What is automatism?
Generally, at common law, a voluntary act is required to establish the actus reus of a crime. The defendant should also be conscious of their acts so that mens rea can be established. A defendant may, however, plead automatism, which if successful will result in an acquittal. Automatism as a defence seeks to show that a crime was the result of an involuntary act caused by an external factor.
Automatism distinguished from insanity
If automatism is the result of a disease of the mind then this is properly insanity and a trial judge is entitled to withdraw the defence of automatism from the jury following Bratty v Attorney-General for Northern Ireland . On the other hand, a diabetic who overdoses on insulin causing them to suffer hypoglycaemia will not be suffering a disease of the mind and may be able to plead automatism. (Compare this to R v Hennessy , where a diabetic suffering from hyperglycaemia was judged to have a disease of the mind and consequently was criminally insane. For a better understanding, hypoglycaemia is a state induced by a lower than normal level of blood glucose. Conversely, hyperglycaemia is when there is an excess of glucose in the blood plasma.) The jury will have to decide whether the defence is made out and that the defendant was acting involuntarily due to an external factor (see R v Quick ).
Drunkenness and drugs
Automatism induced by alcohol or drugs is, as a matter of public policy, incapable of being a legal defence. This is generally because there is a high number of crimes committed under the influence of alcohol, and increasingly nowadays drugs, which would otherwise go unpunished. In R v Lipman  the defendant was convicted of the manslaughter of a girl who he had killed whilst under the influence of LSD. In confirming the conviction, the Court of Appeal held that mental states induced by drink or drugs are no defence to a charge of manslaughter, which, in any event, is an offence that does not require specific intent.
Although automatism induced voluntarily through alcohol or drugs may indeed cause the defendant to act involuntarily or be unaware of what he was doing, it cannot normally be a complete defence. However, there may be a partial defence in such circumstances if the accused is charged with an offence that requires a specific intent which the alcohol or drugs negated. Often, however, serious charges which require specific intent are often coupled with lesser charges which do not. So, a murder charge may be reduced to manslaughter if it can be shown that, due to a voluntary intake of alcohol or drugs, the defendant did not have the specific intent to kill or cause grievous bodily harm which is required for murder. As long as the defendant had the basic intent to commit an unlawful act then a guilty verdict on a charge of manslaughter should be reached. In DPP v Majewski  the House of Lords ruled that unless the offence charged was one of specific intent an alcohol or drugs defence could not apply.
The alcohol/drugs defence cannot apply in situations where the defence of mistake is made. In R v O’Grady  the Court of Appeal held that as far as self-defence is concerned it is not possible to rely on a mistake, induced by voluntary intoxication, as to the nature of the attack which required the person to act in self-defence.
Drunkenness or drugs not self-induced
In Ross v HM Advocate  the accused was charged with attempted murder. On the day in question he had drunk a can of lager which, unbeknown to him, had been spiked with several temazepam tablets and some LSD. Shortly afterwards the defendant started attacking various strangers with a knife. His defence was that he had no self-control and so no mens rea. His appeal against his conviction was allowed because his absence of self-control was not self-inflicted. The case can be compared with that of R v Kingston . In this case the defendant was a paedophile who had been drugged by another man who wanted to take photographs of the defendant in a compromising sexual situation with a 15 year old boy for the purposes of blackmail. The defendant pleaded a defence of involuntary intoxication. The House of Lords, however, ruled that the drugs had not sufficiently negated the defendant’s intent, as the defendant still had the intent to commit acts of indecency with the boy even though he was under the influence of drugs.