Automatism defence poses unique challenge for justice system

Cases where people commit crimes while unconscious pose a unique challenge for the criminal justice system, legal experts say.

Michael Jackson, head of the law department at the University of Hong Kong, said criminal law required that people were acting voluntarily when they did whatever it was that got them into trouble. It was on that premise that the defence of automatism - acting without conscious control or knowledge - was developed.

Mr Jackson said automatism was split into two categories: non-insane automatism, caused by an external factor, such as a blow to the head; and insane automatism, which had its root in some kind of internal trigger.

'The difficulty with sleepwalking is the variety of possible causes,' he said. In the case that ended yesterday, three psychiatrists testified that while they were convinced Zheng Wei-dong was in a state of automatism, they could not be sure what sort it was - sleepwalking or epilepsy or something else - or what caused it.

'Courts have taken the view that when a person suffers a condition that is potentially violent and is seemingly caused by an internal condition, then that person should be classified legally insane.

'The concern of the law in such a case is that people who are potentially dangerous should not be let go. [But] the area is complicated by the fact that sleepwalking has been treated differently in different jurisdictions.'