Violent behaviour can be made more likely by sociological, biological and psychological factors. All of these affect families as well as individuals and it is no surprise that violent crime tends to run in families. Researchers from the Karolinska Institutet in Stockholm looked into this in a study of 12.5 million people. They found that first degree (i.e. from the same nuclear family) relatives of people who had committed a violent crime were 4.3 times as likely as average to have been violent themselves while more distant relatives were 1.9 times as likely. By comparing people who lived with their biological families to those who had been adopted the researchers found that both genetic and environmental factors raised the risk of developing violent behaviour. A family risk of violence was stronger among women; in higher socio-economic groups and in those who had behaved violently early in life. Some crimes like arson - where being the sibling of an arsonist made people 22.4 times as likely to commit arson themselves - ran particularly strongly in families.
Frisell, T., Lichtenstein, P. and Langstrom, N. - Violent crime runs in families: a total population study of 12.5 million people Psychological Medicine (2011), 41, 97–105