11 Forensic psychiatry11.2 Psychiatry and the criminal justice system
11.2.3 Diminished responsibility, insanity and automatism
3 min read
Written by Resident Doctor Committee · Published
New in the 2027 syllabus
The College added this topic for exams from January 2027.
Separate legal questions
Criminal responsibility is about the person's mental state at the time of the alleged offence. Fitness to plead is about whether they can take part in their trial now.
A psychiatric diagnosis can support the evidence. It does not establish a defence unless the functional and legal criteria are also met.
England and Wales
Defence | Core requirement | Outcome if established |
|---|---|---|
Diminished responsibility | A qualifying abnormality of mental functioning substantially impairs a specified ability and explains the killing | Murder is reduced to manslaughter |
Insanity | A disease of the mind causes a defect of reason that meets the M'Naghten test | Special verdict of not guilty by reason of insanity in the Crown Court |
Non-insane automatism | Totally involuntary conduct arising from an external cause | Acquittal |
Diminished responsibility
Section 2 of the Homicide Act 1957 requires:
An abnormality of mental functioning that arose from a recognised medical condition.
The abnormality substantially impaired the person's ability to understand the nature of their conduct, form a rational judgement or exercise self-control.
It caused, or significantly contributed to, their conduct in the killing.
The defence must prove diminished responsibility on the balance of probabilities (more likely than not). It applies to murder, not to offences in general.
Substantial impairment means impairment of real importance or weight. Something merely more than trivial is not enough.
Acute voluntary intoxication on its own does not establish the defence. A coexisting qualifying condition must still be assessed separately.
Insanity
The M'Naghten test requires a defect of reason caused by a disease of the mind, so that the person either did not understand the nature and quality of the act or did not know it was legally wrong.
Disease of the mind is a legal idea, not another word for psychosis. It can include neurological and metabolic conditions.
Believing the act was morally justified does not meet the wrongness limb if the person knew it was unlawful.
The usual burden is on the defence to prove insanity on the balance of probabilities.
Automatism
Automatism needs a total loss of voluntary control. Anger, reduced inhibition, impaired control or amnesia for the event afterwards are not enough on their own.
The cause decides the route. An internal cause counts as a disease of the mind and leads to insanity. An external cause leads to non-insane automatism. Diabetes shows the difference:
Insulin-related hypoglycaemia (low blood sugar caused by the insulin) can support non-insane automatism.
Untreated diabetic hyperglycaemia (high blood sugar caused by the disease itself) can support insanity when the relevant criteria are met.
Whether the episode was foreseeable, or self-induced, can affect the defence. So the assessment reconstructs treatment, substances, warning symptoms and behaviour around the event.
Non-insane automatism needs a proper evidential foundation before it is put to the jury. The prosecution must then disprove it beyond reasonable doubt.
Jurisdiction and the psychiatrist's role
Establish which jurisdiction applies before applying any legal test. English terms and procedures do not transfer automatically elsewhere.
Scotland has statutory mental-disorder and diminished-responsibility provisions in sections 51A and 51B of the Criminal Procedure (Scotland) Act 1995. There, diminished responsibility reduces murder to culpable homicide.
The psychiatrist explains the diagnosis, its functional effects and the limits of the evidence. The court decides whether the legal defence is established.