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In criminal law, insanity is a defence that arises from a claim that the perpetrator was acting under the influence of an 'internal' derrangement of the mind. An 'external' derrangement may constitute Automatism. Note that insanity has been held to include epilepsy.
Insanity can be raised in a jury trial or a summary trial, although the latter is unusual. Although the procedure for a plea of insanity in a trial on indictment are fixed by statute, the right to plead insanity in a summary trial remains a common law matter, so the procedures are different. In particular, a successful defence of insanity in a magistrates' court results in an Acquital, while in a jury trial there is either a Discharge, or an order for detention in hospital or treatment (as for a claim that the accused is unfit to plead -- see: Insanity and unfitness to plead).
Where insanity is offered as a defence -- rather than as a claim of unfitness to plead -- the relevant question is whether the accused was insane at the time of the offence. It is therefore unlikely that direct medical evidence can be offered. Instead, the court usually decides on the basis of the Mnaghten rules.
Before the abolition of the death penalty, and the introduction of the defence of Diminished responsibility, insanity was often claimed in Murder cases. Although the plea, if accepted, may have resulted in indefinite detention, this was the lesser of two evils. After the introduction of diminished responsibility it became less popular; defendants preferred to risk conviction rather than indefinite detention. Recently, courts have been given the power to discharge defendants who successfully claim insanity, subject to a treatment or supervision order, so it is possible that insanity may become a more useful defence.
In law, insanity is a legal term, not a medical one. There may be no correspondence between what the accused would call insanity and the legal definition.
It is not clear whether a defence of insanity can be raised to any offence. Particularly troublesome is its application to Strict liability offences. If one contends that the defence of insanity asserts lack of Mens Rea, then clearly Strict liability offences will be outside its scope. However, the limb of the M'Naghten Rules that allows the defence if the accused 'did not know he was doing wrong' clearly does not rely on absence of Mens Rea. There is no clear answer to this problem.
Either the defendant or the prosecution may raise the question of the defendant's sanity. Typically the prosecution will do this in response to a plea of automatism. This has sometimes been successful, because the boundary between insanity and automatism is not entirely clear. There is now a right of appeal for the accused who is acquitted on the ground of insanity. Although it may seem strange to appeal against an acquital, many people do not care for the stigma of insanity, even if it does result in an acquital. The introduction of a right to appeal was therefore widely welcomed.