Several areas of criminal law turn on a defendant’s mental state, and in each the psychiatric evidence answers a different question. Confusing them is common. The summary below is the starting point; each has its own page.
Four different questions
Fitness to plead asks whether the defendant can take part in their trial now. It is not strictly a defence: it concerns the defendant’s present abilities, and a finding of unfitness leads to a hearing on whether they did the act, not to an acquittal.
Insanity asks whether, at the time of the offence, a disease of the mind meant the defendant did not know what they were doing, or did not know it was wrong. It is a complete defence, leading to the special verdict of not guilty by reason of insanity.
Automatism asks whether the act was involuntary – done without any control by the mind. Where the cause was external, the result is an outright acquittal; where it was internal, the law treats it as insanity.
Diminished responsibility is a partial defence available only to murder. Where an abnormality of mental functioning arising from a recognised medical condition substantially impaired the defendant’s abilities and explains the killing, the conviction is for manslaughter rather than murder.
Related questions
Psychiatric evidence may also bear on whether the defendant formed the intent the offence requires, on the partial defence of loss of control, on effective participation and special measures for a defendant who is fit but vulnerable, and on sentence – including hospital, restriction and hybrid orders under the Mental Health Act 1983. These often arise in the same case, and a single report may need to address more than one.
Timing matters
Fitness to plead concerns the defendant’s condition at the time of trial. Insanity, automatism and diminished responsibility concern their mental state at the time of the offence, which often has to be reconstructed months or years later from records, witness accounts and interviews. The two can be answered differently in the same case: a defendant who was legally insane at the time may now be well enough to stand trial, and the reverse.