Considering the acres of newsprint devoted daily to the exploits of cold-blooded killers, our ignorance of their underlying personality disorder is rather shocking. More than a quarter of the inmates of many high-security prisons can be classified as psychopaths — individuals with an emotional deficit that renders them fearless and lacking in empathy. Many psychiatrists now accept that the underlying condition is biological, and can even be viewed as a 'disease'.

Unfortunately, our understanding of the biological basis of psychopathy remains rudimentary. There has been but a smattering of research into its physiological correlates, and attempts to apply the techniques of neuroimaging to study the brains of psychopaths have so far generated as much heat as light (see page 296). But, at present, we do not even know the incidence of psychopathy in the general population. Could it be true, as some psychopathy experts have only half-jokingly hypothesized, that the traits of psychopathy are actually an advantage in some careers, such as politics?

A comprehensive programme of research — ranging from neuropharmacology, through brain imaging, to traditional psychiatric and behavioural approaches — could help to identify what makes criminal psychopaths notorious recidivists, and how best to treat them.

Such arguments lack popular appeal. The public seems more concerned about punishing violent criminals than rehabilitating them. The idea of sending such people for treatment in secure mental hospitals, rather than to prison, is widely disparaged as a 'soft' option.

We can argue about whether criminal psychopaths are mad, or bad. But there is no doubt that they can be extremely dangerous to know. And the current system, in which violent criminals may be released without having addressed the personality disorder that predisposed them to offend in the first place, serves no one well. There must be a better way, and research into the biological roots of psychopathy may lead us there.