Mass shootings appear to be spiraling out of control. Are they?

The San Bernardino attack Wednesday marked the 355th mass shooting of 2015. The question, which divides experts, is whether such shootings are on the rise or whether the public is more acutely aware of them.

Responding to the mass shooting in San Bernardino, Calif., on Wednesday – the deadliest since the Newtown, Conn., school massacre three years ago – President Obama warned Americans that "we should never think that this is something that just happens in the ordinary course of events."

Unfortunately, the 355^th shooting that involved four or more victims this year suggested that already may be the case: In 2015, such occurrences have averaged more than one a day. In fact, the San Bernardino attack was Wednesday's second mass shooting – the first was in Savannah, Georgia.

The immediate aftermath of the attack, which killed 14 people and injured 21, took on a familiar script, from the political rhetoric of the gun control debate to what some called empty calls for “thoughts and prayers.” Those reactions have become familiar during a span of two years in America when not a calendar week has gone by without an act of mass gun violence.

“We’ve reached a critical saturation point with these mass shootings, and that’s part of the numbness and confusion we feel,” says Ron Astor, a University of Southern California professor of social work who has studied mass violence for more than 30 years. “But it’s not like we’ve accepted [the level of violence]. The problem is, we haven’t figured out what to do with that moral outrage that we all have.”

Overall gun violence has declined, along with violent crime, over the past two decades. But the frequency with which mass shootings have been reported this year have led to perceptions that this one, particularly shocking, type of violence may be experiencing a surge. The question, which divides experts, is whether such shootings are, in fact, running above historical norms, or whether the public is simply more acutely aware of them, given the rise of social media and the 24-hour digital news cycle.

“The only increase has been in fear, and in the perception of an increase,” says James Alan Fox, a criminologist at Northeastern University, told The New York Times.

A lack of a common definition of what encompasses a mass incident complicates the ability of criminologists to reach a consensus on how much of an increase 2015’s tally represents – or what the historical average for mass shootings has been. 

The FBI, in a report last year, counted 160 “active shooter” situations between 2000 and 2013, including the Sandy Hook school shooting in Connecticut; the Aurora, Colo., theater shooting; and the massacre at Fort Hood in Texas. Its findings suggested a steady increase in frequency, with a peak in 2010 of 26.

Research by Harvard University suggests that the frequency of mass shootings has increased threefold since 2011, based on a survey of news reports by the liberal-leaning Mother Jones magazine. That research looked at incidents in which the victims and shooter were strangers, and where the shooter killed four or more people. The Harvard researchers found that such shootings had occurred an average of every 200 days from 1982 to 2011. Since then, they've happened once every 64 days, on average.

But Professor Fox, co-author of “Extreme Killing,” slices the data differently. He says that by including situations where the shooter knows the victims – such as domestic or gang violence – the rate of annual mass shootings actually declined slightly from 2011 to 2014 when compared with the previous four years. (Addressing critiques that his method is flawed, Fox told Huffington Post that to those slain “it hardly matters whether they were killed in public or in a private home.”)

Two online databases that use news reports to track shootings where four or more are wounded or killed found that there have been 355 such shootings since January, spanning 221 cities and 47 states. That’s up from 337 mass shootings in 2014, according to one, shootingtracker.com. Those databases, which also include incidences of domestic and gang violence, were created in 2013 and differ from the FBI’s old definition of mass murder, which exclusively counted fatalities, making it difficult to compare historical trends.

Where criminologists do agree is that, similar to the awareness of killings of unarmed citizens by police that spiked after Michael Brown’s death in Ferguson, Mo., the public’s attention to such events has been heightened and led to a desire to take action – although there is no consensus on either side of the political aisle what such action should be.

“The extent that fear and anxiety increases, that may make it harder for us to come to some sort of resolution, because people feel a greater need to protect themselves, be armed or take defensive action or blame certain people and contain those people,” says Frankie Bailey, a criminologist at the State University of New York in Albany. “And at the same time, we’re dealing with competing perceptions of causation of what’s happening. And until we can agree on that, even though there’s a broader moral imperative to stop it, it’s difficult to find common ground for deciding how to deal with it.”

But in fact, says Mr. Astor, US cultural norms have shifted in other ways, specifically around the acceptance of violence. After all, he says, in just the past century, the US has eliminated lynching, reduced the number of police killed on the job, and dramatically decreased domestic violence.

And while media coverage has increased awareness, he argues that it hasn’t resulted in public apathy.

“Media coverage has, ironically, raised our intolerance for these kinds of events,” says Astor. “We already know through social science and religion that the way to get people to respond differently is to go against our instincts, to not only talk about [the perpetrator] and their motives, but to spend an inordinate amount of time on the totality of each of the victims and who they were. We move more to action if we actually feel more closely each of the individual lives that were lost.”

While the polarization surrounding the gun control debate has made political solutions difficult to attain, others also argue there are areas where progress can be made.

“There’s a sense of helplessness, but it’s in some ways misplaced,” says Jonathan Metzl, director of the Center for Medicine, Health and Society at Vanderbilt University, in Nashville, Tenn. “The fact is, we are going to have a very hard time, given our gun culture, stopping mass shootings. But that doesn’t mean that we can’t stem the tide of gun violence, because everyday gun violence is incredibly preventable.”

Correction: This article has been updated to change Ron Astor's title. He is a professor of social work at the University of Southern California.