Before even beginning a dive into the possible causes of mass shootings, I want to make it abundantly clear that mass shootings are a much different animal than gun violence overall. Mass shootings are relatively rare compared to gun homicides, and as such ought to be discussed as a unique phenomenon. In this post I hope to address some of the most frequently asked questions regarding mass shootings and their causes. Specifically, I hope to focus on one or two misconceptions, and discuss things that don’t do a good job of explaining why mass shootings occur.
Are mass shootings more frequent today than in the past?
Nope. Definitions matter when answering this question, specifically whether it is “mass shootings” or “public mass shootings” that are being examined. When discussing all incidents in which a shooter claimed the lives of four or more persons (the typically accepted definition of a “mass shooting”), the total number of mass shootings that have occurred each year over the last four decades has been remarkably stable (1), despite a growing population (See Figure 1), indicating that the rate at which all mass shootings occur, has actually been declining.
On the other hand, when factoring out shootings related to “terrorism, drug-trafficking, gang activity, and domestic violence,” it appears there has been an increase in the rate of public mass shootings since the turn of the century. Importantly, however, when we go back further, and examine public mass shooting rates per 100 million in the 80s and 90s, they are comparable to today’s rates (2). (See Figure 2)
That said, the apparent consistency of public mass shootings is striking, given that overall rates of gun homicide have significantly decreased since the 1980s. By that measure, the rate of mass public shootings has increased relative to the rate of gun homicides, though due only to the decrease in the rate of those homicides.
Are mass shootings more deadly today than in the past?
It can certainly seems that way.
Using data collected and maintained by Stanford University’s “Mass Shootings in America” project, I looked at the average and median number of all mass shooting fatalities per decade over the last five decades (See Figure 3). I also looked at the average and median number of all mass shooting victims (those killed or injured) per decade over the same time period (See Figure 4)
The data used comes with some important caveats. The Stanford data only goes through 2016, meaning that the “10s” columns don’t represent an entire decade worth of shootings, and more importantly, they don’t include incidents like the Las Vegas shooting last year, or the Parkland shooting earlier this year. Based on the data, however, median deaths per mass shooting do appear to be trending upward, even as the median number of total victims has been decreasing.
The number of victims (injured and killed) in mass public shootings appear to be on the rise as well. Again, data only shows through 2016, but the average number of victims per mass public shooting is at historic highs (2) (See Figure 5). It’s worth noting that, as of now, all five of the deadliest mass shootings in U.S. history have occurred since 2007, and three have occurred in just the last two years (4).
Do bans on “assault” weapons correlate with less deadly shootings?
The evidence we have (though admittedly limited) would seem to indicate that, at least in the case of mass public shootings, they do. For instance, when re-examining Figure 5, there appears to be a dip in the average number of victims in public mass shootings, one that coincides with the period during which the Federal Assault Weapons Ban was in effect. Interestingly, the year after the ban lapsed, the average number of victims in public shootings immediately began to increase, and has been trending upwards ever since.
One 2015 study set out to specifically test for a relationship between state and federal assault weapons bans and deaths in mass public shootings. Using 30 years of data, the study found:
…that both state and federal assault weapons bans have statistically significant and negative effects on mass shooting fatalities but that only the federal assault weapons ban had a negative effect on mass shooting injuries. (5)
Another study published in 2017 found the following:
… Assault weapons and other high-capacity semiautomatics appear to be used in a higher share of firearm mass murders (up to 57% intotal), though data on this issue are very limited. Trend analyses also indicate that high-capacity semiautomatics have grown from 33 to 112% as a share of crime guns since the expiration of the federal ban—a trend that has coincided with recent growth in shootings nationwide. (6)
Some may be quick to point out that these findings are merely correlative. That’s a fair point. But the fact that 4 out of 5 of the deadliest mass shootings in US history have involved shooters using semi-automatic “assault” style weapons should give anyone pause.
Does mental illness correlate with the occurrence of mass shootings?
The answer to this question is “not really,” and certainly not in the broad way it’s often spoken of (as though people with a mental illness are the main reason mass shootings occur). It does appear to be true that in instances of mass murder, that the percentage of perpetrators suffering from a severe mental illness is larger than the percentage of persons with severe mental illness in the general population(7,8). That said, shooters with confirmed psychotic disorders are found in higher rates among the perpetrators of mass public shootings, where they still account for less than 15% of all shooters (9). While some shooters may have a history of severe mental illness, mental illness alone cannot explain why mass shootings are perpetrated almost exclusively by males, and why so many more happen in the United States than in other countries around the world.
