2020 wasn’t just the year of the COVID-19 pandemic, but it was the year of another pandemic in the US: gun violence. Almost 20.000 Americans died last year of gunshots, according to the Gun Violence Archive. The total number of gun violence deaths (excluding suicides) was 30% higher in 2020 than in the past years.

With almost 400 million firearms in civilian possession, the US is the world’s leading country when it comes to the number of guns held by its inhabitants.  

Shootings are up 95% from the same time last year in New York, up 67% in Philadelphia, and In Chicago, where homicides are up 56%, shootings also increased by 54%.

A closer look at Chicago's gun violence

tracking assaults and robberies using a firearm in the past 5 years in Chicago

*zoom on the map for specific neighbourhoods.

How does gun violence spread like a virus?

Yale university sociologist Andrew Papachristos conducted a study of shootings in Chicago and found that for every 100 people who were shot, contagion would result in 63 follow-up attacks.

The study

The study was an epidemiological analysis of a social network of individuals who were arrested during an 8-year period in Chicago, with connections between people who were arrested together for the same offense. The spread of gunshot violence over the network was assessed using a probabilistic contagion model that assumed individuals were subject to risks associated with being arrested together, in addition to demographic factors, such as age, sex, and neighborhood residence.

The subjects

9.773 of them were subjects of gun violence


Social contagion accounted for 63.1% of the 11.123 gunshot violence episodes; subjects of gun violence were shot on average 125 days after their infector (the person most responsible for exposing the subject to gunshot violence). Some subjects of gun violence were shot more than once.

From tracing gunshot violence episodes through the network, the researchers detected 4.107 separate cascades (connected chains of infection through the network), ranging in size from cascades with a single subject to a cascade involving 469 subjects, with 680 cascades involving multiple subjects.

The study presents 3 representative
cascades, containing 12 people, 34 people, and 64 people, all shot during the study period, and illustrating the pathways of diffusion between individuals. 

12 People shot between May 2009 and December 2012

34 People shot between May 2009 and December 2012

64 People shot between May 2009 and December 2012

Gunshot violence is spreading like an epidemic, through social interaction. The researchers highlight the fact that violence prevention efforts that account for social contagion, in addition to demographics, have the potential to prevent more shootings than efforts that focus on only demographics.