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Why Is Violence So Contagious?

Violence is a contagion that can go viral among social networks and communities.

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Violence is contagious. Just like altruism and kindness can create an upward spiral of well-being for both the giver and receiver—being the perpetrator or victim of violence is a contagion that often spreads like wildfire.

This week, two different studies on how violence spreads like a communicable disease within social networks and communities put the phenomenon of "violence as a contagion" back in the national spotlight.

These new findings add fresh insights to a growing body of interdisciplinary research that helps us understand what makes violence so contagious. For example, earlier this year, researchers at Yale University led by Nicholas Christakis, reported that a close-knit network of friends who are prone to violence is a prime driving force for collective acts of group violence.

Once violence begins within a social network, the researchers found that violent tendencies tend to propagate throughout the entire group's population. These findings may also be applied to potentially violent activities associated with terrorism, political revolutions, and gang warfare.

This study, "Formation of Raiding Parties for Intergroup Violence Is Mediated by Social Network Structure" was published in an October 2016 online issue of the Proceedings of the National Academy of Sciences.

For this study, the researchers observed the social dynamics of a nomadic tribal group in East Africa called the Nyangatom. This tribe is regularly involved in violent raids. For over a three-year period the researchers mapped the interpersonal dynamics among Nyangatom men. The researchers were primarily focused on how friendships and social networks affected the initiation of raids and someone’s participation in those raids.

Christakis is a sociologist, physician, and professor who conducts research on biological predicates and consequences of social phenomena. He also directs the Human Nature Lab. In a statement to Yale News, Christakis described aspects of his research:

"Social interactions in networks are crucial for the emergence of positive phenomena, like cooperation and innovation, but they also play a role in other sorts of collective behavior, like the seemingly spontaneous emergence of violence . . . People go to war with their friends, and the social network properties of such violent activities have rarely been explored."

Christakis’ lab focuses on how social networks form “connection” and also how these connections can transfer behavioral “contagions” that spread within groups. His team's ongoing investigations around the globe also explore how the application of social network principles can change population-level behaviors.

Violence Within U.S. Adolescent Social Networks Is Highly Contagious

The first new study on the contagiousness of violence from this week was conducted by Robert Bond and Brad Bushman of Ohio State University (OSU).

The OSU researchers found that the influence of one person’s violent act can spread up to two degrees of separation (friend of a friend) for hurting someone badly, three degrees (friend of a friend’s friend) for pulling a weapon on someone, and four degrees for serious fights.

This study, "The Contagious Spread of Violence Among US Adolescents Through Social Networks" was published online in the December 2016 American Journal of Public Health.

Bond and Bushman identified that in the United States, adolescents are up to 183 percent more likely to perpetrate acts of violence if one of their friends had also committed a similar violent act. Although the contagiousness of violence seems to decline with each degree of separation, the researchers say this is the first study to show just how far violent behavior can spread within a social network.

The OSU researchers hypothesize that adolescents are more likely to commit acts of violence if their friends are violent is related to a phenomenon called the “clustering effect” in which people with similar interests, including the use of violence, tend to cluster together in social networks.

That said, the researchers also observed that inadvertent exposure to violence also increased the odds that someone would go on to commit more acts of violence than they would had they not been brought into a contagious cycle of being a perpetrator or victim of violence.

In a statement to OSU, Bond said, “If we can stop violence in one person, that spreads to their social network. We’re actually preventing violence not only in that person, but potentially for all the people they come in contact with.”

The empirical evidence on the contagiousness of violence reaffirms the importance of anti-violence programs and public service announcements such as the White House “1 is 2 Many” initiative featuring President Barack Obama and VP Joe Biden which is aimed at reducing campus-based sexual assault.

Mental Health Issues Often Drive Contagious Violence

The second new study on the contagiousness of violence from this week pinpointed the leading indicators of community-based violence among adults with mental illness. In particular, the researchers deconstructed how a singular violent event could ricochet and snowball into increasing incidences of violence within a community.

The team of researchers from RTI International, North Carolina State University, Arizona State University, and Duke University Medical Center published their findings in the December 2016 journal Psychological Medicine.

This study highlights the importance of specific interventions that treat mental-health problems in order to reduce community violence and exacerbated instances of mental-health crises.

In an NC State News statement, Richard Van Dorn, a researcher at RTI and lead author of this paper describing indicators of contagious violence said,

"This work builds on an earlier study that found almost one-third of adults with mental illness are likely to be victims of violence within a six-month period . . . It's a complex series of interactions that spirals over time, exacerbating substance use, mental health problems, and violent behavior.

In this study, we addressed two fundamental questions: If someone is victimized, is he or she more likely to become violent? And if someone is violent, is he or she more likely to be victimized? The answer is yes, to both questions."

For every single violent event in which a person was a victim of violence, the researchers identified an average of seven various domino effects that could cascade into about 39 additional violence-related consequences. In the statement, Sarah Desmarais, an associate professor of psychology at NC State and co-author of the paper, said,

"We found that all of these indicators mattered, but often in different ways. For example, drug use was a leading indicator of committing violence, while alcohol use was a leading indicator of being a victim of violence. The more pronounced affective symptoms were, the more likely someone was to both commit violence and be a victim of violence. By affect, we mean symptoms including anxiety, depressive symptoms and poor impulse control.

This is particularly important because good practices already exist for how to help people, such as therapeutic interventions or medication. And by treating people who are exhibiting these symptoms, we could reduce violence. Just treating drug or alcohol use—which is what happens in many cases—isn't enough. We need to treat the underlying mental illness that is associated with these affective symptoms."

Social Bonds Based on Prosocial Behaviors Can Break the Cycle of Violence

In December 2014, a team of researchers from the Yale School of Medicine presented community-based research which found that social bonds that tie a neighborhood together can help shield community members from gun violence at the annual Institute of Medicine's Means of Violence workshop.

In a statement to Yale News, Emily Wang, assistant professor of internal medicine at Yale School of Medicine, said,

"Violence results in chronic community-level trauma and stress, and undermines health, capacity, and productivity in these neighborhoods. Police and government response to the problem has focused on the victim or the criminal. Our study focuses on empowering communities to combat the effects of living with chronic and persistent gun violence."

Ann Greene, community research liaison at Yale and chair of the West River Community Resilience Team, added,

"Our study is a community-based and community-driven intervention to prevent and reduce the negative effects of gun violence in the communities affected by high rates of gun violence by strengthening social ties, bonds, resilience, or in other words, by 'putting neighbor back in hood.'"

These thought leaders are working with the Yale investigators to share data with their communities and request any input other people might have about ways to strengthen neighborhood social ties.

If you have any suggestions or input on ways to improve the social ties within American neighborhoods or social networks, please reach out to any of the researchers involved in these studies by clicking on the link associated with his or her name.


Robert M. Bond, Brad J. Bushman. The Contagious Spread of Violence Among US Adolescents Through Social Networks. American Journal of Public Health, 2016; e1 DOI: 10.2105/AJPH.2016.303550

R. A. Van Dorn, K. J. Grimm, S. L. Desmarais, S. J. Tueller, K. L. Johnson, M. S. Swartz. Leading indicators of community-based violent events among adults with mental illness. Psychological Medicine, 2016; 1 DOI: 10.1017/S0033291716003160

Luke Glowacki, Alexander Isakov, Richard W. Wrangham, Rose McDermott, James H. Fowler, and Nicholas A. Christakis. Formation of raiding parties for intergroup violence is mediated by social network structure. PNAS 2016 113 (43) 12114-12119; published ahead of print October 10, 2016, doi: 10.1073/pnas.1610961113