On another terrible day, I hate to introduce even more pessimism, but when we discuss mass shootings, one of the first questions we ask is the simplest and also the hardest to answer. Why? Why does this keep happening? Those who advocate for gun control have an immediate answer — the prevalence of guns in the United States. Yet guns have been part of the fabric of American life for the entire history of our republic. Mass shootings — especially the most deadly mass shootings — are a far more recent phenomenon.

Writing in 2015, Malcolm Gladwell wrote what I think is still the best explanation for modern American mass shootings, and it’s easily the least comforting. At the risk of oversimplifying a complex argument, essentially he argues that each mass shooting lowers the threshold for the next. He argues, we are in the midst of a slow-motion “riot” of mass shootings, with the Columbine shooting in many ways the key triggering event. Relying on the work of Stanford sociologist Mark Granovetter, Gladwell notes that it’s a mistake to look at each incident independently.