Part of the reason our public discussion of public mass shootings in the US tends to devolve into arguments that we should just “Keep the guns away from the crazies!” is that we have a poor collective understanding of what mental illness actually is.
“Mental illness” is, technically speaking, psychiatric diagnosis, and psychiatric diagnosis is, in this part of the world, determined by the DSM-5 The DSM-5 is a diagnostic manual created by the American Psychiatric Association, and the diagnoses therein are based on descriptions of symptoms that tend to cluster together.
Whether a person has these symptoms is determined by self-report (often on a simple form) and observed behavior.
There is no blood test to determine mental illness, and the brain imaging studies that have been done so far aren’t definitive. If that seems surprising, consider that describing Picasso’s Guernica in terms of it being made of certain tones of oil paint on canvas won’t help you understand the Spanish Civil War, fascism, or cubism, for that matter, nor will it help you understand Picasso’s peculiar genius.
There is some preliminary association between these observed behaviors and reported states with certain genetic profiles, but there’s little understanding of the epigenetics involved (why people with the same genetic profile can have very different psychological experiences).
And none of this stuff matches exactly with what people actually experience when they’re suffering psychologically or going through extreme states of mind.
In other words, mental illness is descriptive, no prescriptive; the idea that we can just screen somebody and divine the next ticking time bomb is a convenient Hollywood plot device, not reality.
There’s little daylight between the notion that people “just snap” and “start shooting” and the idea of demon possession. What’s compelling about this Vanderbilt report is that it begins to see what leads to public mass shootings as something that’s probably complex, that is, like so many human behaviors, a cultural and “biopsychosocial” experience, not a simplistic bit of myth.