What is a “Mass Shooting”?

For those who don’t want to read 1,000+ words on the issue, here is the bottom Line: In my considered opinion, it makes sense to use the following terms to describe different events, from most specific to most general:

Mass public killing: Event in which a large number of people (mass) in a public or quasi-public space (public) are murdered (killing), regardless of the mechanism of death.

Mass murder: Event in which a large number of people (mass) are killed (murder). Again, the mechanism is unimportant.

Mass shooting: Event in which a large number of people (mass) are injured by gunfire (shootings). Some of those wounded will die and others will live. Here the mechanism of injury is emphasized.

These different phenomena occur with different frequencies and have different causes. Distinguishing between them, whether in conversation with your neighbor, in scholarly research, or in public policy debates will bring clarity to a currently muddled situation.

Read on for more…

What is a “Mass Shooting”?

Is this even a question? Yes, it is. If you are reading about or discussing “mass shootings” with someone this week, especially if you are throwing statistics back and forth, you do well to make sure you agree on what you’re talking about because there is no commonly accepted definition of mass shooting.

Any time you address definitions the possibility of trying to obscure issues with semantics arises.  But I think it was Socrates who said, “The beginning of wisdom is the definition of terms.” In the case of mass shootings it is vitally important to consider definitions so when we make comparisons of the phenomena across time and space we can be sure we are comparing apples to apples.

No Accepted Government Definition

Different agencies of the U.S. government do not agree on a definition of “mass shooting”:

  1. Congressional Research Service: “a multiple homicide incident in which four or more victims are murdered with firearms—not including the offender(s)—within one event, and in one or more locations in close geographical proximity.”
  2. Department of Justice Office of Community Oriented Policing Services: “pre-planned multiple causality violent events within the United States, excluding terrorist acts, killings in conjunction with the commission of other crimes, and domestic violence incidents in which only family members are killed.”

The former is the more common definition, but it has some specific aspects that may in the end not be helpful (see below).

No Accepted Scholarly Definition

To my knowledge, no scholar even attempted a definition of “mass shooting” until Gary Kleck in 1997: An “incident in which six or more victims were shot dead with a gun, or twelve or more were wounded.”

Other scholars like Adam Lankford have employed a four-fatality minimum (excluding the killer) rather than six, and focus on deaths rather than injuries.

Again, these differences are consequential not semantic when it comes to counting and comparing events.

No Accepted Media Definition

Following the Aurora theater and Newtown massacres, Mother Jones magazine attempted to track the number of “mass shooting” in America using the following criteria:

  1. Attack is in a single, isolated incident in a public space
  2. At least four people murdered by a lone gunman
  3. Motive is indiscriminate (excludes gang strife, armed robbery, domestic violence)

By contrast, USA Today used a broader set of criteria: Any firearm incident “where four or more people are killed.” This would include multiple gunmen, private settings, additional criminal motives, and possibly also the killer as one of the victims (not sure and no time to double check this).

Of What Consequence are Different Definitions

To take just one example, the different definitions are consequential. Using their criteria, Mother Jones identified 80 mass shootings from January 1982 to February 2016, just over 2 per year on average.

Using their broader criteria, USA Today finds nearly three times as many incidents (211) in a much shorter period of time (January 2006 to June 2015) – an average of over 22 per year on average.

Broadening Definitions of Mass Shootings

The issue of definitions has become even more complex of late because some are broadening the definition of mass shooting to include not just deaths but injuries as well (recall Kleck’s early scholarly definition).

In his book Rampage Nation, Louis Klarevas defines a mass shooting as: “Any violent attack that results in four or more individuals incurring gunshot wounds.”

From there, he distinguishes between three categories of mass shootings:

  1. Nonfatal: Mass shootings in which no one dies.
  2. Fatal: Mass shootings in which at least one victim dies.
  3. High-Fatality/Gun Massacre: Mass shootings in which six or more victims die.

Klarevas’s primary interest is in #3, and by this definition, he counts 111 gun massacres in the 50 years from 1966 to 2015, with 904 fatalities (2.22 a year, like the Mother Jones numbers not surprisingly).

A Proposal to Describe Different Events Going Forward

These different definitions of mass shootings reflect the fact that people are often concerned about different phenomena when they appear to be discussing the same thing. Based on this, I have some suggestions for using specific terms to describe specific events going forward.

(1) Mass public killings

Arguably, in the wake of an event like Las Vegas, what people are most concerned about is mass public killings. These are the kind of (seemingly) random murders of a large number of people that take place in public or quasi-public spaces.

Here it is less important that the death toll is at least 4 or 6, or whether that number includes the killer. The randomness and publicness is what makes them particularly frightening. You are at a concert (in Las Vegas or Paris or Manchester) and suddenly you are dead.

