In the immediate aftermath of the mass shootings in San Bernardino, CA and Savannah, GA earlier this week, several infographics circulated around social media and internet news sites showing that the US averages around one mass shooting per day. That, to say the least, is an eye-opening statistic. Second Amendment rights advocates balked at that figure, contending that the actual number of mass shootings was far lower, and at any rate once gang violence, drug deals gone bad, family kidnappings, and so forth were taken into consideration, our mass shooting rate would be no different than that found in European countries.
The truth of the matter is that there is apparently no agreed-upon definition of the term mass shooting. Some of those sources tracking mass shootings count only those incidents in which a single shooter kills four or more people (although here there is some dispute as to whether or not to include gang-related shootings, etc. - see Mother Jones' tracker, for example). Other trackers, such as Reddit's Mass Shooting Tracker and the Gun Violence Archive count any shooting with four or more victims who were injured or killed, excluding the shooter or shooters. The Gun Violence Archive defines the incidents in which four or more people are killed as mass murders, a distinct sub-class within mass shootings. The debate over the appropriate operational definition of mass shooting incidents matters to the extent that it frames the extent to which there is a problem and the extent to which it is worsening. If we use Reddit's Mass Shooting Tracker or the Gun Violence Archive, one will come away with the impression that mass shootings are now a daily occurrence, and that the number of victims each year is in the thousands. The mass shooting definition used by Mother Jones, which is perhaps the most conservative of all the definitions, suggests a far lower number of incidents but that they have increased in frequency in recent years. USA Today's tracker which includes gang-related incidents, but only counts those with four or more murdered, suggests that the frequency is actually going down.
In the behavioral and social sciences, disagreements about operational definitions are quite common, and often those definitions that eventually become accepted are those that are able to produce consistently replicable findings. In the case of mass shootings, since there is a very heated debate regarding whether even mild regulation of firearms in the US should be considered, the debate over how to define them is tied quite strongly to ideology. Those who are in favor of stricter gun laws are likely to gravitate toward those definitions that seem to best make their case (Mother Jones' tracker, or either of the other two trackers), whereas those in favor of further liberalizing gun laws are likely to gravitate toward those definitions that make their case (e.g., the USA Today tracker). In other words, the debate over how to define mass shootings is not merely an academic debate, but one that is very politically loaded at the moment, suggesting that arriving on an agreed-upon definition that will satisfy most interested parties is not likely any time soon.
Regardless of the operational definition of mass shootings, the question of just how uniquely American this phenomenon is one that appears to have an answer. Using a fairly restrictive definition of mass shootings (four or more murdered, excluding the shooter or shooters), Adam Lankford discovered that the frequency of mass shootings was considerably higher in the US than any other developed nation as well as other nations that make up the Global South. Lankford found that approximately one third of mass shootings in his data set were committed in the US. Generally, these findings make some sense, given that firearms are more readily available in the US than in much of the rest of the world (see this article comparing gun laws in the US to a selection of other nations). Not all will agree with these findings, of course, and I am sure that there are ways of finessing the data to make particular political points.
The question of why mass shootings occur is one that is also politically charged, but in principle should be answerable. Often, commentators view mental illness as a factor. However, those making that claim fail to take into consideration that people who are mentally ill are usually less likely to commit acts of violence (including gun-related violence) than those who are not mentally ill - a topic I touch on briefly in my recently published book chapter on aggression. It may turn out that mental health status of mass shooters, however defined, may turn out to be truly unique among those who perpetrate gun-related violent incidents. Without the necessary research, the "mental illness" explanation remains untested, and should be treated with some skepticism. It would also be useful to know the extent to which extreme religious and ideological beliefs factor in to mass shootings, given that a number of the perpetrators of high profile shootings (those that received a great deal of media coverage) have had religious or political motivations. Finally, some have floated the idea of a Columbine effect, suggesting that many of the mass shootings in the US may be copycat crimes inspired by the two teenagers who killed and injured a large number of people at Columbine High School in Littleton, CO in 1999. It is certainly an interesting idea that may explain those incidents that have occurred on high school and college campuses since 1999. The extent to which those incidents outside school grounds are related to the Columbine shooting is for now debatable.
There does appear to be a problem, and one that is more so in the US than elsewhere as a general rule. Skeptics are certainly correct in pointing out that stricter gun regulations would not guarantee an end to mass shootings or other forms of gun-related violence. Even in those countries with stricter laws, gun-related violence still happens, and mass shootings are not entirely out of the question (see what happened in Norway a few years ago as one example). However, in those nations, such incidents are considerably less frequent elsewhere, and one can even go through a significant portion of one's own life without ever having experienced a mass shooting on one's own soil - something that probably has some effect on the social psychological makeup of those who have the chance to live without such occurrences. In Australia, for example, there is now an entire generation that has never experienced a mass shooting. For those wondering what life might be like without the worry of when and where the next shooting rampage might happen, the Australian experience is quite edifying.