For instance, females in the U.S. suffer from severe mental illness at higher rates than males do (8), but perpetrate less than 10% of mass shootings (3); and while rates of severe mental illness in the U.S. are similar to those in other countries(10), the U.S. has a mass shooting rate that is substantially higher than those found in all but one other county (11).
It should also be noted that violence of any kind among those suffering from severe mental illness, is typically exacerbated by drug abuse (12).
All this to say that if mental illness is correlated with the occurrence of mass shootings in a statistically significant way, it’s effects appear to be much smaller than people tend to think (13), and mediated by both drug abuse and gun access.
Does bullying correlate with the occurrence of mass shootings?
There’s not really much of a case for this, but the argument gained some attention in the wake of the Parkland, Florida shooting. In any case, it depends again on how you look at it.
As has already been noted, public mass shooting rates are at similar levels now as in the 80s and 90s, meaning that if bullying rates were a good explanatory variable for public mass shootings, we would expect bullying rates to also be similar to those of the 80s and 90s. While we don’t have reliable data from the 80s, the data going back to the early 90s show that bullying rates fell by more than 70% between 1992 and 2010 (14) (See Figure 6), and that they have continued to fall since then (15). There is also some data to show that while bullying has decreased, weapon carrying at schools has increased among some segments of the population(16).
These data suggest that there is no relationship between bullying and public mass shootings (at least not on a national level), but what about other mass shootings? What about the ones that occur in the context of gang activity, drug trafficking, or domestic violence? Does bullying cause those shootings? There don’t appear to be any studies on that particular question, so some inferences will need to be made, inferences that should definitely, definitely be taken with rather large grains of salt.
There is evidence to substantiate the claim that those who are bullied are more likely to commit crimes than those who aren’t (17). That said, actual bullies still appear to be significantly more likely to commit crimes and engage in risky behavior than victims of bullying (18) (See Figure 7).
Figure 7: Outcomes by bully/bullied type
These results hold when looking specifically at the relationship between bullying and the perpetration of violence: i.e. while both bullies and victims of bullying show an increased tendency to be violent when compared to others, this tendency is markedly larger among bullies than victims (19). Bullies are also significantly more likely than non-bullies to perpetrate intimate-partner violence (20), a variable connected to “about a fifth of mass public shootings and arguably nearly all of the familicide mass shootings” (upwards of 50% of mass shootings overall) (21).
This evidence suggests that while bullying of any kind may increase the odds of a mass shooting occurring, it is bullies, not their victims, who are at the highest risk of perpetration.
Does “the cultural decline of society” correlate with the occurrence of mass shootings?
A lot of things could conceivably be used to measure “cultural decline.” Let’s look at a few.
Some people suggest that mass shootings in the U.S. are the result of religion’s declining role in society. This theory implies that countries that are less religious than the U.S. should experience more mass shootings. In reality, whether measuring religious affiliation (22), Christian affiliation (23), or overall religiosity (24), the U.S. is comparable to or more religious than several developed nations, but still experiences mass shootings at a significantly higher rate than any country but Yemen. (11).
Others have suggested that mass shootings are a product of the decline of the nuclear family. One way to measure the decline of the nuclear family is to measure the percentage of children born outside of marriage. It is an uncontested fact that the rate of these births has increased substantially since the 80s (25) (See Figure 8).
Pornography consumption has also increased since the 80s (26), as has the use of violent video games (27). All three of these increases conflict with the “society’s decline” theory of mass shootings. Indeed, if any relationship exists between these measures and the occurrence of mass shootings, it would seem that these measures of “societal decay” are actually making mass shootings less frequent.
What you should take away:
- The rate of public mass shootings today is similar to that of the 80s and 90s, not larger.
- The rate of all mass shootings today has decreased since the 80s.
- Mass shootings have gotten more deadly over time.
- “Assault Weapon” bans correlate with less deadly shootings.
- Mental illness, bullying, and various measures of “cultural decline” do not provide satisfactory explanations for the cause of mass shootings; and to the extent that they offer any explanation, those explanations largely contradict popular narratives.
Fox, J. A., & DeLateur, M. J. (2014). Mass shootings in America: moving beyond Newtown. Homicide studies, 18(1), 125-145.
