In criminal law, automatism is a rarely used criminal defence. It is one of the mental condition defences that relate to the mental state of the defendant. Automatism can be seen variously as lack of voluntariness, lack of culpability (unconsciousness) or excuse. Automatism means that the defendant was not aware of their actions when making the particular movements that constituted the illegal act.

For example, in 1859, Esther Griggs threw her child out of a first floor window believing that the house was on fire, while having a sleep terror. In 2002, Peter Buck, lead guitarist of the band R.E.M., was cleared of several charges, including assault, which resulted from automatism brought on by a bad interaction between alcohol and sleeping pills. In a 2009 case in Aberporth in west Wales, Brian Thomas strangled his wife in their camper van, also during a sleep terror, when he mistook his wife for an intruder. The defence of automatism is denying that the person was acting in the sense that the criminal law demands. As such it is really a denial-of-proof – the defendant is asserting that the offence is not made out. The prosecution does not have to disprove the defence as is sometimes erroneously reported; the prosecution has to prove all the elements of the offence including the voluntary act requirement. Automatism is a defence even against strict liability crimes like dangerous driving, where no intent is necessary.

There are several limitations to the defence of automatism in English law. Prior fault generally excludes automatism. Intoxication generally excludes automatism, even when involuntary. Any defence that rests on insanity comes under the M'Naghten rules. Under English law internal causes of automatism are generally judged to be insane automatism and so result in the special verdict (not guilty by reason of insanity) rather than simple acquittal.

Scope Automatism is arguably the only defence that excludes responsibility by negating the existence of the actus reus which uniquely allows it to be a defence to both conventional and strict liability offences (although this argument could be extended to the status defence of insanity, too). Strict automatism is a denial of actus reus and therefore most commonly used as a defence against strict liability offences. There are a number of reasons why a person may go into a state of automatism, including dissociation or hypo or hyperglycemia.. Unconsciousness is the defence of denial of mens rea, which is easier to prove and hence more commonly used for non-strict liability crimes. For example, in cases of homicidal sleepwalking the illegal act is typically not denied but the intent to kill is. The defendant will typically be perplexed and confused and will not cover up the episode. Kenneth Parks, after killing his mother-in-law and severely injuring his father-in-law, drove to the police station stating that he thought he had killed some people. The person's movements seem purposeful - the sleepwalker interacts with their environment in a limited way. Nonetheless the sleepwalker is not conscious of their actions. The use of the term "automatism" for these situations causes some confusion, as in these cases it is really the lack of intent on the part of the defendant which denies the mens rea of the offence rather than the actus reus (although this distinction is problematic in many instances), better called "unconsciousness". Intention is a problem in crimes of strict liability. Very few people intend to crash their vehicles, so clearly something better than intent is required to define automatism.

Another issue with automatism is that when the issue is raised by the defence as a realistic defence (an evidentiary basis), the prosecution then has to prove beyond reasonable doubt that the defendant was acting voluntarily. This is the case for several other defences e.g. duress. The justification for this is that voluntary action is part of the definition of the offence, and therefore something under the presumption of innocence the prosecution has to prove. The evidentiary burden was laid down in Hill v Baxter where the defence of automatism failed because there was no good evidence for the alleged blackout.Hill v Baxter 1 QB 277. Evidentiary burden means that the defendant needs to provide evidence to satisfy the judge that the issue should be put to the jury, which normally requires medical evidence (although R v Woolley,R v Woolley CLY 914 in which an HGV driver crashed after sneezing, proved an exception).

Exclusions Because automatism is such a comprehensive defence, there are various exclusions to an automatism defence. The person must not be at fault. The classic example of this is falling asleep at the wheel of a car (Kay v Butterworth). Although one is not responsible for acts done while asleep, one can be held responsible for driving in a state where one would fall asleep at the wheel. The issue of prior fault applies to many diabetics who suffer hypoglycaemia while driving. Voluntary (and often involuntary) intoxication cannot cause legal automatism.

In many jurisdictions, there is a distinction made between "sane automatism" and "insane automatism". Where the involuntariness is caused by a mental illness, or "disease of the mind", as per the M'Naghten rules, it will be regarded as "insane automatism" and will often result in a special verdict of "not guilty by reason of insanity". This can have significant practical effects for the defendant, as they still may be detained after a special verdict as opposed to the straight acquittal available through sane automatism..

