Nearly every year since the TSA's inception, more guns have been found in carry-on luggage than the year before. The number increased 40 percent between 2012 and 2014 alone, and 2015 broke the record again with weeks to spare.

The big question: Why? This isn't just an increase in air travel; the number of airline passengers has been growing rather slowly in comparison. It also doesn't seem to be a rise in gun ownership, which is steady or even declining in surveys. (Gun sales are brisk, but the people buying the weapons mainly seem to be expanding their collections rather than becoming gun owners.)

One theory: More and more Americans have acquired permits to carry concealed weapons lately, and some small percentage of them are bringing their guns in their carry-on bags, intentionally or not, when they fly.

This theory came up during libertarian journalist Ari Armstrong's recent interviews with gun researchers Gary Kleck and John Lott. Kleck suggested that concealed-carry laws don't even increase the carrying of guns; instead, they just legalize what people were doing anyway. In response, Lott pointed out that the rising number of concealed-carry permits has roughly tracked the rise of guns found by the TSA, suggesting that permits do make a difference.

For his part, Armstrong wrote that he personally knows people who carried guns after getting their permits but not before. I do as well — in fact, I'm one of them. And a closer look at the TSA data reveals a stark pattern: More guns are being found in states with permissive concealed-carry laws, but not in states where it's hard to get a license.

TSA's gun finds are reported weekly on the agency's blog. The Medill National Security Journalism Initiative has done the yeoman's work of aggregating all this information into convenient spreadsheets for 2012, 2013, and 2014. (These data are slightly less accurate than the TSA's final year-end totals, but those totals are not broken down by airport.) I matched each airport in Medill's data to the state in which it is located, allowing me to tally up the guns found in each state and year.

Then I looked for a connection to concealed-carry policies. I combined all the states with reasonably permissive laws (as approved by the NRA) into a single unit. I excluded Illinois entirely, because its gun laws have been in upheaval over this time period. I grouped the eight states with stricter laws — California, Delaware, Hawaii, Massachusetts, Maryland, New Jersey, New York, and Rhode Island — plus D.C. together as well.

The places with strict laws are home to more than one-quarter of the U.S. population, but they accounted for just 8 percent of the guns TSA found, mostly in massive California. Of course, this isn't just carry laws at work — these states also have lower gun ownership and harsher penalties for people who run afoul of gun laws, even accidentally. (See New Jersey, especially, on that latter point.)

The trends are much more convincing. The stricter states accounted for 157 guns in 2012, 141 in 2013, and 165 in 2014 — a pattern consistent with randomness, with a decrease between the first two years. The states that grant permits liberally, by contrast, saw their gun count climb steadily and dramatically, from 1,334 to 1,652 to 1,945. Many of these states saw substantial increases individually as well. Texas's number rose from 292 to 417.

To be sure, you can come up with alternative explanations for this pattern that are hard to rule out. Maybe something crazy is going on in gun-loving red states. Or maybe carry permits make it more likely for people to bring guns to the airport — perhaps they (stupidly) think they're allowed to carry them on the plane — without affecting whether people carry in places where their belongings won't be searched. (TSA, however, says that "in many cases, people simply forgot they had these items.")

Yet the Occam's Razor explanation, one must admit, is that people are carrying guns more frequently because they have been licensed to do so.

Robert VerBruggen is editor of RealClearPolicy. Twitter: @RAVerBruggen



Notes: Here is my spreadsheet paring down Medill's data, here is a spreadsheet of airports and their locations (mostly from Expedia), and here is the R code I used to match and crunch the data. The R code also includes some bonus statistical comparisons between years. For convenience, I've made a spreadsheet with the totals by airport and year, and another with totals by state and year.

Ideally, we'd also look to see whether the states with the biggest permit increases also saw the biggest jumps in TSA guns, but appropriate data for this are hard to come by. Lott's organization, the Crime Prevention Research Center, keeps track of the best numbers available, but these are not always current and are not always collected at even annual intervals. These numbers may be usable with the proper adjustments and exclusions, but I'll leave that to others.