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In the Criminal Law, automatism is a defense to liability. Except in the case of strict liability offences, a crime must contain two elements: the actus reus or "guilty act", and the mens rea or "guilty mind". This defense seeks to prove that the criminal defendant made only physical movements and did not "act" as required to prove the actus reus. The term describes movements that are characteristic of an automaton, i.e. a machine that moves.

The criminal defendant is charged because he or she was involved in a situation where consequences prohibited by law occurred. The substance of the defense is that the accused should be excused from liability because these consequences resulted from movements that were not within the defendant's control, such as reflexes, or movements made while sleepwalking. All the other conditions raised as defenses relate to the mens rea element, e.g. the defense of drunkenness claims that the accused could not form the mens rea, a mistake of fact, if of sufficient substance and honestly held, would give the accused a non-criminal set of intentions, etc. Automatism is the only defense that excludes liability by negating the existence of the actus reus which uniquely allows it to be a defense to both conventional and strict liability offences.

For example, Homicidal somnambulism.



In order to find a criminal guilty, the prosecution must be able to show that the accused "committed" an actus reus. The literal translation of this Latin term is "guilty act" although, for the purposes of the law, both actions and omissions can be criminalised. It is therefore necessary to consider what constitutes an action in law. For these purposes, the word "action" is taken to include both acts and omissions.



The simple definition would be a "willed and voluntary muscular movement". But this reference to "muscular" movements does not match everyday experience. If asked what muscles are involved in standing up, only a very few individuals with detailed medical training would be able to give a comprehensive answer. Most people have no idea which individual muscles are being used in achieving a particular result. Hence, a better definition of an action would be "a set of movements that represent an act of will". When people move, they have needs and intentions, and the movements they make are designed to satisfy the needs and fulfil the intentions. To that extent, therefore, the body must mirror the mind to constitute an action. This is not the same test as for mens rea, but simply a two-stage test:

  1. to ensure that nothing is interfering with the ability of the mind to control the way in which the body is moving; and
  2. that the loss of control that occurred was not foreseeable and therefore not preventable.


As to an omission, the test is the same as for an "action" to determine whether, at the material time, the failure to move was under the control of the mind. Hence, if a person suffers an attack of cramp and is therefore unable to move at a critical point in time, this could theoretically fall within the scope of the defense. But, in the real world, there are usually symptoms preceding the attack which should alert the ordinary person to the possibility of an attack. If that person was engaged in a safety-critical activity and knew that control might be lost, the defense will be denied if that person failed to take due precautions.


The essence of the defense requires the court to consider the precise nature of the actus reus as defined by either the common law or statute. The definition will usually be based on a verb. For example, a "driving" offence. The definition of the verb to "drive" requires that the appropriate person has conscious control over the speed and direction of the vehicle. Suppose that, without warning, there is a failure in the steering linkage so that, no matter how the steering wheel is turned, the vehicle will not respond. During the few seconds that it takes the driver to use the brakes to stop the vehicle, he or she is not driving and, if an accident occurred, the defendant should not be liable. However, if the steering linkage had been intermittently weakening and the driver had done nothing to have the system checked by a competent repair person, the defense will be denied because the loss of control was foreseeable. Indeed, continuing to drive knowing of the risk that the steering might fail makes the defendant reckless. Similarly, a person who seeks to raise the defense for drunkenness or any other self-induced loss of conscious control will also be denied because the loss of control will be taken as intentional (i.e. if the defendant continues to drink, loss of control is within the scope of his or her intention). However, there are some inherent conditions that may, without warning, disable a person. For example, some incidents of hypoglycaemia may cause erratic and uncontrolled movements for which the person would not be responsible. But, if the diabetic had failed to eat regularly or consumed alcohol, both of which make hypoglycaemic attacks more likely, the defense would be denied.

In a famous English case, a defendant claimed to have been asleep at the time that a death occurred. The explanation for the circumstances of the death were that, while sleeping, the accused turned over in bed and covered a child sleeping beside him. The child was smothered by the accused's body weight. Because this was not a foreseeable loss of control and entirely involuntary, the defense was allowed. The relationship between automatism and the M'Naghten Rules used to be problematic but, with the Rules slowly slipping into irrelevance in most legal systems, there is now very little risk that an accused invoking automatism might find him or herself confronting a prosecutor advocating the position that the evidence of loss of control means that the accused did not know the nature and quality of his actions and was therefore insane. Nevertheless, some care is required in invoking automatism where the statutory defenses of insanity or diminished responsibility might also be relevant.


Hart, H. L. A. "Punishment and Responsibility: Essays in the Philosophy of Law". (1968) Oxford University Press. ISBN 0-19-825181-5

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