Mass murders less frequent than we think: Column

• We often wonder why mass shootings have become an all-too-frequent occurrence.
• Yet%2C over the last 30 years%2C the average number of mass killings have remained about the same.
• As a result%2C questions fail to address the larger issues surrounding these tragic events.

"Yet another mass shooting," lamented President Obama on Monday before giving a speech marking the fifth anniversary of the financial crisis. Meanwhile, the cable news channels featured wall-to-wall coverage of the D.C. Navy Yard massacre, complete with eyewitness interviews and telestrator-aided analysis of how the events had unfolded.

Commentator after commentator pondered the reasons why mass shootings were becoming an all-too-frequent horror of American society and asked what could have prevented the senseless loss of 13 lives, including the gunman. Angry and frustrated citizens are left questioning the backbone of lawmakers at the state and federal levels in failing to stand up to the NRA's political muscle.

Whatever the political response, it is important to dispel the widely held notion that mass shootings are on the rise. Over the past 30 years, there has been an average of nearly 20 mass shootings a year in the U.S., each involving at least four victims killed, but with no upward or downward trajectory. Of course, most were not of the large-scale public slaughter variety that grabs the attention of the news media and has millions of Americans glued to their television sets.

There have been other occasions when similar headlines have noted a repetition of tragedy. In the 1980s, a stunning series of post office shootings helped to create "going postal" as a catchphrase for what we now call "active shooters." The 1990s witnessed a barrage of school shootings at the hands of angry and dispirited adolescents, prompting then CBS anchorman Dan Rather to declare a new epidemic. Mass murder -- be it at post offices, public schools, shopping malls or military bases – are exceptionally rare events, which occasionally cluster together within a close time frame but still do not reflect an epidemic.

Every episode of extreme bloodshed brings searing questions about who missed the warning signs and who failed to take appropriate action that may have averted the tragedy. As details surfaced on Monday about 34-year-old Aaron Alexis, the alleged shooter, critics were quick to point to possible breaches in security.

How could someone with disciplinary issues during military service and arrests for gun-related incidents be cleared to work as a contractor on a military base? How could someone get a firearm into a building on a military base without being searched for contraband?

Although popular avenues for Monday morning quarterbacks, these questions fail to address larger issues surrounding mass murder. After all, most mass shootings do not take place on military bases, despite the recent trial of Nidal Hasan for his attacks at Fort Hood in Texas. In fact, most places in which mass murders have occurred would not and should not require security clearance or maintain access controls. This is an open society in which we enjoy the ability to move freely almost anywhere and in which misfits and malcontents are not detained just because they make us uncomfortable.

But then there is the troubling matter of how someone like Alexis with a documented troubled past is able to acquire a concealed carry license and sufficient weapons to wreak havoc on innocents. The unfortunate and largely unavoidable fact is that most would-be mass murderers are able to acquire a gun legally.

Most mass murderers do not have criminal records that would disqualify them from purchasing a firearm from a licensed dealer. And most mass murderers, despite whatever mental health issues are driving them, do not have a history of institutionalization that would knock them from eligibility. In addition, proposals to restore the assault weapons ban, although well-intentioned and potentially helpful in curtailing gun crime generally, will not appreciably impact violence in its most extreme form. Most mass murderers do not use weapons that would be characterized as "assault weapons"; unfortunately, they can do tremendous damage using more conventional firearms.

In the coming days, we'll surely learn more details about Alexis' troubled past. But while no consolation for the victims and their families, the frequency of mass shootings should be kept in perspective lest we become a nation that constantly fears for its safety and over-responds to the risk.

James Alan Fox is the Lipman Family Professor of Criminology, Law and Public Policy at Northeastern University and author of Extreme Killing. He is a member of USA TODAY's Board of Contributors.

In addition to its own editorials, USA TODAY publishes diverse opinions from outside writers, including ourBoard of Contributors. To read more columns like this, go to the opinion front page or follow us on twitter @USATopinion or Facebook.