Insanity
The M'Naghten Rules element by element, the special verdict and what follows it.
Psychiatric defences
How mental disorder bears on criminal liability in England and Wales - insanity, automatism and diminished responsibility - and how each differs from fitness to plead.
Each of these asks a different question, at a different point in time, with a different burden of proof and a different outcome. Confusing them is the commonest error in this area.
| Insanity | Non-insane automatism | Diminished responsibility | Fitness to plead | |
|---|---|---|---|---|
| The question | Did a disease of the mind mean the defendant did not know the nature and quality of the act, or that it was wrong? | Was the act involuntary - a total loss of control caused by something external? | Did an abnormality of mental functioning, from a recognised medical condition, substantially impair the defendant's abilities and explain the killing? | Can the defendant take part in their trial? |
| When | At the time of the offence | At the time of the offence | At the time of the offence | At the time of trial |
| Available for | Any offence | Any offence, subject to the rules on self-induced automatism | Murder only | Any trial |
| Burden | Defence, on the balance of probabilities | Defence must raise it with evidence; prosecution must then disprove it beyond reasonable doubt | Defence, on the balance of probabilities | Defence, on the balance of probabilities, where the defence raises it |
| Medical evidence | Required by statute: two registered medical practitioners, one approved under section 12 | No statutory requirement, but essential in practice | No statutory requirement, but essential in practice | Required by statute to find unfitness: two registered medical practitioners, one approved under section 12 |
| Outcome | Not guilty by reason of insanity; hospital order, supervision order or absolute discharge | Acquittal | Conviction for manslaughter rather than murder; sentence at the court's discretion, including a hospital order | A hearing on whether the defendant did the act; hospital order, supervision order or absolute discharge |
Where automatism has an internal cause, it is treated as insanity. Fitness to plead is covered in depth at pritchardcriteria.uk.
What the defences are not
A diagnosis is not a defence
Mental illness does not, by itself, mean a defendant is not guilty. Each defence asks a specific question about how a condition affected the defendant at the time, and many defendants with serious mental illness meet none of them. The psychiatric evidence matters only insofar as it answers the legal question.
Insanity and diminished responsibility are not the same
Insanity is a complete defence to any offence, but its test is narrow: it turns on what the defendant knew. Diminished responsibility is available only for murder, reduces the conviction to manslaughter rather than acquitting, and asks a broader question about the defendant's understanding, judgment and self-control. A defendant may fail the insanity test and still establish diminished responsibility.
Intoxication is not a route in
Voluntary intoxication with alcohol or drugs cannot found a defence of insanity or automatism, and acute intoxication is not a recognised medical condition for the purposes of diminished responsibility. Alcohol dependence syndrome is different, and can found diminished responsibility. Intoxication may still be relevant to whether the defendant formed the intent the offence requires.
These are not questions about now
All three defences concern the defendant's mental state at the time of the offence, which the expert usually has to reconstruct months later from records, witness accounts and interviews. Fitness to plead concerns the defendant's condition at the time of trial. The two can be answered differently in the same case.
The defences in depth
Each defence element by element, with the leading authorities and what the psychiatric evidence has to address.
Automatism
Involuntary acts, insane and non-insane automatism, and the conditions involved.
Diminished responsibility
The partial defence to murder: the 2009 test, intoxication, and sentence.
Instructing an expert
Instructions on insanity, automatism and diminished responsibility are accepted across England and Wales through Forensic Psychiatry Experts. This site is written by Dr Nicholas Taylor, Consultant Forensic Psychiatrist.