The M'Naghten rules require a "disease of mind", which requires an internal cause. This is medically nonsensical, and does not always bear much relationship to continuing risk which is the main justification. This means that insane automatisms do not require total loss of voluntary control (see below). Thus they are easier to prove in some circumstances, but conversely the burden of proof is on the defendant. Sleepwalking was initially an exception to the internal-external doctrine until the case of R v Burgess.R v Burgess 2 QB 92; 2 WLR 1206

The most contentious qualifier is that there must be a total loss of control. In Attorney-General's Reference No 2 of 1992,Attorney-General's Reference No 2 of 1992 3 WLR 982. this definition of legal automatism was confirmed. A lorry driver had crashed, and his defence (backed up by expert evidence) was that the monotony of motorway driving had caused him to go into a state of "driving without awareness" where, although he could make minor adjustments to follow the road, he was not truly conscious of driving. This followed the decisions of Watmore v JenkinsWatmore v Jenkins 2 QB 572 and Broome v PerkinsBroome v Perkins RTR 321. where diabetic drivers who had driven three miles or more were held to not have the total loss of control necessary for the defence of automatism. This definition is problematic, and the Law Commission, Butler Committee and leading legal academic R. D. Mackay have all argued that this definition is too restrictive.

Classically automatisms in the legal sense have been defined as spasms, reflexes, convulsions or acts committed in a state of unconsciousness e.g. sleep. However, there have been cases where the automatism defence was successful when none of these apply. In R v TR v T Crim LR 256. the defendant had been raped a few days prior to committing a robbery. She was clearly conscious of what she was doing, but in a dissociative state due to post-traumatic stress disorder from being raped. However, in R v Isitt,R v Isitt (1978) 67 Cr App R 44. when the defendant drove away from a collision and evaded a police roadblock in a dissociative state, the defence was not successful. The Ontario Court of Appeal expressed a logical way of distinguishing such cases in Rabey v The Queen.Rabey v The Queen (1981) 79 Dominion Law Reports 435 (Ontario Court of Appeal). There, the defendant went into a dissociative state due to being spurned. It was held that such a commonplace occurrence was not the sort of external stimulus that would cause legal automatism (although the insanity plea would be open to him).

Omissions The problem with omissions and automatism is that the strict legal definition requires total loss of control. A person may well not be able to avoid a crash even though they have some residual control. In this case, the law is imposing liability for failing to do the impossible. Several commentators, including H. L. A. Hart, have suggested that responsibility for omissions must be framed with reference to the actor's capabilities at the time, rather than the objective test.

Voluntariness La Forest J. in the Canadian Supreme Court case of R v Parks,R v Parks (1992) 75 CCC (3d) 287 at 302. asserted that automatism is "conceptually a subset of the voluntariness requirement". One of the main rationales of criminal law is to use the threat of punishment as a deterrent to future wrongdoing. But, if an individual is to be deterred, he or she must be acting under voluntary control. If something is interfering with this control, automatism may be available as an excuse. Duress is not an example of involuntary action as although the choices faced by the person under duress may be difficult, nonetheless they are still acting voluntarily. Some would describe action under duress as non-voluntary as opposed to involuntary. This distinction is emphasized by the exclusion of the defense of duress for murder. In the words of the Queensland Court of Criminal Appeal in R v Milloy,R v Milloy (1991) 54 A Crim R 340 at 342–343. Thomas J says, that for automatism to succeed:

Reflex movements One of the difficulties is defining what a voluntary action is and is not. Words like "willed" have the same difficulty: a voluntary action is one that is willed, whatever that means. In Australia, Ryan v The Queen,. the defendant entered a shop with a loaded rifle for a robbery. In a sudden attack, the shop assistant caught the appellant by surprise, causing him by a reflex action to discharge the gun, killing the assistant instantly. The Crimes Act 1900 (NSW) requires that "murder shall be committed where the act of the accused ... causing the death charged".. Barwick CJ said: "That a crime cannot be committed except by an act or omission is axiomatic. It is basic, in my opinion, that the 'act' of an accused ... must be a 'willed', a voluntary act which has caused the death charged. It is the act which must be willed, though its consequences may not be intended."

Concerning whether the firing of the gun was willed so as to constitute an "act" for the purposes of the murder charge, Elliot commented that "his reaction was like the sudden movement of a tennis player retrieving a difficult shot; not accompanied by conscious planning, but certainly not involuntary". (1968) 41 Australian Law Journal 497. Despite accepting that the actual discharge was involuntary, Barwick CJ confirmed the murder conviction because "the act causing death" included the general circumstances in which the gun was fired. The judge and jury

In the U.S., in People v. Decina (1956) 2 NY2d 13 3, 143 the defendant had epilepsy. While driving his car, he had an epileptic seizure and the car went out of control, killing four people. Decina was convicted of negligent homicide because he had voluntarily driven an automobile without assistance knowing that a seizure was possible, breaching Penal Law 1053 on the negligent operation of a motor vehicle: