The truly astonishing thing isn't that America's mass shootings are quickly becoming ever more deadly. It's that there aren't far more, and far bloodier, massacres in the United States.

Think of all the crowds: On city streets. At sporting events and concerts and move theaters. On campuses. At the mall. Dozens, hundreds, even thousands of people gathered together in public spaces — unarmed, unprotected, vulnerable to advanced technologies of violence, available for purchase in countless varieties all around us.

All it takes is the intent, the will, to make a purchase and then pull the trigger, over and over again.

Yet most of us pass through such crowds all our lives without having to endure the kind of horror that erupted on Sunday night in Las Vegas, either as a victim or a witness.

Some might attribute it to the innate goodness of human beings, with the rare perpetrators of mass violence standing as outliers. But given the bloody behavior recorded on nearly every page of human history, I'm more inclined to look elsewhere — to the absolutely vital, and nearly always very fragile, norms of civilization.

Where such norms prevail and are deeply rooted in a culture, the eruption of homicidal violence is rare. But where civilization is thin, or significant numbers of people reject its strictures, such eruptions will be more common, more frequent.

In our time, in the largely peaceful and ordered and wealthy Western world, explosions of barbarism generally take two forms: terrorism (acts of violence perpetrated against civilians with a political aim) or psychosis.

The terrorist who engages in mass murder does it because he believes the methods of barbarism will help him to advance his aims. The psychopath does it because his mental state places him beyond the distinction between civilization and barbarism altogether. He lives within civilization but is not of it. One way in which this paradox plays itself out with deadly consequences is in the ability of the psychopath to purchase weaponry that only a highly advanced civilization could produce. This product of civilization allows him to inflict deadly violence (meaning: to cut himself loose from the norms of civilization) with ruthlessly vicious efficiency. (As I write, the death toll in Las Vegas is 58, with 515 injuries, many of them severe.)

One important factor in determining the frequency and severity of such outbreaks of barbarism is the availability of weaponry. But another is motive: How many terrorists and how many psychopaths do we have in our midst? In the years since the 9/11 attacks, Europe has suffered more, and more spectacular, terrorist attacks, whereas the U.S. has endured more massacres perpetrated by so-called "lone wolf" psychopaths. (Though it's also true that when we include, as we should, right-wing and left-wing domestic terrorism along with Islamic terrorism on our list of attacks, the discrepancy between the U.S. and Europe is less stark.)

The fact is that the United States is a violent country, and it always has been. From the lawlessness and vigilante justice of the frontier, to the extrajudicial killings of the Jim Crow South, to off-the-charts murder rates in American cities, to the cold-blooded murder of children and their teachers in the classrooms of an elementary school, to the charnel house on the Vegas strip — the veneer of civilization is somewhat thinner here than elsewhere. The prevalence of guns is both an expression of this reality and a response to it, as people opt to arm themselves against the barbarism that threatens to erupt around them at any moment.

It's an understandable choice, but also one that increases the likelihood that the veneer of civilization will remain thin, and perhaps even grow thinner over time, with eruptions of barbarism becoming more deadly and more frequent with each passing year.

There's nothing inevitable about the norms and constraints of civilization. We learn that lesson anew every time we receive word of another round of mass death. Yet we seem incapable of breaking out of the cycle, of making a change for the better, of shoring up the foundations of civilized life against the barbarism that threatens always to engulf it. Instead we produce and purchase ever more weapons, and arm ourselves to the teeth.

Understandable, yes. But also horribly foolish. It's America's somewhat less than civilized answer to the crumbling of civilization. And all of us are forced to live — and some of us to die — with the consequences.