Violent crime runs in families: a total population study of 12.5 million individuals

Background. Etiological theory and prior research with small or selected samples suggest that interpersonal violence clusters in families. However, the strength and pattern of this aggregation remains mostly unknown. Method. We investigated all convictions for violent crime in Sweden 1973-2004 among more than 12.5 million individuals in the nationwide Multi-Generation Register, and compared rates of violent convictions among relatives of violent individuals with relatives of matched, non-violent controls, using a nested case-control design. Results. We found strong familial aggregation of interpersonal violence among first-degree relatives [e.g. odds ratio (OR)(sibling) 4.3, 95% confidence interval (CI) 4.2-4.3], lower for more distant relatives (e.g. ORcousin 1.9, 95% CI 1.9-1.9). Risk patterns across biological and adoptive relations provided evidence for both genetic and environmental influences on the development of violent behavior. Familial risks were stronger among women, in higher socioeconomic strata, and for early onset interpersonal violence. There were crime-specific effects (e.g. ORsibling for arson 22.4, 95% CI 12.2-41.2), suggesting both general and subtype-specific familial risk factors for violent behavior. Conclusions. The observed familiality should be accounted for in criminological research, applied violence risk assessment, and prevention efforts.