Detecting the contagion effect in mass killings; a constructive example of the statistical advantages of unbinned likelihood methods

A Sequence Analysis of the Behaviors and Experiences of the Deadliest Public Mass Shooters

Researchers of public mass shooters are increasingly focusing on the pre-attack behaviors and experiences of these offenders. Varying in scope from consideration of individual factors to more generalized life course and threat assessment analyses, their scholarship has identified behavioral and experiential factors associated with public mass shooters. However, what is generally missing from this body of research is consideration of the order in which the offender encounters these factors, and prior related research has shown that analyzing sequences allows for insights not available from a catalogue of common characteristics. To address this shortcoming, we use a sample of offenders from 1999 to 2020 for whom the most detailed data is available and conduct a sequence analysis of their stressors, antisocial behaviors, mental health issues, and planning and preparation activities. We calculate proximity coefficients for each variable in relation to all others, capturing both local and distant connections between and among them. Our findings reveal relatively coherent phases in the lives of attackers, highlight the centrality of family problems and interest in past mass killings in the overall sequence, and provide new context to understanding the effect of mental health issues and firearms acquisition in the lives of offenders. A discussion of findings illustrates potential intervention points that may have been overlooked, as well suggestions for future research.