as forms of insanity. Diabetics however can also make a person an automaton. The crucial distinction depends on whether the impairment of mental facilities was caused by an „external factor‟ or an „internal factor‟. In R v Quick, the accused suffered from hypoglycaemia, which is a deficiency in blood sugar levels. In order to maintain the appropriate level of blood sugar he should have taken a certain amount of insulin. In the event he took too much, which meant the blood level was too low. As a result, the assault with which he was charged occurred while he was suffering from an external factor (the injection of insulin) and so the appropriate defence was
automatism. By contrast, in R v Hennessy, a case that also involved diabetes, The defendant suffered from hyperglycaemia (when the blood sugar level is too high). Here, it was held that the offence occurred while the accused suffered from a purely internal factor, and hence the appropriate defence was insanity.
The defence of diminished responsibility was introduced by the Criminal Law (Insanity) Act 2006. This defence is a partial defence to murder, reducing the verdict of murder to manslaughter. Section 6 of the 2006 Act provides that a verdict of guilty of manslaughter on the ground of diminished responsibility shall be returned where the jury find that the accused:
did the act alleged; was at the time suffering from a mental disorder; and the mental disorder was not such as to justify finding him or her not guilty by reason of insanity, but was such as to diminish substantially his or her responsibility for the act.
Automatism occurs where a defendant suffers a complete loss of self-control caused by an external factor such as being hit on the head and then losing all awareness of their actions. Essentially automatism involves more than a claim that the individual lacked mens rea (which he or she did); it involves a claim that he or she is not acting – it is a complete denial of the actus reus.
Therefore, in order for a defendant to plead automatism it is necessary to show that they suffered a complete loss of voluntary control, that this loss of self-control was caused by an external factor and finally that they were not at fault in losing capacity. With regard to the first requirement, some commentators argue that complete loss of self-control appears to be very harsh. It would deny a defence to a person who had a vague awareness of what was happening.
The requirement of loss of self control being caused by an external factor is an important aspect and ultimately distinguishes inanity from automatism. If the loss of self-control is caused by an internal factor the person is classified as insane. It can be extremely difficult at times to distinguish between internal and external factors. Examples of external factors include a blow to the head or the taking of prescribed medication.
A defendant cannot plead automatism if he or she is responsible for causing the condition. For example, if the defendant‟s mental state is caused by taking alcohol or an illegal drug he or she cannot plead automatism. Similarly, if the defendant is a diabetic and is aware that if he or she does not eat an adequate amount of food he or she may enter a state of lack of awareness, and may still be held responsible for their actions