If a person commits a violent criminal act, there is a higher chance that a younger sibling will follow in their footsteps than an older one, according to a new paper.

It's been common sense for centuries that violent criminal behavior runs strongly in families but why is unclear. Blame is attributed to shared environmental factors such as poverty, divorce and poor parental supervision. 
This 'social transmission' of violent behaviors suggests that environmental factors within families can be important when it comes to delinquent behavior. 

The paper in Psychological Medicine 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.