In addition to adding the “public,” I suggest we drop the “shooting.” The mechanism used in the killing is not important. Do the 3 knife victims of the Isla Vista murderer matter less than the 3 gunshot victims? Are the 49 dead from the shooting at the Pulse Night Club more dead than the 22 dead from the bombing in Manchester or the 86 dead from the truck attack in Nice? Not to mention the 2,996 killed in the World Trade Center attack or the 168 at the Oklahoma City Federal Building? If certain mechanisms of death in mass public killings are more common, that will emerge in the data analysis.

If you are concerned about events in which a large number of people (mass) in pubic or quasi-public spaces (public) are murdered (killing), you are talking about mass public killings.

(2) Mass murders

Accepting definition #1 will allow us to use the term mass murder to refer to ANY event in which a large number of people are killed. Whether the death toll is at least 4 or 6 or some other threshold is less important than that we use a term that focuses on the two key dimensions: a lot of people (mass) were killed (murder).

Again, the mechanism is unimportant. If certain mechanisms (e.g., guns) are used more commonly in mass killings, that will emerge in the data analysis. If certain mechanisms (e.g., bombs) have higher death tolls per incident, that will come out, too. But there is no reason to exclude mass killings using other mechanisms from the analysis a priori.

(3) Mass shootings

If you believe that those who are shot are categorically different than those who are injured by other mechanisms, this is the category for you. This is in line with Klarevas’s definition: “Any violent attack that results in four or more individuals incurring gunshot wounds.” Some of those wounded will die and others will live.

Here the most important things are that a lot of people (mass) were injured by gunfire (shootings).

I know there are some people who will object to the redefinition of “mass shooting” to include those who are only injured but not necessarily killed. This breaks from previous definitions of “mass shooting” and could introduce confusion, especially when making comparisons to the past. As sociologist Otis Dudley Duncan said, “If you want to measure change, don’t change the measure.”

But I think the terms I am proposing have the potential to clarify discussions about violence in American society – including the prevalence and causes – by being more specific and closer to reality in terms of ordinary language.

In a following post I will look at some research that shows the benefit of making these distinctions.


  1. From a criminology perspective, I do think Mother Jones is correct in their use of “indiscriminate motive,” if we are going to discuss mass killings in terms of actual risk to the public. Risk, real and perceived, is what we are really discussing, and motives are key in accurately calculating that risk. If any research doesn’t distinguish between motives as a primary factor, it definitely needs to be up front and center in the data reporting, particularly if the intent of collecting the data is to look at solutions. The ideologically-motivated attacker with no prior criminal record will not be addressed by any of the known mitigation efforts for gang members, career criminals, nor lethal domestic abusers.

    Mother Jones’ criteria is: “Motive is indiscriminate (excludes gang strife, armed robbery, domestic violence)”

    The research shows gang members are motivated to kill each other primarily over territory, affiliation, and, primarily, in ongoing retaliation for real and perceived slights. While most of their attacks fall into the same category of “non-felony” (not directly related to an underlying crime) homicides, those attacks -are- aimed at a known and knowable subset of society, opposing gang members, not a subset known only to the ideologically or psychologically motivated “random” mass killer. While innocents may be hurt or killed, these attacks are localized and the risk is concentrated in very small geographic locations.

    Similarly, while mass killings do occur in the course of (primarily) armed robbery, career criminals who kill (or rape / assault) multiple people during their crimes are mostly motivated to do so either by predatory impulse – acting on awareness of having absolute power over others-, disposal of witnesses, or due to an underlying particularized antagonism like gang members. Again, while armed robbery is all too common, robberies that end in any murder, much less a mass killing, are rare events that can be addressed, as much as any crime can be addressed, by normal crime mitigation measures.

    Finally, while a domestic violence-motivated mass killing may end up with unrelated innocents dying, lethal domestic violence does not “come out of nowhere.” It is usually characterized by ongoing and escalating domestic violence and those who are associated with persons in that situation are at much greater risk of being harmed in such an attack than the average person. Certainly when it comes to a prototypical “public” mass killing. The killing of friends and associates at a football party, or a victim’s workplace, is “public” in the sense it doesn’t occur where the -attacker- lives, and, given the realities of domestic violence often being hidden by the victim, the potential threat may be unknown to those friends and associates, but it is uniformly localized to a place associated with the -victim-, not the public at large.

    As a side note, I think it is useful to continue, as much as possible, the distinction between spree mass killings, “single event” mass killings that occur over a wide area (usually a route) and over a relatively prolonged period, and mass killings that occur in a single location and in a constrained amount of time. Port Arthur, Isla Vista, and several mass killings here and abroad covered a decent amount of ground and could have been impacted by a host of legal and policy decisions. Single location, short-term attacks, like Vegas, Pulse, and VT, likely lead to a different possible solution set.

    Liked by 1 person

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