The defence of Insanity


The defence of insanity:

  • This is a special defence - the D has to prove that they come within the legal rules of insanity.
  • The defendant must prove on the balance of probabilities he was insane at the time of the offence. If the prosecution wishes to raise the issue of the defendant's insanity it must be proved beyond reasonably doubt.
  • This defence does not acquitt the defendant, instead the verdict is 'Not guilty by reason of insanity'.

For the defence of insanity to be established, the D must prove that at the time of committing the act-

'he 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, he did not know what he was doing was wrong' 

The M'Naghten(1843) rules:

These are a test for the legal sanity of the defendant in a criminal case and are still used in cases today.  The 3 elements are;

  • A defect of reason - This means that the defendant's power of reasoning must be impaired. If the defendant is capable of reasoning but has failed to use those powers, then this is not a defect of reason. In Clarke (1972), it was decided that defect of reason must be more…


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