If a sibling commits a violent criminal act, the risk that a younger sibling may follow in their footsteps is more likely than the transmission of that behavior to an older sibling, according to a new study conducted by researchers at Virginia Commonwealth University and Lund University in Sweden.

The findings provide insight into the social transmission of violent behaviors and suggest that environmental factors within families can be important when it comes to delinquent behavior. Down the road, the results may be used to inform strategies for prevention and treatment programs.

For some time, experts have recognized that violent criminal behavior runs strongly in families due to shared environmental factors such as poverty, divorce and poor parental supervision.

In a study, published online April 28 in the journal Psychological Medicine, researchers examined a series of national databases from Sweden linking full sibling pairs and criminal conviction. The team conducted two analyses – one that looked at age differences in siblings, and one that examined the difference in the risk of being a younger sibling versus an older sibling of a proband with violent criminal behavior.

Researchers found that older siblings more strongly “transmit” the risk for violent criminal behavior to their younger siblings, rather than vice versa. The team also found that the closer in age that siblings are, the greater the risk for the transmission of violent behavior. The authors write, “Because older siblings often exert more influence on siblings than younger, the risk for violent criminal behavior should be greater when the older sibling has violent criminal behavior as compared to the younger sibling. However it is not just mere closeness in age, but rather the nature of the sibling relationship that often occurs when siblings are closer in age.”

“Our findings strongly support the importance of familial-environmental influences on violent criminal behavior and provide some insight into the possible mechanisms at work,” said first author Kenneth S. Kendler, M.D., director of the VCU Virginia Institute for Psychiatric and Behavioral Genetics.

Kendler, professor of psychiatry, and human and molecular genetics in the VCU School of Medicine, collaborated with Nancy A. Morris, Ph.D., assistant professor in the Department of Criminal Justice in the L. Douglas Wilder School of Government and Public Affairs. Researchers from Lund University were led by Jan Sundquist, M.D., Ph.D., professor and director of the Center for Primary Health Care Research, and Kristina Sundquist, M.D., Ph.D., professor of family medicine at the Center for Primary Health Care Research, and Sara Larsson Lönn, Ph.D., with the Center for Primary Health Care Research. Sundquist and Sundquist also have affiliations with the Stanford Prevention Research Center at the Stanford University School of Medicine.

This work was supported by the Ellison Medical Foundation, the Swedish Research Council (K2012-70X-15428-08-3), the Swedish Research Council for Health, Working Life and Welfare (In Swedish: Forte; Reg.nr: 2013-1836), the Swedish Research Council (2012-2378) as well as ALF funding from Region Skåne.