There's this graph on the internets:

illegal and legal weapons in mass shootings (graph)

But I was unable to find any sources of the data. Most of the sites that present the graph seem to be politically oriented. Is this backed by any hard data?

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    If you created another graph showing "Mass shooting killers with previous criminal history", I'd guess it'd look a lot like the graph above. Jun 15, 2016 at 12:25
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    @QuestionMarks I can't tell. Can someone with criminal history legally have a gun in US? Either way, how is that relevant to the question? Jun 15, 2016 at 12:30
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    All the mass shootings I can remember were carried out by people with no previous criminal history, and criminal history is a big part of how its decided if someone can or can't purchase a gun. There's no obvious reason (before the shootings) to have prevented those individuals from buying the gun. Jun 15, 2016 at 13:11
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    @QuestionMarks I thought the law was that you already can't buy guns if you have committed a felony? The proposed changes I've read about have been things like tightening rules on people with mental health issues, people investigated or watched on suspicion of terrorist affiliation, people with histories of violence other than felony convictions, etc. Trying to understand the relevance of your comment. Jun 15, 2016 at 14:19
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    Please use Skeptics Chat unless suggesting improvements to the question
    – user30557
    Jun 15, 2016 at 14:54

2 Answers 2


The source of the graphic in the OP is Mother Jones.

This is what they consider a "mass shooting".

Our research has focused on seemingly indiscriminate rampages in public places resulting in four or more victims killed.

A Guide to Mass Shootings in America

Mother Jones considered a specific list of 56 US mass shootings starting with the 8/20/1982 machine shop shooting where the shooter was killed by civilian using a car to hit the shooter and run over his body and ending with the 7/20/2012 Aurora theater shootings.

Mother Jones originally reported that of these 56 shootings, the data is as shown in the OP, but updated/revised the data to say out of 62 shootings through 2012, in 49 cases the gun(s) was/were obtained legally, 12 illegally, and 1 unknown.

Looking at the data table spread sheet, in many cases where they acknowledge that it is "unknown" where the gun was obtained, they assume it was obtained legally.

Also the list is incomplete. For example it does not include the 01 March 1983 fatal shooting of 6 people by Louis D. Hastings in McCarthy, Alaska.

See 6 KILLED IN ALASKA IN SHOOTING SPREE New York Times March 3, 1983

For another source see Where'd They Get Their Guns, An Analysis of High Profile Shootings 1963-2001.

This report gives more-detailed explanations of the legality of the gun purchases in the first 20 years of the Mother Jones data set.

  • Good answer, one possible useful addition: what criteria did they use to select these cases? Jun 15, 2016 at 14:20
  • @user568458 in the original data set, the used the criteria that 4 people other than the shooter were kill in a single incident, but in the post 2012 data the mix in case with fewer people killed.
    – DavePhD
    Jun 15, 2016 at 14:22
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    @DavePhD I've added the definition in your answer so we can take it out of the equation.
    – Sklivvz
    Jun 15, 2016 at 14:36
  • @Sklivvz ok, but they stopped following that definition at some point, including 5 incidents with "only" 3 fatalities, all since 2014 and none before.
    – DavePhD
    Jun 15, 2016 at 14:46
  • The graph is up to 2012 though so, does it matter?
    – Sklivvz
    Jun 15, 2016 at 14:48

EDIT: Ignore comments, significant changes after discussion and change to question.

This appears to be based on the data in a Mother Jones data set: US Mass Shootings, 1982-2016

The graph is absolutely based on reasonably good data but the conclusions someone might draw from the graph alone is lacking context.

"Crimes primarily related to gang activity, armed robbery, or domestic violence in homes are not included."

There is a slight chance that these criteria might bias the fraction of illegally obtained weapons involved in the included cases.

The graph covers up to 2012 but motherjones expanded their dataset using the same criteria. We can compare to a database which doesn't have the same exclusion criteria

Data from Shooting Tracker tends to include far more cases for some of these time periods.

They use a very simple definition

Gun Violence Archive has always used the FBI derived definition:

FOUR or more shot and/or killed in a single event [incident], at the same general time and location, not including the shooter.

And do not exclude gang activities or robberies. So, for example, in 2014 motherjones include 4 events while gunviolencearchive includes 280.

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    @Sklivvz the list is incomplete because it does not include the 01 March 1983 fatal shooting of 6 people by Louis D. Hastings in McCarthy, Alaska. It's only a question of how incomplete the list is.
    – DavePhD
    Jun 15, 2016 at 15:36
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    @DavePhD I was going to include your example but it apparently is classified as a "spree" shooting, apparently if someone goes from house to house killing without a cooling off period that's a spree but doesn't kill 4 people in 1 place then it's not included in the motherjones data.
    – Murphy
    Jun 15, 2016 at 16:08
  • It wasn't house to house, it was 3 killed in 1 house and 3 outdoors. He was trying to kill all the people in the village (which was only about 12 people at the time) as they went to meet the mail plane which only came once a week.
    – DavePhD
    Jun 15, 2016 at 19:39
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    Note that the definition they claim is "FBI-derived" is very different from the FBI's own definition (which is much closer to that of Mother Jones.) The FBI (in their definition of "mass murder") and Mother Jones (in their definition of "mass shooting") consider only indiscriminate killings. By contrast, nearly all of the cases listed on the "shooting tracker" are domestic violence or gang violence, both of which have very different causes and patterns from the indiscriminate killings that most people think of when you say "mass shooting" or "mass murder."
    – reirab
    Jun 16, 2016 at 4:38

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