Dr. Martin Luther King, Jr. noted in 1958 that “violence begets violence,” recognizing the contagious aspect of violence. More recently, there is hard science to back up King's observations.
In 2013, the National Academies of Science published a 153-page report titled “The Contagion of Violence,” which looked at how violence occurs. This analogy—that violence is like a contagious disease that spreads from person to person—can shed light on how to reduce and prevent violence.
Social networks are especially important to youth. Violence is also especially high among youth. Youth violence is a particularly serious problem everywhere, but especially in the U.S. In the U.S., more youth die from homicide each year than from cancer, heart disease, birth defects, flu and pneumonia, respiratory diseases, stroke, and diabetes combined. Youth homicide rates in the U.S. are 3 to 40 times higher than rates in similarly high-income countries.
My colleague Robert Bond and I recently conducted a study to see how violence spreads like a contagious disease in social networks, using a nationally representative sample of American youth. We also tested how far the violence spreads.
Violence: It's Catching
When people are exposed to a contagious disease, it increases the likelihood that they will contract the disease.
Contagious diseases can spread quickly or slowly, depending on a host of factors. For example, measles spreads quickly, whereas tuberculosis spreads slowly.
Likewise, some violence can spread quickly or slowly, depending on a host of factors. For example, gang wars and riots spread quickly, whereas other violence spreads much more slowly. Victims of child abuse may become perpetrators of family violence many years later.
Both contagious diseases and violence tend to cluster. A cluster is an “aggregation of cases of a disease that are closely grouped in time and place” (p. 154). Some people think that violent acts occur somewhat randomly, but the research evidence does not support this assumption. Violence occurs in clusters, often among individuals in the same social network.
People exposed to contagious diseases can develop a wide spectrum of possible outcomes, from no disease at all, to chronic or relapsing syndrome, to disability and death. The same is true for people exposed to violence. Not everyone exposed to violence becomes violent themselves, but people exposed to violence are much more likely to become violent themselves than people not exposed to violence.
When Imitation is a Sincere Form of Danger
One key mechanism of violence is imitation, or copying a model’s behavior. According to social learning theory, people learn aggressive and violent behaviors the same way they learn other social behaviors—by direct experience and by imitating the behaviors they observe others perform.
And according to social cognitive theory, observers don’t just imitate the specific behaviors they observe. They also make cognitive inferences based on their observations. These inferences lead to more general patterns of behavior.
Using Social Networks to Analyze the Spread of Violence
Robert Bond and I used these concepts from social learning theory and social cognitive theory to try to gain insights into how and whether violence spreads through social networks like a contagious disease.
A social network is a network of social interactions and personal relationships, such as friendships, familial relationships, romantic relationships, and so on. (Note that we are not talking about social media networks.) In our study, the social network started with the participant, and the spread was measured by the number of degrees of separation between the participant and their friends.
One degree is the participant’s friend, two degrees is the participant’s friend’s friend, three degrees is the participant’s friend’s friend’s friend, four degrees is the participant’s friend’s friend’s friend’s friend, and so on.
Our data came from a nationally representative sample of 5,913 adolescents who participated in the National Longitudinal Study of Adolescent Health and who were interviewed three times, a year apart.
To measure social networks, participants were asked to name up to five male and five female friends from their school at both of the two interviews. To measure violence, participants were asked how often in the past 12 months they had been in a serious physical fight, how often they had pulled a knife or gun on someone, and how often they hurt someone badly enough to need bandages or care from a doctor or nurse.
We analyzed whether each student’s friends (and friends of friends, and so on) had said they committed these three violent acts within the previous 12 months.
The results showed that adolescents were more likely to commit acts of violence if their friends had done so. Participants in the study were 48 percent more likely to have been in a serious fight if their friend had been in a serious fight, and the influence spread up to four degrees. Participants were 140 percent more likely to have pulled a weapon on someone if a friend had pulled a weapon on someone, and the influence spread three degrees. Participants were 183 percent more likely to have hurt someone badly enough that they needed medical care if their friend had also hurt someone badly, and the influence spread two degrees.
The effects were stronger for males than for females, perhaps because males are more likely to commit violent acts than females are.
Implications of our Study's Findings
Considerable evidence shows that observing violence in the home, school, community, or even in the mass media can increase the likelihood of violence in observers, both inside and outside the home. Considerable evidence also shows that having been a victim of violence increases the likelihood that one will also become a perpetrator of violence.
There is good news, though. Violence, like other contagious diseases, can be prevented or treated.
Prevention efforts can be directed at preventing individuals from being exposed to violence, or they can be directed at inoculating individuals against the effects of exposure to violence. Because friends have a significant impact on the violent behavior of adolescents, parents and other adults should encourage adolescents to interact with nonviolent peers.
Already aggressive youth can be taught alternative nonviolent ways to solve conflict (e.g., negotiation, compromise, cooperation). Aggressive youth can also be taught how to be more empathic, because empathy is one of the best predictors of pro-social behavior. Increasing supportive and pro-social climates in schools and communities can help decrease violence.
Dr. Martin Luther King, Jr. was correct when he noted that "violence begets violence." The results from our study show the violence can spread up to four degrees of separation. However, the treatment of violence should also spread through social networks.
 Patel, D. M., Simon, M. A., & Taylor, R. M. (2013). Contagion of violence: Workshop summary. Retrieved from The National Academies Press at http://iom.edu/Reports/2012/Contagion-of-Violence.aspx
 David-Ferdon, C., & Simon, T. R. (2014). Preventing youth violence: Opportunities for action. Atlanta, GA: National Center for Injury Prevention and Control, Centers for Disease Control and Prevention.
 Bond, R. M., & Bushman, B. J. (2017). The contagious spread of violence through social networks in U.S. adolescents. American Journal of Public Health, 107(2), 288-294. doi:10.2105/AJPH.2016.303550
 CDC (2016). Contagious diseases and disasters. Retrieved from https://www.cdc.gov/about/facts/cdcfastfacts/contagious-diseases-disasters.html
 Bandura, A. (1977). Social learning theory. New York: Prentice Hall.
 Bandura, A., (1986). Social foundations of thought and action: A social cognitive theory. Englewood Cliffs, N.J.: Prentice-Hall.
 Harris, K.M. (2009) The National Longitudinal Study of Adolescent Health (Add Health), Waves I & II, 1994-1996; Wave III, 2001-2002; Wave IV 2007-2009. Chapel Hill, NC: Carolina Population Center, University of North Carolina at Chapel Hill.