Is the so-called "gun show loophole" dangerous? According to this study appearing in the Annals of Internal Medicine, the answer is yes, if only modestly.

For the video version, click here.

But to really understand what's going on, let's look at the methods this study uses, because it’s not entirely straight-forward.

OK first things first.  The "gun show loophole" is a bit more complicated than it sounds. Federal law states that anyone buying a gun from a licensed dealer needs to undergo a background check, regardless of where they purchase the gun. But private sales are not subject to those federal rules, they are governed by state laws and most states (in red on this map) do not require background checks.

Gun shows are a convenient place for private gun sales to take place, but not the only place.

OK – so the concern is as follows: what if gun shows lead to an influx of guns in a local area, procured under less stringent guidelines, which would then subsequently be used malevolently?

To test that effect, researcher Ellicott Matthay and colleagues used a convenient natural experiment. California has strict state laws regarding gun shows – all private sales must proceed through a licensed dealer, who will perform a background check, and the purchaser will receive the gun after a 10-day waiting period. In Nevada – just next door – no background check, no waiting period.

The researchers examined the rates of gun-related injury and death in areas of California close to a California gun show. They looked at the two weeks before the gun show, and compared that to the two weeks after the gun show. Would the show lead to a spike in gun violence in the local area?

In short? No. No effect.

Gun shows in California were not associated with a local increase in gun violence, even after accounting for the 10-day waiting period.

But the plot thickened when the researchers looked at those free-wheeling Nevada gun shows.

Comparing the two-weeks before to the two-weeks after a Nevada gun show, gun violence increased significantly.

The intuitive explanation here is that California's laws are serving to mitigate gun violence after gun shows.

But we need to be really careful with conclusions like that. For one thing, the areas near a California gun show might be different than the areas near a Nevada gun show.

After all, the rates of gun violence near Nevada gun shows were lower than in areas near California gun shows even after the uptick associated with the show. Does this suggest that, overall, Nevada’s free-wheeling gun show regulation decreases violence, albeit at a price of occasional surges in violence? Isn’t that just the plot to The Purge?

But look, gun-violence really did go up in the two weeks after a Nevada gun show. Non-gun violence didn't go up, by the way – a nice negative control. So it seems that there is something different about Nevada vs. California gun shows. Assuming that something different is just the law, though, is a bit of a shot in the dark.