Scientists have, for the first time, shown how violence can spread within a social network, similar to a contagious disease. They say an adolescent can be influenced to commit a violent act by a relative with whom he or she has four degrees of separation.

The idea that violence spreads quickly in a community, just like an illness, is not new. Renowned scientists and health professionals have studied the phenomenon of contagious violence among young people for years, with anti-violence programs created as a result.

The example propagated by Dr. Gary Slutkin, an epidemiologist who spent decades fighting epidemics in Africa, is of particular interest. He used his medical experience to reduce violence in the US through behaviour change and disease control methods, founding a groundbreaking program known as Cure Violence.

However, violence continues to plague many cities across the US, particularly affecting adolescents. In a new study, published in the journal American Journal of Public Health, scientists have analysed the data of thousands of young Americans to better assess the role of social relationships in spreading violence and map out the connections between relatives who engage in violent behaviours.

More at risk of committing violence

The team, led by Robert Bond from Ohio State University, used the data from 5,913 young people who participated in the National Longitudinal Study of Adolescent Health (ADD Health) and who were interviewed in-depth in 1994-95 and again in 1996. These participants were pupils in grades 7 to 12 at the time (12 to 18 years old) from 142 schools across the US.

One of the questions involved them saying the names of five male and five female friends from their school. They were then asked how often in the past 12 months they had been in a serious physical fight, how often they hurt someone badly enough to need bandages or care from a doctor or nurse, and how often they had pulled a knife or gun on someone.

Analysing all this information, the researchers found out that the adolescents were 48% more likely to have been in a serious fight, 183% more likely to have hurt someone badly, and 140% more likely to have pulled a weapon if a friend had done the same.

This shows that adolescents are more likely to commit violent acts if violence is prevalent in their social network. The surprising finding however was that they were not influenced to be violent only by their close friends – they could also be influenced by friends of friends and relatives as far removed as four degrees of separation.

While a participant was 48% more likely to have participated in a serious fight if a friend had been involved in one, he or she was still 18% more likely to have done so if a friend of a friend had.

The results of this study emphasise that anti-violence programs like Cure Violence could have very important effects, as stopping one person from acting violently could prevent him from influencing his relatives – not only his close friends, but his more distant acquaintances too.