Thursday, February 20, 2014

The Mass Murderer Triad

People used to talk about a "serial killer triad" -- behaviors observed in children that "predicted" future nasty behavior. These were bedwetting (enuresis), fire setting, and cruelty to animals/other children. Like most models like these, the predictive power is vanishingly small, mostly because most of these behaviors are commonly found among children who do not grow up to become serial murderers.

To show how easy it is to come up with a model that purports to predict an extremely low base rate (rare) complex behavior, I made the Venn diagram below. The components are: 1) Mother was a nurse; 2) Parents divorced; and, 3) Played video games. I plugged in the names of some notorious mass murderers. For example, Marc Lepine's mother was a nurse, and Tim McVeigh's parents were divorced, while Cho was rumored to play video games. The Aurora, Colorado movie theater shooter played video games AND his mother was a nurse. The Sandy Hook shooter played video games AND his parents were divorced. And the (arguably) most notorious of all these murderers, played video games, had a mother who was a nurse, and had parents who were divorced.

(If someone out there could get me the name of a mass murderer whose mother was a nurse and whose parents were divorced, but who didn't play video games, I'd be much obliged. That empty section of the Venn diagram really bugs the compulsive side of me.)

Now, let's be clear -- I don't think that these factors are causative to any significant degree. But when I arrange them like this, they appear to be causative. To most journalists, I think this would be the basis for feature article. It certainly reflects the same level of scientific thinking evidenced in most journalistic accounts of these type of crimes. It could probably also convince the same congressmen who want to ban violent video games to prohibit divorced mothers from obtaining nursing degrees.

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