The study was conducted by a group called The Violence Project, a nonpartisan think tank founded by psychologists at Minnesota's Hamline University, with the goal of reducing violence in society. The study consisted of several components, which included the creation of a a database of the 171 shootings — defined per FBI guidelines as involving the killing of four or more people in a public setting — that have taken place between 1966 and 2019. The database is coded on 99 “life history variables,” including mental health, trauma, and “interest in past shootings.” You can find the database
here.
The study also explored larger sociological factors in the areas where the shootings occurred, exploring crime rates, inequality, and prevalence of household guns. Furthermore, the authors of the study conducted interviews with incarcerated shooters to determine what led to their actions.
In looking at the shootings, the authors of the study identified
four key traits that tied together the perpetrators. Typically, they concluded, shooters had experienced childhood trauma, had identified a personal grievance, and had developed a “script” based on prior cases. They also, of course, could get their hands on a gun.