This study used merged Swedish nationwide longitudinal registers, including data on parents and criminal convictions, over a span of 32 years, to describe the distribution of violent crime in the total population and personal risk factors for individual persistence in violence among the offenders. Since these levels of violent crime and their judicial resolution (case clearance) are similar to those in most countries in the western EU and in Canada , our results for prevalence and distribution could be generalizable within this cultural sphere, while the identified risk factors for persistence in especially violence-prone individuals are probably more widely generalizable across cultures. Socioeconomic factors would most certainly have proved to be important had they been included in the analyses, but they were excluded in this study in favor of individual risk factors to focus on the relation between potentially treatable or predictive mental health factors and persistence in violence. Further investigations of the relationship between socioeconomic factors and the individual risk factors considered here are clearly warranted. Separate studies of individual risk factors and of class or socioeconomic factors will have to be made side by side and interpreted for the different purposes of individual treatment or prevention versus societal efforts.
Comments to main findings
In summary, we confirmed the important parental and psychiatric risk factors for any violent crime suggested in prior research primarily based on clinical samples  and in a small body of population-based, epidemiological studies [9, 15, 32]. In relation to the persistence of violent crime (defined by the number of violent convictions per individual), we identified male sex, early onset of violent criminality, personality disorders, and substance use disorders as the most important risk factors specific to persistence in violence.
Distribution and preventive potential
About one quarter of all those who were ever convicted of a violent crime (i.e., the offender group), corresponding to the most persistently violent 1 % of the study population, were responsible for a total of 63 % of all violent crime convictions in the country, while almost three quarters of all violent offenders, corresponding to 2.9 % of the population, were convicted only once or twice during the study period. This is in line with previous research that shows a small group of criminally active young adult men to be responsible for a large part of all violent crimes [3, 4, 33] and the high impact of a very small group of especially violent individuals indicated by the high reconviction rates among former prison detainees . The numbers (and relative proportions) of violent convictions that theoretically could be prevented if individuals were stopped from committing further crimes at specific stages of relapsing into violence may be used to plan preventive strategies against violent crime (Table 2). If all violent crime careers could come to a stop after a third conviction (which would require interventions directed at 1 % of the total population), more than 50 % of all convictions for violent crime in the total population would be prevented.
The fact that females represented a mere 11 % of all those convicted and 6 % of the persistently violent concurs with previous findings. When considering persistence in violent crime and convictions for aggravated violent crimes, females represented an even smaller part of the group. The female group was too small for detailed subgroup analyses of risk factors. Other studies have reported more complex and severe mental health problems, including higher frequencies of combinations of substance use disorders, personality disorders, major mental disorders and other susceptibility factors in female offender groups .
Risk factors, prevention, and treatment strategies
First offenses are particularly difficult to predict, especially due to the low base rates of violent crime overall. By contrast, the majority of violent crimes are committed by a group of offenders who may be identified by rather easily observable features, such as having already been convicted of violent crimes several times already in adolescence, and having problems with substance abuse.
These statistics seemingly support the catchphrase and model employed in California and several other states in the USA, “three strikes and you’re out.” This model, however, does not seem to have been as successful as was hoped when it was initiated in the mid-1990s [35, 36]. The “three strikes law” was intended to act as a deterrent and keep offenders from committing aggravated and violent crimes, but its effect is difficult to ascertain; overall prevalence of crime has remained quite stable in the USA, while the prison population has increased to quite dramatic levels in some places and in some social groups . One explanation could be that the “three strikes law” also applies to nonviolent crimes, thus creating too large a group of detainees to be given sufficient shares of scarce treatment and rehabilitation resources. Proponents, however, have argued that a major increase in criminality may have been prevented through the “three strikes law” . The group identified here as medium- or high-persistent violent offenders, those with three or more convictions, could be small enough to single out for more costly treatment efforts for their often clear-cut psychosocial and mental health needs—efforts that might also potentially yield a substantial reduction in overall societal violence. Ethically, it is easier to defend treatment efforts that reduce autonomy for people who have actually committed a number of violent crimes than for everyone considered likely to do so in the future. If a “three strikes” policy were applied only to cases of violent crime rather than to all types of crime, a smaller group of individuals would be targeted and could therefore be given more resource-intensive treatment and preventive efforts. Specific prevention of violent recidivism may include detention or intensive surveillance programs including electronic surveillance, intensive care management, and monitoring for drug and alcohol use. But a wide array of other treatment and support efforts, such as education, work training, housing, mental and somatic health care, psychotherapy, and other social care interventions could also be used. For a detailed and current overview of the field of treatments for violent propensities, see Raine . The cut-off point of three convictions may also work as a simple pedagogic tool for more effective preventive efforts, as it is easily explained to adolescents and includes two opportunities for increasingly severe “warnings” to a young person on the path of violence.
