together through a series of co-offending events. Overall this group
included 138,000 people, or about a third of the dataset.
Papachristos’s team started by checking whether homophily or
environmental factors could explain the observed patterns of gun
violence. They found that it was unlikely: many shootings occurred in
a linked way that couldn’t be explained by homophily or environment,
suggesting contagion was responsible. Having identified the
shootings that were likely due to contagion, the team carefully
reconstructed the chains of transmission between one shooting and
the next. They estimated that for every 100 people who were shot,
contagion would result in 63 follow-up attacks. In other words, gun
violence in Chicago had a reproduction number of about 0.63.
Fifty simulated outbreaks of shootings, based on the dynamics of
violence contagion in Chicago. Dots show shootings, with (grey)
arrows indicating follow-up attacks. Although there are some
superspreading events, most outbreaks involve a single shooting and
no onward transmission.If the reproduction number is below one, it means that an outbreak
might spark but it rarely lasts very long. The Yale team identified over
four thousand outbreaks of gun violence in Chicago, but most were