- Duwe, G. (2017). Mass Shootings are Getting Deadlier, Not More Frequent. Politico.
- Stanford Mass Shootings in America Project. Accessed March, 2018.
- Ahmed, S. (2018). Parkland Shooting is Now Among the 10 Deadliest Mass Shootings in Modern US History. CNN.
- Gius, M. (2015). The impact of state and federal assault weapons bans on public mass shootings. Applied Economics Letters, 22(4), 281-284.
- Koper, C. S., Johnson, W. D., Nichols, J. L., Ayers, A., & Mullins, N. (2017). Criminal use of assault weapons and high-capacity semiautomatic firearms: an updated examination of local and national sources. Journal of urban health, 1-9.
- Stone, M. H. (2015). Mass murder, mental illness, and men. Violence and gender, 2(1), 51-86.
- National Institute of Public Health. Mental Health Statistics. Accessed April 2, 2018.
- Fox, J. A., & Fridel, E. E. (2016). The tenuous connections involving mass shootings, mental illness, and gun laws. Violence and gender, 3(1), 14-19.
- Kessler, R. C., Aguilar-Gaxiola, S., Alonso, J., Chatterji, S., Lee, S., Ormel, J., … & Wang, P. S. (2009). The global burden of mental disorders: an update from the WHO World Mental Health (WMH) surveys. Epidemiology and Psychiatric Sciences, 18(1), 23-33.
- Fisher, M. and J. Keller. Nov. 2017. “What Explains U.S. Mass Shootings? International Comparisons Suggest an Answer.” New York Times.
- Swanson, J. W., McGinty, E. E., Fazel, S., & Mays, V. M. (2015). Mental illness and reduction of gun violence and suicide: bringing epidemiologic research to policy. Annals of epidemiology, 25(5), 366-376.
- Olmsted, M. Feb. 2018. “More Americans Blame Mass Shootings on Mental Health Than on Gun Laws, New Poll Finds.” Slate.
- Finkelhor, D. (2014). Trends in Bullying and Peer Victimization. University of New Hampshire.
- Waasdorp, T. E., Pas, E. T., Zablotsky, B., & Bradshaw, C. P. (2017). Ten-year trends in bullying and related attitudes among 4th-to 12th-graders. Pediatrics, e20162615.
- Perlus, J. G., Brooks-Russell, A., Wang, J., & Iannotti, R. J. (2014). Trends in bullying, physical fighting, and weapon carrying among 6th-through 10th-grade students from 1998 to 2010: findings from a national study. American journal of public health, 104(6), 1100-1106.
- Turner, M.G. (2013) Repeat Bully Victimizations and Legal Outcomes in a National Sample: The Impact Over the Life Course.
- Wolke, D., Copeland, W. E., Angold, A., & Costello, E. J. (2013). Impact of bullying in childhood on adult health, wealth, crime, and social outcomes. Psychological science, 24(10), 1958-1970.
- Ttofi, M. M., Farrington, D. P., & Lösel, F. (2012). School bullying as a predictor of violence later in life: A systematic review and meta-analysis of prospective longitudinal studies. Aggression and Violent Behavior, 17(5), 405-418.
- Falb, K. L., McCauley, H. L., Decker, M. R., Gupta, J., Raj, A., & Silverman, J. G. (2011). School bullying perpetration and other childhood risk factors as predictors of adult intimate partner violence perpetration. Archives of pediatrics & adolescent medicine, 165(10), 890-894.
- Krouse, W.J. and Richardsen, D.J. (2015). Mass Murder with Firearms: Incidents and Victims, 1999-2013. Congressional Research Service.
- “List of Countries by Irreligion.” Wikipedia. Accessed April 10, 2018.
- “Christianity by Country.” Wikipedia. Accessed April 10, 2018.
- “Importance of Religion by Country.” Wikipedia. Accessed April 10, 2018.
- “Births to Unmarried Women.” Child Trends. Accessed April 12, 2018.
- Perry, S. L., & Schleifer, C. (2017). Race and Trends in Pornography Viewership, 1973–2016: Examining the Moderating Roles of Gender and Religion. The Journal of Sex Research, 1-12.
- Cunningham, S., Engelstätter, B., & Ward, M. R. (2016). Violent video games and violent crime. Southern Economic Journal, 82(4), 1247-1265.