The cases of Anders Breivik (who shot 69 people on Utoya Island in Norway) and James Holmes (who shot 12 people in a cinema in Aurora, Colorado) raise again the issue of when a criminal defendant should be able to take advantage of a defence of insanity. As laid down in the M’Naghten Rules (‘MR’, for short), the defence of insanity will be available to a defendant if, at the time he acted, he was ‘labouring under such a defect of reason, from a 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 what he was doing was wrong.’

One criticism that it is often made of the defence of insanity is that it is medically outdated – that our notions of when someone can be said to have been medically insane in acting as he did have developed since 1843, when the MR were laid down, and the law has failed to keep pace with those developments. However, this criticism is unjustified: in my view, the MR are not intended at all to reflect clinical or medical ideas as to when someone should be said to be insane.

Instead, I think the MR are directed at identifying cases where: (1) it was impossible for the defendant to alter his conduct to make it conform to the law (which accounts for the requirement that the defendant must not have known what he was doing or must not have known that what he was doing was wrong), and (2) the defendant was unable to alter his conduct to make it conform to the law because he suffered from an internal condition which means that the defendant is likely to do what he did again, and therefore constitutes a public danger that needs to be dealt with (which accounts for the requirement that the defendant’s incapacity arises out of a ‘disease of the mind’).

On this view, then, the MR are not at all concerned to reflect modern medical notions of when someone can be said to be insane: they arise out of a dual concern to uphold the requirements of the rule of law – which would dictate that someone not be held liable for breaking the law if he was incapable of being guided by its dictates – and to ensure that people who get the treatment they need to deal with conditions that they have that, if unaddressed, will prevent them from complying with the law in the future.

So it really does not matter whether Breivik or Holmes were medically insane when they acted as they did (which they probably were). They were both clearly capable of conforming their conduct to the law, and as such can be properly held accountable for their failures to exercise that capacity. If – having been imprisoned for their crimes – they also require medical treatment to deal with their psychiatric conditions (as Peter Sutcliffe, the Yorkshire Ripper, did, eventually being incarcerated in Broadmoor Hospital under the Mental Health Act 1983) then they should receive it; but their psychiatric states cannot excuse them from being punished for their failures to obey the law.

All of this is not to say that the law on when someone will be found not guilty by reason of insanity is perfect. Four criticisms of the law can be made.

(1) Stigma. There is a concern that defendants who fall within the MR might be put off pleading that they are ‘not guilty by reason of insanity’ because of the stigma attached to being labelled as ‘insane’. This concern gets particularly strong in cases like Sullivan (epilepsy) and Hennessy (diabetes) where what triggered the defendant’s incapacity to comply with the law was an internal condition that no one would ordinarily categorise as ‘insanity’. If the MR are really not concerned with medical notions of insanity, there is a case for saying that we should not use the term ‘insanity’ to refer to the defence to which the MR apply. However, it is difficult to think of what alternative name we could give this alternative defence. ‘Not guilty by reason of internal incapacity’ feels a bit too clinical.

(2) Civil liberties. Another reason why defendants might be unwilling to plead ‘not guilty by reason of insanity’ is that if the plea is successful, then the court is then empowered under s 3 of the Criminal Procedure (Insanity and Unfitness to Plead) Act 1991 to make an order requiring the defendant to be restrained in hospital for an ‘unlimited period of time’. Now – the court does not have to make such an order (though it does in the case where the sentence for the offence which the defendant was charged with is fixed by law (effectively, murder cases)) – it could make a more limited order, or even an absolute discharge. But the possibility that a court might make such an order in their case would make a defendant – particularly one who is charged with a relatively petty offence such as shoplifting – hesitate long and hard before making a plea of ‘not guilty by reason of insanity’ for fear of being trapped in a One Flew Over the Cuckoo’s Nest situation of being trapped in an institution where you are not insane, but are treated as though you are, and are eventually driven crazy by the experience. One way of getting round this problem would be to amend the 1991 Act to require a defendant who has been committed to hospital having pleaded ‘not guilty by reason of insanity’ to appear before a judge at regular intervals so that the judge (preferably the one who presided over the defendant’s trial) can evaluate their treatment and condition and ensure that they are not being treated inappropriately.

(3) Falling between two stools? The defence of ‘not guilty by reason of insanity’ exists alongside the ‘defence’ of non-insane automatism – which is not really a defence, but a denial that the defendant committed the actus reus of the crime because an external cause (for which the defendant was not at fault) meant that the defendant was unaware of what he was doing (the defendant ‘blacked out’) or was aware of what he was doing, but was unable to control his bodily movements (the defendant was acting reflexively). However, there may be one situation which falls between these two defences, because it does not come under either of them. This is the situation where due to an external cause (for which the defendant was not at fault) the defendant thinks that there is nothing wrong – either legally or morally – with acting in a particular way, and he goes on to act in that way, where acting in that way amounts to the actus reus of an offence. For example: D is hit on the head by a stone. When he comes to, he thinks that he is the emperor of the world, and that all women are required to have sex with him, if he so wishes. He proceeds to have sex with V against her will. D won’t be able to rely on a defence of non-insane automatism because he was not acting as an automaton in having sex with V – he knew what he was doing, and was in control of his body. But will he be able to take advantage of a defence of ‘not guilty by reason of insanity’? The difficulty is that his delusions were triggered by an external cause – that of being hit on the head by a rock. Perhaps it could be argued that the external cause triggered a ‘disease of the mind’ and in that way allow him to take advantage of the defence of insanity. But this seems hard to reconcile with cases like Bailey, where the defendant acted under the influence of an insulin injection, and was held not to be able to take advantage of the defence of insanity. (Though it looks like the defendant knew what he was doing in that case – though he might not have been aware of the effects of his actions.)

(4) Strict liability. In DPP v Harper, the Court of Appeal ruled that the defence of insanity did not apply to strict liability offences – in that case, driving with excess alcohol in one’s bloodstream. This seems wrong. If the MR are directed at someone’s being unable to conform their conduct to the law, then there seems no reason why they should not apply as much to strict liability offences as they do to offences which require some degree of mens rea for their commission. A defendant who seeks to take advantage of the MR is not denying that he had the mens rea for the offence with which he is charged: he is making a much more basic point – that he should not be held responsible for his actions because they were either not his actions or they were performed in ignorance of the fact that they might be contrary to the law (and he was not at fault for that ignorance – so the maxim ignorantia legis non excusat does not apply). (Having said that, there are a couple of cases which suggest that a strict liability offence will capture someone who was completely incapable of avoiding committing that offence, and on that basis they refused to find that the offence in question was a strict liability offence. They are Sherras v De Rutzen (refusing to find that the offence of selling alcohol to an on-duty policeman was a strict liability offence as it would be impossible for someone selling alcohol to know whether a policeman was on-duty or off) and Lim Chin Aik v R (refusing to find that the offence of entering Singapore when having been prohibited from doing so was a strict liability offence as the notices prohibiting someone from entering Singapore were only published in Singapore, and so it would be impossible for someone outside Singapore to know whether they were prohibited from entering Singapore). These cases may be confusing absolute liability – where it is no defence to say ‘It was impossible for me to avoid the prohibited conduct’ – and strict liability, where in principle it should be a defence to say that it was impossible to avoid doing the thing which is prohibited.)