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Criminal Liability: Insanity and Automatism


When should a person not be held criminally liable because of their mental condition at the time they committed an offence? This is the question posed by the defence of ‘insanity’ in criminal law. Similarly, for a reason other than their mental condition, a person might not be held criminally liable because they lacked conscious control of their actions at the time of committing the offence. This might amount to a defence of ‘sane automatism’ under the common law. Sane automatism (referred to as ‘automatism’) and insane automatism (referred to as ‘insanity’) are incapacity defences, which means that their conduct was involuntary and thus should not be criminalized. Automatism is also defined as the performance of actions without conscious thought or intention.


Some key factors to be applicable for the defence of automatism are:

  1. The defendant’s actions must be involuntary.

  2. The defendant must have a complete loss of self-control.

  3. The defendant’s actions must be caused by external factors.

Lord Denning, the judge for the Bratty v Attorney General for Northern Ireland case (1963), defined automatism as either an act which is done by a person who is not conscious of what one is doing (for instance, while suffering from a concussion or while sleepwalking) or an act which is done by the muscles without any control of the mind (like a spasm, a reflex action or a convulsion).

There were a few amendments after this definition, with the most prominent one being the Attorney General's Reference (No. 2 of 1992) which specified that the defence of automatism requires a “total destruction of voluntary control” on the defendant’s part―impaired, reduced, or partial control is not enough.

When referring to an external cause, it means that the defendant’s normal mind was temporarily affected by some external factor. For example, if a person is driving and a swarm of bees enter the car, causing the driver to swerve and hit a pedestrian, the defence of automatism would apply.

The application of the law to diabetics demonstrates the boundary between “internal” and “external” factors (which determines which defence can be used) most starkly. Diabetics may suffer excessively high blood sugar (hyperglycaemia) or excessively low blood sugar (hypoglycaemia), and both states may be caused by “external factors” (too much insulin) or “internal factors” (lack of food or insufficient insulin).

An example of a case that used the defence of automatism successfully is R v Quick (1973). Quick was a diabetic nurse who assaulted a patient after injecting too much prescribed insulin on his doctor’s orders; he had no recollection of the assault and pleaded automatism as the doctor’s orders were the reason for his hypoglycemia and resultant loss of control. Whilst the effects of the insulin were internal, the injection of insulin was an external factor. The appeal court stated that if the hypoglycemia was self-induced through negligence, it would not have been a defence. But since Quick’s loss of control was involuntary and caused by the external factor of the dosage of insulin prescribed by his doctor, the defendant was entitled to have his defence of automatism left to the jury.

A case that was unsuccessful in using the defence of automatism was R v Hennessy (1989). The defendant, also a diabetic, had stolen a car in a hyperglycaemic state. Hennessy had not been taking his insulin as he was in an emotional state as his wife had just left him. He could not remember taking the car and thus raised the defence of automatism. However, the trial judge ruled that the appropriate defence would be insanity as the defendant acted recklessly in self-inducing his state of automatism by not taking his insulin. Furthermore, the defence of automatism was not available because the defendant was able to exercise some level of control by driving the car―albeit not much. Only a total lack of awareness and a completely involuntary act can make the defence of automatism available.

In summary, if you acted recklessly in self-inducing your state of automatism, then you cannot raise the defence of sane automatism.


When a defendant is found to be criminally insane, the jury is directed to give a special verdict of 'not guilty by reason of insanity' under Section 2 of the Trial of Lunatics Act (1883). The defence of insanity is a criminal defence defined by M’Naghten Rules (1843). After the M’Naghten case, the House of Lords compiled rules which now define the defence of insanity.

Some key factors to be applicable for the defence of insanity are:

  1. The defendant must be suffering from a “disease of the mind” (i.e. mental defect or disease) at the time of the crime.

  2. The defendant must not have known the nature or quality of the criminal act that they committed or that the act was wrong because of the mental defect or disease.

There are problems with the defence of insanity which is why this defence is rarely used presently. Defendants do not want to be found ‘not guilty by reason of insanity’ because of the stigma and negative connotations associated with the term ‘insanity’. The label of ‘insane’ is outdated as a description of those with mental illnesses, and simply wrong as regards those who have learning disabilities and/or difficulties, or those with epilepsy or diabetes. Many of these cases instead want to use the defence of sane automatism but that is very often not open to them because the availability of that defence is determined by external causes, not internal.

