Contagion in Mass Killings and School Shootings

Abstract

BackgroundSeveral past studies have found that media reports of suicides and homicides appear to subsequently increase the incidence of similar events in the community, apparently due to the coverage planting the seeds of ideation in at-risk individuals to commit similar acts.MethodsHere we explore whether or not contagion is evident in more high-profile incidents, such as school shootings and mass killings (incidents with four or more people killed). We fit a contagion model to recent data sets related to such incidents in the US, with terms that take into account the fact that a school shooting or mass murder may temporarily increase the probability of a similar event in the immediate future, by assuming an exponential decay in contagiousness after an event.ConclusionsWe find significant evidence that mass killings involving firearms are incented by similar events in the immediate past. On average, this temporary increase in probability lasts 13 days, and each incident incites at least 0.30 new incidents (p = 0.0015). We also find significant evidence of contagion in school shootings, for which an incident is contagious for an average of 13 days, and incites an average of at least 0.22 new incidents (p = 0.0001). All p-values are assessed based on a likelihood ratio test comparing the likelihood of a contagion model to that of a null model with no contagion. On average, mass killings involving firearms occur approximately every two weeks in the US, while school shootings occur on average monthly. We find that state prevalence of firearm ownership is significantly associated with the state incidence of mass killings with firearms, school shootings, and mass shootings.

The Link Between Mental Illness and Firearm Violence: Implications for Social Policy and Clinical Practice

“…Similar guidelines could be developed and adopted by major media outlets for ethical reporting of mass shootings and similar events. Links between sensational reporting of mass shootings and copycat events are becoming better established (Cantor et al 1999, Towers et al 2015), and early proposals for media guidelines are already being developed (Perrin 2016). …”

Putting homicide followed by suicide in context: Do macro‐environmental characteristics impact the odds of committing suicide after homicide?*

“…The infrequency with which homicide-suicide occurs poses several challenges. First, until now, researchers have mainly discounted the utility of studying homicide-suicide as a distinct behavior, instead regarding it as a variant of either homicide or suicide and studying it accordingly (Liem & Nieuwbeerta, 2010), despite the knowledge that homicide-suicide has a devastating impact on families and communities (Liem, 2010), garners national attention that can be used to shape the sociopolitical landscape (McPhedran et al, 2015), and inspires copycat crimes (Towers, Gomez-Lievano, Khan, Mubayi, & Castillo-Chavez, 2015). Second, and relatedly, many studies on homicide-suicide have methodological and statistical shortcomings, including small sample sizes, low statistical power, inflated effect sizes, overestimation of both type I and type II errors, inefficiency of model estimates, and low reproducibility of results (Button et al, 2013).…”

School shootings during 2013–2015 in the USA

“…However, we documented an increase where there were 154 episodes in a 3-year span as compared with 44 incidents from 1966 to 2008 in the USA. This spike in school shooting may be explained by the theory of contagion; evidence has shown that mass killings involving firearms are motivated by similar events in the immediate past 31. To our knowledge, there has been no formal documentation of the spatial or temporal spread of school shooting episodes after Sandy Hook Elementary shooting.…”