By Ben Bowley-Drinnan
The defence of insanity can be found in s 23 of the Crimes Act 1961. In practice, the defence means that the defendant did not have the mental capacity to commit an offence, and hence results in complete exculpation. However, in recent years the insanity defence has been increasing in use, and groups such as the Sensible Sentencing Trust have called for reform on the basis that the defence does not adequately deal with, and in many cases ignores victims’ rights.
The question is whether the insanity defence is working as intended, or is it broken beyond repair?
The current insanity law is generally found within the defence of s 23. However, there is first a general presumption of sanity, in that everyone is presumed sane. In general, criminal law requires both proof of a committed crime (actus reus) and the mental requirement, often termed mens rea. Insanity in particular goes to the mental element of the offence, in contrast to other defences, as it requires a “mental imbecility or disease of the mind” as described in the Crimes Act.  Mental imbecility has been compared to an intellectual disability, and has been described as rarely relied upon, as the general permanence of such a disability means a defendant is often unable to stand trial.
However, the term ‘disease of the mind’ has been noted as an amorphous, imprecise term that “defies precise definition”. However, there have been two general tests that have been provided from overseas jurisdictions that provide some assistance.
Firstly, there is the recurrence test, as proposed by Lord Denning in Attorney-General v Bratty in the United Kingdom: “Any disorder which has manifested itself in violence and is prone to recur is a disease of the mind.” In addition, Lord Denning also emphasised the role of medical evidence in determining insanity.
Secondly, there is the internal/external cause test, as proposed by the Supreme Court of Canada in Rabey v R. In this case, the Court made a distinction between a cause that is internal to the accused, within their emotional/physiological makeup, which would be classed as insanity, as contrasted to one that is caused by an external factor such as a psychological blow, which would be classed as sane automatism.
In addition to the definition of ‘disease of the mind’, any defendant must be rendered incapable of understanding the “nature and quality of the act or omission… or of knowing the act or omission was morally wrong, having regard to the commonly accepted standards.”
Insanity results in a special verdict of ‘Not Guilty by reason of insanity’ (NGRI), in contrast to sane automatism, which results in a complete acquittal.
Problematic areas with the insanity defence
One major issue with the insanity defence as it currently stands relates to the tests used to determine legal insanity. The internal/external cause test has been criticised as resulting in irrational distinctions, particularly in diabetes cases as Lord Justice Hughes stated the distinction can produce “strange results at the margin”.
This can be evidenced by the cases of R v Quick,  where the defendant was acquitted on the basis of an external cause, this being his excessively low blood sugar levels. However, in R v Hennessy, excessively high blood sugar was found to be an internal cause sufficient to allow insanity.
These cases, despite the seeming differences to judges and lawyers, or indeed with people who possess access to expert medical evidence, are likely too subtle for the ordinary person to understand. It is perfectly possible that someone would only see two cases of diabetics, with one found insane and one not guilty, yet it is likely that such a person would not appreciate the subtle differences that gave rise to the verdicts. This has been noted by Professor Andrew Ashworth, who has stated “there can be no sense in classifying hypoglycaemic states as automatism and hyperglycaemic states as insanity, when both states are associated with such a common condition as diabetes”.
The sheer confusion and loss of clarity generated by the distinction between causes of insanity does little to educate the public about why a defendant has been found insane, which could contribute to the negative views regarding the defence by groups like the Sensible Sentencing Trust.
Another issue with the defence is conversely that it is too rigid and is only utilised in rare situations. It has been provided by groups like the Auckland District Law Society that the current insanity rules could mean that mentally impaired offenders are not found insane under the law, but could potentially be impaired enough to cause significant difficulties in prison.
With these issues then, can the insanity defence be said to be functioning as intended, or should the defence be significantly altered in order to bring it into the current societal climate?
Whilst it is easy to claim that the insanity defence is too easy to obtain, as groups have called the defence “problematic” and even “shambolic”, the defendant must always provide enough evidence to leave insanity to the jury or the judge, as per the presumption of sanity in s 23 (1). Indeed, an attempt to lower the burden of proof to only require a ‘reasonable doubt’ of sanity was rejected by the courts, favouring the current balance of probabilities test.
Whilst there is an exception to this, as provided by the Criminal Procedure (Mentally Impaired Persons) Act 2003, this is heavily limited, as it requires agreement from the prosecution that insanity is the only verdict available, and the judge is satisfied. Furthermore, whilst it is possible for critics of the insanity defence to point to the ability of a prospective defendant to ‘shop around’ for a willing psychiatrist to support their insanity defence, it must be remembered that the question of insanity is always for the jury and is never left to competing psychiatrists to decide. Moreover, the insanity defence is always founded on the evidence before the jury, and the jury is prevented from dismissing expert evidence on the basis of their own prejudices or views about the defendant.
Significantly, the New Zealand Law Commission has rejected any changes to the insanity defence, claiming “there is nothing really to indicate that the defence has posed major problems in practice in New Zealand”.
Overall, whilst there are issues with the insanity defence in New Zealand, it is arguable that the law, whilst problematic and slightly rigid in some areas relating to definitions, on the whole functions as intended, to ensure the defence is available to those who require it, whilst proposing a high enough bar to discourage frivolous usage. Indeed, any law change would likely have to be made by Parliament, and would likely have to be done with significant care to avoid prejudicing victims of mental health illnesses, whilst also maintaining a balance with law and order. Such a change would likely be slow, due to the extensive consultation and deliberation that would be required for such a change. Whilst this would likely not be sufficient for groups such as the Sensible Sentencing Trust, would likely ensure the balance is struck with care and decency.
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 Crimes Act 1961, s 23
 Tommy Livingston “Record number of insanity verdicts in New Zealand courts” Stuff (online ed, New Zealand, August 23 2017)
 Graham Moyle “Insanity Acquittals Antiquated” Sensible Sentencing Trust “http://sst.org.nz/insanity-acquittals-antiquated/
 Crimes Act, s 23 (1)
 AP Simester and WJ Brookbanks Principles of Criminal Law (3rd ed, Brookers, Wellington, 2007) at 20
 Section 23 (2)
 Law Commission Mental Impairment Decision Making and the Insanity Defence (NZLCR 120, 2010) at 20
 R v Cottle  NZLR 999 (CA).
 Bratty v Attorney-General  AC 386 at 412 per Lord Denning
 At 414
 Rabey v R  2 SCR 513
 Crimes Act 1961, s 23(2)
 C  EWCA 223 at 
 R v Quick  QB 910 (CA)
 R v Hennessy  1 WLR 287
 A Ashworth and J Horder Principles of Criminal Law (7th ed, Oxford University Press, Oxford, 2013) at 94
 Graham Moyle, above n 3
 Crimes Act, s 23 (1)
 Adams on Criminal Law (online loose-leaf ed) at [CA 23.02]
 Criminal Procedure (Mentally Impaired Persons) Act 2003, s20.
 R v Makahili HC Auckland CRI-2008-092-1873, 4 September 2008.
 Adams on Criminal Law, above n 20, at [CA 23.35]
 New Zealand Law Commission, above n 3 at 29