In the United States, there are several different, but common, definitions of mass shootings. The Congressional Research Service defines mass shootings, as multiple, firearm, homicide incidents, involving 4 or more victims at one or more locations close to one another. The FBI definition is essentially the same. Often there is a distinction made between private and public mass shootings (e.g., a school, place of worship, or a business establishment). Mass shootings undertaken by foreign terrorists are not included, no matter how many people die or where the shooting occurs.
These formulations are certainly workable, but the threshold of 4 or more deaths is arbitrary. There are also important exclusions. For example, if 10 people are shot but only 2 dies, the incident is not a mass shooting. Homicides by other means also are not counted. If 5 people are purposely run down and killed by an individual driving motor vehicle, the deaths do not count because a firearm is not involved. There also are inclusions that can seem curious because the motives of perpetrators are not considered when defining a mass shooting.
For example, multiple homicides that result from an armed robbery gone bad are included. So are multiple homicides that result from turf wars between rival drug gangs. The heterogeneous nature of mass shootings needs to be unpacked as well. There are important differences between mass shootings in schools, places of worship, business establishments, outdoor rock concerts, private residences, and other settings. At the very least, there is reason to suspect that each is characterized by different kinds of motives.
Differences in how mass shootings are defined make it difficult to arrive a consensus about the number of victims or the kinds of incidents that are more common. A very rough estimate is that over the past decade, there have been about 40 deaths per year. Virtually all perpetrators were male (just as in most violent crime). Mass shootings associated with intimate partner violence apparently were the most common type. An estranged husband, for instance, kills his wife, their children, and perhaps her parents. There also is some indication that the number of mass shooting deaths has been increasing over time. The increase seems to result from greater lethality per incident, not a greater frequency of mass shootings.
What might be done? Some claim that mass shooting perpetrators suffer from severe forms of mental illness. What we need are better mental health services. However, many shooters do not survive the shooting incident, and there is often very little earlier information about their mental health. In addition, the vast majority of people in need of mental health services pose no threat of violence, let alone of committing a mass shooting. We have, therefore, no evidence one way or the other that mental illness is at the heart of most mass shootings.
But even if mental illness were a key factor, prospective mass shooters would need to already have been receiving mental health services for their hostile intentions to be identified. In the past, at least, most mass shooting perpetrators were not receiving such services. Perhaps the most promising venue for mental health interventions is high schools, where regular contact with counselors could be universal. However, there are a host of cost and privacy complications, and a very large number of false positives is a likely result.
Others claim that the problem is easy access to firearms, especially semi-automatic handguns and assault rifles. The United States, just like all countries, has a large number of individuals who for many reasons are prone to violence. Lethality from semi-automatic firearms can turn a brawl into a mass shooting. The 2nd amendment, coupled with the sheer number of semi-automatic weapons throughout the country, make gun control options very challenging.
Even well designed and implemented background checks only can work if prospective mass shooters have disqualifying attributes. One important instance may be perpetrators convicted of intimate partner violence or who are under a court order prohibiting possession of a firearm. More surgical interventions, such as banning high capacity magazines, may be a better approach in general.
Still, others claim that “target hardening” is the answer. For institutions such as high schools, target hardening in principle might help. But that means determining exactly what target hardening entails and what works, both informed by real evidence, not by sales pitches from security firms or fact-free ideological assertions. There are also major challenges in scaling up to the approximately 20,000 high schools in the United States, the vast majority of which will not experience, and are not in danger of ever experiencing a mass shooting.
High school students are far more likely to die in a fatal, automobile accidents than to be killed in a school mass shooting. For other venues, such as business establishments, shopping centers, outdoor concerts and places of worship, the challenges are greater, with the most difficult setting being private residences. What could target hardening mean there?
There is yet another possibility. Mass shooters need to prepare. They require at least one, and often more than one, semi-automatic firearm and many rounds of ammunition. Some acquire bullet-resistant vests. There also has been a tendency to broadcast motives, intentions, and even exact targets on social media. These indicators often materialize shortly before a mass shooting is undertaken and can, in principle, can be monitored. For example, a victim of intimate partner violence may be able to alert police or providers of domestic violence support services that her husband/boyfriend has credibly threatened fatal violence. Various kinds of surveillance can then be undertaken, assuming that criminal justice agencies (i.e., police, prosecutors, courts) properly follow through.
Each homicide is a tragedy, but deaths from mass shootings should be understood within the broader context of gun violence in the United States. There were 351 homicides in Philadelphia in 2018, the vast majority of which were deaths from firearms. That is over 8 times more deaths than for all of the mass shooting homicides in that year. Also in 2018, over 120 Philadelphia of children school age or younger were shot while not in school. Most survived, but the number of shootings for that year exceeded the number of mass school shooting victims across the entire country, including those who survived their wounds. And Philadelphia is not an outlier.