The defence of insanity
The defence of insanity

The following Corporate Crime guidance note provides comprehensive and up to date legal information covering:

  • The defence of insanity
  • What is the defence of insanity?
  • Does a defendant have an absolute right to a trial on the issue of insanity in the magistrates' court?
  • The defence of insanity in the Crown Court

What is the defence of insanity?

Insanity is a common law defence to a criminal charge. A defence of insanity is available in both the magistrates' court and the Crown Court.

At the heart of the rationale for the insanity defence is the principle that criminal punishment should only be imposed upon those who are responsible for their conduct.

What constituted insanity was debated until resolved by the rule in M'Naghten's Case. In that case, the defendant, Daniel M'Naghten shot and killed the secretary to the politician Sir Robert Peel, intending to kill Sir Robert. He was tried for murder but acquitted on the ground of insanity. High Court judges were subsequently required to answer questions in the House of Lords as to the law governing the defence of insanity. Their answers were reported, as if they constituted judgments and precedent, as the M'Naghten Rules and included the following key principle:

‘[T]o establish a defence on the ground of insanity, it must be clearly proved that, at the time of the committing of the act, the party accused was labouring under such a defect of reason, from disease of the mind, as not to know the nature and quality of the act he was doing; or, if he did know it, that he did not know he was doing what

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