Based on the results of this and previous studies [3, 9], it should be possible to identify individuals most at risk for developing persistent violent criminal behavior as early as adolescence, since an early onset in violent criminality manifested through convictions for violent crimes seems to be one of the strongest predictors of persistence in violence later in life . Therefore, developing and improving preventive strategies for delinquent youth who display violent tendencies should be a priority. Here, as in the overall prevention of violence in society, political efforts are important, since rates of early engagement in violent behavior may be highly dependent on sociocultural factors, which are malleable and may be ameliorated through political effort . Specific efforts targeting the group of especially violent individuals described in this study, along with preventive efforts aimed at youth, may provide a complimentary inroad to achieve a significant reduction in overall levels of violent crime.
Further research to establish evidence-based and cost-effective preventive methods against violence is obviously necessary. Currently, in many penal systems, including the Swedish, many resources are invested in preventing those diagnosed with major mental disorders from relapsing into criminal behaviors, even if their overall contribution to violence is small (3.7 % of all convictions) and only moderately linked to persistence in violence. Several other studies have also shown that people with major mental disorders share the same risk factors for persistence in violent crime as others who have been convicted of violent crimes, especially when substance abuse is involved [31, 40]. Based on the current results, what is needed are research efforts focused on developing suitable preventive measures that specifically address the risk factors with the strongest contribution to persistence in violence.
The skewness in the data may, to some extent, be context specific and could be less extreme in societies with a higher base prevalence of violent crime than Sweden. The fact that socioeconomic factors were not included in this study prevents analyzes the importance of adverse social circumstances to the development of persistent violent behavior. Nevertheless, the results suggest that even in a country such as Sweden, with a long tradition of tax-funded general health and social welfare policies, a very few individuals do repeatedly commit violent offenses and may reach tens of convictions for severe crimes during a study period of just over 2 decades. Importantly, they continue to do so despite negative attitudes in the general population against the use of violence, and despite penal sanctions (including loss of freedom) and other current preventive or treatment efforts .
Since the analyses of mental disorders presented here are based only on inpatient diagnoses from the Hospital Discharge Register, it is likely that we underestimated the actual prevalence of these disorders, especially personality disorders and substance use disorders. The marked difference in these characteristics between the offenders and the non-offenders, however, could represent a difference in the severity of these disorders, rather than differences in actual group prevalences. Another limitation of the analyses of personality disorders in this study is that this category includes all types of personality disorders. Therefore, it is possible that the increased risk detected was mitigated by the admixture of personality disorders that actually reduce the risk of violence.
As this study was cross-sectional and the only dates included in the current data file were for year and month of birth and year and month of first conviction for a violent crime, the exact order between risk factors and violent crime could not be teased out. For instance, it is possible that some subjects were given their psychiatric diagnoses after their first convictions. Studies focusing on the causality between psychotic disorders and criminality have shown an increased risk for homicide in the primary stages of the disorder (before diagnosis and treatment) . Moreover, a recently published study by Coid et al.  may show important associations between anger-related delusions and both minor and serious violence, even in the year prior to first contact with health care. Therefore, our results on the role of major mental disorders in violence should be interpreted with caution. On the other hand, many of the psychiatric diagnoses are considered life-long, with problems arising early, and the results that we present in relation to persistence in violence concur with numerous case–control studies . Since no data on behavior, mental health, or criminal acts before the age of criminal responsibility are available, we did not have the opportunity in this study to consider or discuss the specific pathway of life-course persistent antisocial behavior.
In addition, one might question our use of the number of violent convictions, rather than the actual number of crimes per individual—the so-called “dark figure” of crime—retrieved from self-reports or collateral interviews. This “dark figure” haunts many types of research on crime, but as no accurate measures of unreported and/or unconvicted crimes are available, it seems reasonable to assume that official data on crime are the tip of the iceberg and thereby provide a crude estimate of what is below the surface. In support of this, different indices of violent crime are highly related, and different sources of information alter the actual base rates but not the strength of association between variables .
This study did not include data on sentences carried out, so it is possible that some of those convicted once or twice were given long prison sentences, and were thereby prevented from committing additional crimes. Almost 60 % of those convicted once or twice included an aggravated violent crime, which carries 4 years to life imprisonment. Further investigation is needed to address the effect of detainment, even if studies of violence also have to consider periods in the penal system, where new violent crimes are not especially rare .
As this study did not consider socioeconomic factors, and because of the great differences in law and efficacy of law enforcement globally, the generalizability of the criminological results presented might be restricted to the countries in the western EU and to Canada.