Contagion in Mass Killings and School Shootings

Abstract:

Background Several 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.
Methods Here 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. Conclusions We 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