How the law determines 'Not guilty by reason of insanity'

How the law determines 'Not guilty by reason of insanity'
Photo credit: Getty.

"Not guilty by reason of insanity" is one of the most recognisable legal phrases in the world, but what does it actually mean and how does it work?

Legal insanity in New Zealand is determined by a judge, based on relevant medical evidence. Broadly speaking, it means the defendant is deemed not mentally capable of being legally responsible for their actions.

Before 2003, insanity was determined at the end of the trial. However, changes to the  Criminal Procedure (Mentally Impaired Persons) Act mean that if the judge, defence and Crown solicitor all agree, an insanity ruling can now be reached without any trial needed.  

Section 23 of the Crimes Act 1963 outlines the insanity defence:

(1) Every one shall be presumed to be sane at the time of doing or omitting any act
until the contrary is proved.
(2) No person shall be convicted of an offence by reason of an act done or omitted
by him when labouring under natural imbecility or disease of the mind to such
an extent as to render him incapable—
(a) Of understanding the nature and quality of the act or omission; or
(b) Of knowing that the act or omission was morally wrong, having regard to the
commonly accepted standards of right and wrong.

The wording of the law is dated by today's standards, and what constitutes a "disease of the mind" or "natural imbecility" is subjective, but they are generally understood to mean mental illness and mental disability respectively.

What exactly determines a mental illness, and whether that illness means the person can’t be held responsible for their actions can be dependant on the judge's interpretation.   

If the person is found guilty, they are usually held at a psychiatric facility until they are no longer thought to be a danger to themselves or others.

One problem with this is that there’s no set time frame for how long someone may be hospitalised for, unlike a prison sentence. This means that someone who commits a crime may spend less time in hospital than they would have in prison if they’d been found guilty.

Conversely, they could feasibly spend the rest of their life in a mental institution if they continue to be considered dangerous.

Another problem is that a defendant who is legally insane is not given a guilty verdict, so they will not have a criminal record, regardless of the act they committed.

Much of the legal wording and general concept behind the insanity defence originated in Britain in 1843, with the trial of Daniel M'naghten. He was found not guilty of murdering a man named Edward Drummond, whom he believed to be the Prime Minister of Britain, because he was deemed insane.

With minor adjustments, the rules established in the M'naghten case are still referenced in common law around the world and remain virtually unchanged in Britain.

Between 2007-2017, 1324 people were charged with homicide in New Zealand. Of those, 21 were ruled not guilty by reason of insanity.

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