The number of murders committed by people with a mental illness has been falling steadily since the late 1970s according to an analysis of official Home Office figures. Matthew Large and his colleagues who made the observation in the the British Journal of Psychiatry said they believe the trend is likely due to improvements in treatment.

The researchers identified overall homicide rates in the UK between 1946 to 2004 and compared these with rates of homicide with a verdict of diminished responsibility, not guilty by reason of insanity, unfit to plead and infanticide.

Rates of homicide due to mental disorder rose between 1957 and 1979, as did homicides in general. However, since 1979 homicides due to mental disorder have fallen consistently while other homicides have continued to rise.

The fall has occurred despite the fact that there have been no changes to the official definitions of the defences to murder since the mid-1950s. Moreover, one would expect the improved detection of mental illness in more modern times to have inflated the rates of murder attributed to mental illness, not reduced them.

'The introduction and increasing use of antipsychotic medication, the greater awareness of the treatment of psychosis by primary care providers after deinstitutionalisation, and the creation of regional health authorities with responsibility for defined populations, may all have contributed to the observed decline in abnormal homicide since the 1970s,' the researchers wrote.

Dr Large told The Psychologist that he was surprised by how little media coverage his paper had received. 'The British Journal of Psychiatry issued a press release and it was covered in the BMJ, New Scientist and BBC Online - but no print news, ' he said. 'And yet I was in the UK recently for a month and murders by the mentally ill came up quite often. It seems to me this is proof positive that the press are more interested in one murder than the lack of 50-70 per year. I believe the undue attention on very rare events can drive bad treatment and policy and is a serious issue.'