BMJ  2006;333 (2 September), doi:10.1136/bmj.333.7566.0-e

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People with psychoses commit about 5% of the violent crimes in Sweden

Research question What is the impact of serious mental illness on violent crime in Sweden?

Answer People with psychoses commit about 5% of violent crimes.

Why did the authors do the study? We already know that people with a serious mental illness are substantially more likely than others to commit violent crime. But this doesn't tell us much about the absolute risk they pose to the public. These authors wanted answers to two different but related questions: what proportion of violent crimes are these people responsible for, and how much would the crime rate go down if serious mental illness were eliminated or if all patients were locked up indefinitely?

What did they do? They linked data from two high quality Swedish databases using a unique identification number given to all Swedish residents. The first database records a discharge diagnosis for all hospitalisations. The second records all criminal convictions. The authors included in their analysis anyone discharged from hospital with a psychotic mental illness, and anyone convicted of committing a violent crime in the 13 years between 1988 and 2000. The authors defined violent crime as homicide or attempted homicide, aggravated and common assault, robbery, threatening behaviour, harassment, arson, or sex crime.

They calculated the overall rate of violent crime (offences per 1000 people) in the general population and the rate among individuals with no admissions for severe mental illness. The difference between the two was the rate attributable to people with a history of psychotic mental illness. They then did separate analyses for men, women, different age groups, different crimes, and different mental illnesses.

What did they find? During the 13 years of the study there were 45.2 violent crimes per 1000 people in Sweden. Of these 2.4/1000 were accounted for by people with a psychotic mental illness (the population attributable risk), so they committed 5.2% of the violent crime during this period. Breaking the figures down by crime showed that people with a psychotic mental illness were responsible for 18.2% of murders or attempted murders, 15.7% of arson attacks, 6.3% of aggravated assaults, 4.9% of sex offences, 3.6% of robberies, and 3.1% of common assaults. The impact of mental illness on violent crime was substantially greater for men than for women (population attributable risk 4.3/1000 v 0.6/1000) and was slightly lower for schizophrenia than for other psychotic illnesses (1.0 v 1.4)

What does it mean? This data linkage study shows that people with psychoses are responsible for a small proportion of violent crime in Sweden. Eliminating serious mental illness or locking up patients indefinitely would result in about one less violent crime per 1000 residents every five years, assuming there's a straightforward causal link between psychoses and violent crime. The authors admit this assumption could be simplistic. They also note that these results won't translate well to societies, such as the United States, where violent crime is more common than it is in Sweden and where other risk factors, such as gun ownership, are likely to distort the impact of mental illnesses on violent crime, particularly homicide.

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Fazel et al. The population impact of severe mental illness on violent crime. Am J Psychiatry 2006;163: 1397-403[Abstract/Free Full Text]

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