Mass shootings are a part of American culture now, and they have been for years — changing the way we think, changing the way we act.

Consider the novelist Don DeLillo, who once explained why the paranoia in his fiction could not have existed before lone gunmen shocked the nation and redefined the 1960s.

First, there was Lee Harvey Oswald assassinating President John F. Kennedy. Then there was the disturbed University of Texas student who shot and killed 14 people and a fetus from a campus clock tower.

“There’s the shattering randomness of the event, the missing motive, the violence that people not only commit but seem to watch simultaneously from a disinterested distance,” DeLillo told the Paris Review. “Then the uncertainty we feel about the basic facts that surround the case — number of gunmen, number of shots, and so on. Our grip on reality has felt a little threatened … . Mainly we have the individual in the small room, the nobody who walks out of the shadows and changes everything.”

DeLillo’s remarks were made in 1993 — long before the 1999 massacre at Columbine High School changed the way we look at school security and provided a mold for future copycats.

But how common are mass shootings? Academics, journalists, congressional researchers and Internet commentators now regularly check data to determine exactly how often mass shootings occur in a country that averages more than 10,000 gun homicides every year.

It’s a tough and surprisingly subjective job. Counts differ depending on who is collecting the data, and how they define mass shooting.

According to a Congressional Research Service analysis released in July, the U.S. from 1999 to 2013 averaged 21 mass shootings per year in which four or more people were shot and killed.

More than 1,500 people were killed in those shootings, with the totals fluctuating from year to year with no clear trend up or down, according to the analysis.

In the least deadly year, 2001, 53 people were killed in such shootings, and in the most deadly year, 2009, 145 people were killed.

In response, a familiar subculture has gradually developed to attacks that are often stunningly senseless and paradoxically routine.

There’s President Barack Obama, who will use such attacks to make repeated and largely fruitless calls for gun control, while conservative lawmakers across the nation might argue for more civilians to be armed in bars and schools.

Then there are the victims’ advocates, who, frustrated by the media’s relentless coverage of such shootings, have launched no-notoriety campaigns to deny gunmen the spotlight and to prevent copycats from getting inspired.

In Roseburg, Ore., residents and officials were so disgusted by a mass shooting striking their community on Oct. 1 that they refused to publicly identify the student who killed nine people in his English class at Umpqua Community College.

“You will never hear me say his name,” Douglas County Sheriff John Hanlin told reporters.

The media, in turn, have gotten so practiced at covering such attacks that newsrooms often repeat their coverage strategies from one shooting to the next.

Americans, meanwhile, have learned to protect against mass shootings with active-shooter drills and precautionary lockdowns.

They do it because new “shooters” — to use a now-popularized term among the public — seem to emerge weekly.

On Wednesday, a quiet young couple who amassed an arsenal of pipe bombs and ammunition used rifles to slaughter 14 people and wound 21 more at a holiday party in San Bernardino. Syed Rizwan Farook and Tashfeen Malik died in a shootout with police.

As often happens with such shootings, their attack puzzled those who knew them, and readers who often read profiles of suspects probably will not be surprised to learn that those who knew Farook said he was a quiet guy.

“If you had told me that he had killed a bird, I would say, ‘No way,’” said Mustafa Kuko, the director of the Islamic Center of Riverside, Calif., where Farook once worshiped.

Two days earlier, Robert Lewis Dear, a 59-year-old loner, was charged with killing three people and wounding nine on Nov. 27 at a Planned Parenthood clinic in Colorado Springs, Colo.

The even more disturbing part is that the Planned Parenthood shooting rampage wasn’t even the first one in Colorado Springs that month. On Nov. 1, a man shot and killed three people in the downtown area before he was shot and killed by police.

The cycles of tragedy are so bitter that they’ve made the Onion parody news outlet an essential post-shooting destination for Americans seeking biting, if bleak, satire.

“Location of Newest Mass Shooting Revealed,” an Onion story blared in 2013 after a gunman killed 12 people at the Washington Navy Yard in the nation’s capital.

A joke 2013 headline would now qualify as an old joke if the topic weren’t so unending. The Onion’s headline this week? “Authorities Say Country Still an Active Shooter Situation.”

When police stormed the Inland Regional Center in San Bernardino on Wednesday, some staffers thought it was one of the facility’s routine mass shooting drills.

Melinda Rivas, 51, a social worker who evacuated from the center, used a phrase often uttered around the nation when a mass shooting claims another community.

“I never thought I was going to be a part of it,” Rivas said. “Today, it was us.”