A case that exemplifies this is the R v Sullivan case (1984). The defendant kicked a man and at the time of the attack, he was suffering from epilepsy. The trial judge ruled that―according to the evidence―the appropriate defence was insanity, not automatism.

When defendants plead 'not guilty by reason of insanity', they are asserting an affirmative defence—they essentially admit that they committed a criminal act, but seek to excuse their behaviour by reason of mental illness that satisfies the legal definition of ‘insanity’. People who are adjudicated to have been insane at the time they committed a crime are neither legally nor morally guilty.


To briefly summarize all aforementioned information, a quick comparison of automatism and insanity is as follows.

Insanity occurs when the condition giving rise to the automatism is intrinsic to the mind of the defendant such as loss of control due to epilepsy. If this defence is successfully used, the defendant is found ‘not guilty by reason of insanity’. However, this is not a full acquittal―the defendant will most likely be given a hospital order (which is mandated for murder).

In automatism, the loss of control is due to some factor external to the mind of the defendant; the defendant has a normal mind that was temporarily affected by some external factor such as being hit across the head with a crowbar and committing a crime whilst having a concussion. If this defence is successfully used, the defendant receives a full acquittal and is no longer under the control of the criminal justice or mental health systems.

Note: This is neither an exhaustive nor a conclusive list. The jury or judge has to consider the relative contributions with reference to burden and standard of proof.


The current law regarding criminal liabilities and specifically insanity and automatism are indeed flawed. The outdated use of the M’Naghten test (1843) and its lack of medical understanding by using broad terms like “diseases of the mind” and stigmatizing labels like “insanity”, causes people in the 21st century with epilepsy, diabetes, and others with conditions that would not be generally described as mental disorders being classified as ‘insane’. As it continues to be used, the M’Naghten test of ‘right and wrong’ represents antiquated and outworn medical and ethical concepts.

Another issue is the distinction between internal and external factors, which is crucial in determining between the defence of insanity and automatism. In “psychological blow” cases where the accused enters into a dissociative state following a traumatic event, a person can commit a crime whilst experiencing Post-Traumatic Stress Disorder (PTSD). The cases are not consistent as to whether the insanity defence or the automatism defence applies. Psychological trauma can be caused by experiencing a traumatizing situation, but also by witnessing something that happened to someone else―the internal/external distinction does not make it easy for courts to methodically reach just results.

The law should protect mentally disordered offenders from arbitrary detention as it protects any other kind of offender. But with the way the insanity defence is currently framed, and the available disposals, mean that it is particularly ill-suited to people with mental health problems and/or learning difficulties. It is important to consider whether the current law has the right test to distinguish between those who should be held criminally liable for their actions and those who should not, depending on their mental condition at the time of committing the crime. While continuing the present emphasis on irrationality as the principal criterion of insanity, amending the current tests to accommodate developing medical knowledge would allow more informed and coherent judgements when determining criminal liability.


Cases Mentioned:

A-G Ref (No. 2 of 1992) [1993] 3 WLR 982

Bratty v AG for Northern Ireland [1963] A.C. 386

R v Quick [1973] Q.B. 910

R v Hennessy [1989] 1 WLR 297

R v Sullivan [1984] A.C. 156

Legislation Mentioned:

Trial of Lunatics Act 1883

Works Cited:

DeFreitas, K., et al (2015). Forensic Psychiatry and Forensic Psychology: Criminal Responsibility, Encyclopedia of Forensic and Legal Medicine (Second Edition), p. 574-578,

ISBN 9780128000557.

Hall, J. (1963). The M'Naghten Rules and Proposed Alternatives, Maurer Faculty of Law.

Lederman, E. (1985). Non-Insane and Insane Automatism: Reducing the Significance of a Problematic Distinction. The International and Comparative Law Quarterly, 34(4), 819-837.

Ormerod, D., et al (2013). Criminal Liability: Insanity and Automatism - A Discussion Paper, The Law Commission.

Padfield, N. (1989). Exploring a Quagmire: Insanity and Automatism, The Cambridge Law Journal, 48(3), 354-357.

Ward, D. (1962). The M'Naghten Rule: A Re-evaluation, Marquette Law Review, Vol. 45, Issue 4.

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