H418/01 - Insanity and automatism

Legal insanity and the starting point

Insanity concerns the defendant’s condition when the alleged offence occurred, rather than whether they can now participate in a trial. The latter question is fitness to plead. A medical diagnosis alone does not settle either legal question.

Insanity is a legal defence based on a defect of reason caused by a disease of the mind, producing the particular lack of knowledge required by the M’Naghten rules. The name is historical; it does not mean that everyone with a mental illness satisfies the defence or that a defendant with epilepsy has a psychiatric illness.

The starting point is that everyone is presumed sane and responsible. When the defendant relies on insanity, they must establish it on the balance of probabilities: more likely than not. The prosecution must still prove that the defendant did the alleged act or omission to the criminal standard. Failure to prove the alleged act calls for an ordinary acquittal, rather than a special insanity verdict.

In the Crown Court, a special verdict requires supporting evidence from at least two registered medical practitioners, one appropriately approved. Experts explain the medical evidence; the legal test is applied by the court. For example, “I have depression” neither proves nor disproves insanity. The important questions concern its effect on reasoning and knowledge at the relevant time.

Disease of the mind and defect of reason

A disease of the mind is a legal category covering an internal condition that affects the faculties of reason, memory and understanding; it need not be permanent or a medically classified mental illness.

In R v Sullivan (1984), epilepsy causing an attack was a disease of the mind for this purpose. In R v Hennessy (1989), a diabetic defendant’s high blood sugar arose from the underlying condition after failure to take insulin. That internal cause belonged to the insanity route. These decisions classify the cause; a defendant must still meet the remaining M’Naghten requirements.

A defect of reason means an inability to use the relevant powers of reasoning, rather than ordinary absent-mindedness or a decision not to think. In R v Clarke (1972), a depressed shopper took goods while absent-minded. The court distinguished a failure to use reasoning from an inability to reason. That mattered because she could dispute dishonest intention without having to prove insanity.

Consider two original examples. A person distracted by a phone call leaves a shop without paying: inattention alone is not a defect of reason. Another person, during an internally caused episode supported by medical evidence, cannot understand what the objects or actions are: that may satisfy the first two elements. The evidence must connect the condition, loss of reasoning and alleged act; simply naming a condition skips the causal link.

The two knowledge limbs

The M’Naghten rules require the defect of reason from disease of the mind to leave the defendant without the relevant knowledge in either of two ways.

First, the defendant may not know the nature and quality of the act: its physical character and consequences. In an original example, a delusion makes a person believe they are cutting fabric when they are cutting another person’s arm. This is different from knowing that an arm is being cut but believing it is justified.

Second, a defendant who understands the physical act may not know that it is wrong. The traditional authority R v Windle (1952) treated knowledge that killing was contrary to law as defeating this limb; the defendant’s reference to being hanged indicated such knowledge. Personal approval of an act does not establish the defence.

For an accurate current account, R v Keal (2022) clarifies that this limb requires the defendant not to know that the act is unlawful and not to know that it is wrong by the standards of ordinary reasonable people. Keal’s psychotic beliefs did not create a separate defence that he knew the acts were wrong but felt he had no choice. An irresistible impulse or command hallucination does not by itself replace the knowledge test.

Apply the alternatives carefully. An original defendant who knows they are striking a person and knows it is unlawful cannot succeed merely by saying a voice ordered it. A defendant who cannot understand the physical act may rely on the first limb even though the second is disputed. Insanity is not simply “no mens rea”: a person may intend a physical movement yet meet the separate defence because of its nature or wrongness as understood during the episode.