Establishing the Defence of Automatism
Automatism is a complete defence with three elements:
- The defendant’s actions were completely involuntary;
- The involuntary action was the result of an external trigger or cause;
- The defendant did not induce their own automatism.
The action must be entirely involuntary, such as a reflex action or a sneeze. If the defendant retains any control over their actions the defence will fail: Broome v Perkins  Crim LR 271.
If the trigger for the automatism was internal, then insanity is the correct defence instead. Non-insane bodily reactions like cramps and sneezes are classified as external for these purposes: R v Woolley  CLY 914.
The automatism cannot have been induced by the defendant’s actions, such as voluntary drinking: R v Lipman  1 QB 152. There appears to be an exception to this where the automatism was not a reasonably foreseeable result of taking proper or prescribed medicine: R v Bailey  3 All ER 503.
Diabetic Automatism: Internal or External?
Diabetes and similar conditions can give rise to two states that may result in involuntary action: hyperglycemia (too much sugar) and hypoglycemia (too little sugar due to an insulin overdose). Hyperglycemia is considered an internal source, while hypoglycemia is an external source: R v Hennessy  1 WLR 287; R v Quick  3 WLR 26.