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.
