R v Hennessy [1989] 1 WLR 287
Facts
Hennessy, who had diabetes, was charged with taking a vehicle without consent. He had not taken insulin for several days and said he was unaware of what he was doing because of hyperglycaemia. He also referred to stress and difficulties in his personal life. The trial judge ruled that the condition relied upon fell within the legal category of disease of the mind rather than external-factor automatism. Hennessy changed his plea and appealed against that ruling, seeking to rely on non-insane automatism instead.
Legal Issue
Was the alleged loss of control caused by an external factor supporting automatism, or by an internal condition to be considered under the insanity rules?
Held
The Court of Appeal dismissed the appeal. The hyperglycaemic condition relied upon arose from the underlying diabetes and the absence of insulin, and was treated as an internal cause for the legal classification. Stress and anxiety did not, on the evidence, convert it into an external cause supporting non-insane automatism.
The ruling concerned which legal framework applied to the alleged impairment. It did not mean that every person with diabetes is legally insane or that a medical diagnosis alone establishes the insanity defence. The required effect on reasoning under the M'Naghten rules still had to be considered. Different facts involving externally administered substances can engage a different analysis.
⭐ Legal Principle
An alleged automatistic state caused by an internal condition may fall under the legal insanity rules rather than non-insane automatism. Hennessy treated hyperglycaemia arising from diabetes and lack of insulin in that way. Classification depends on cause and effect, not a diagnosis alone.
Significance
Hennessy is useful alongside cases involving insulin-induced hypoglycaemia, where an external factor may be central. It illustrates the legal, rather than purely medical, distinction between insanity and automatism. Students should identify the alleged cause of the loss of control and apply the relevant test without labelling all diabetic defendants alike. The case also cautions against treating everyday stress as automatically sufficient to supply an external cause.
Common exam questions about this case
Why was the condition treated as internal?
The alleged impairment arose from diabetes and failure to take insulin, rather than an external substance producing the state. That causal account placed the argument within the legal insanity framework on these facts. The court did not simply infer the defence from the fact that Hennessy had diabetes.
Does Hennessy classify every diabetic defendant as legally insane?
No. The legal question depends on the cause and effect of the particular impairment. Diabetes may feature in different factual situations, including ones involving externally administered insulin. A diagnosis alone does not establish the M'Naghten requirements or determine whether non-insane automatism is available.
Why did stress not supply the external factor asserted?
The court did not regard the stress and anxiety described as the kind of external cause needed to transform this condition into non-insane automatism. Their presence in the background was insufficient. An answer must connect a legally relevant cause to the alleged loss of control rather than rely on a general account of distress.