How the expiration of the assault weapon ban affected Mexico

September 2, 2010
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(This article was first published on Diego Valle's Blog, and kindly contributed to R-bloggers)

There has been a lot of attention paid to the role of US guns exacerbating the violence in Mexico. The assault-weapon ban expired on September 14, 2004, but with the recent spiraling of violence in Mexico the ban has attracted renewed attention. Just recently the Mexican President stood before the American Congress and blamed the assault weapon ban for the rising violence in Mexico, seemingly without proof. This post will try to clarify some of the issues surrounding the controversy.

I think it would be fair to say that the conventional wisdom by scholars who have studied the ban would be the one Christopher Koper, Daniel Woods and Jeffrey Roth stated: "We cannot clearly credit the ban with any of the nation’s recent drop in gun violence." In other words, the assault weapon ban had no significant effect—at least where the United States is concerned. And as far as I know there are no studies of the effect of the ban expiration on Mexico.

As is well known, gun control is politically charged issue, not only in the United States, but also in Mexico. For example, after two students died in a shootout between the army and cartel gunmen, the rector of what is arguably Mexico's best university stated:
For me [Rangel Sostmann] the solution is that Mexico and the United States must change their policies. And the policy right now is for the United States to send the money and weapons, and Mexico puts the dead and the drugs. And that ultimately is not going to work, it's not a question of driving the Army out, the problem is that you have to change the policies, in this case the United States policy.
—Rangel Sostmann, ITESM Campus Rector
Later an investigation by the National Human Rights Commission concluded that the army had used excessive force, impeded the investigation, planted weapons on the students, possibly tortured them for several minutes, and then shot them at point blank range. The day after the report was released, the ITESM bestowed upon the Secretary of Defense the "AdHonorem" award, given to persons whose works have contributed in an outstanding manner to the "the fight against injustice." As you can see gun control has become extremely politicized, to say the least.

With politics hopefully out of the way, let's look at the homicide data with a seasonal decomposition by loess:
As you can see from the charts there was an increase in the number of homicides and homicides with firearm that coincided with the expiration. The big drop at the start of 2007 was due to the military operations in Michoacan and Acapulco, and also to a temporary drop in the homicide rate of Mexico City (which had nothing to do with the drug war).

Since homicides and homicides with firearm can rise at the same time because the overall level of violence in society is increasing, say because of land reform or election disputes, we should also look at how the proportion of homicides with firearm has changed in Mexico:
It looks like the proportion of homicides by firearm started changing about a year after the expiration of the assault weapon ban. Mexico is a big country so it's worth looking at its different regions. There was a coding mistake in the homicide database (Chihuahua was classified as being in the North East), so I divided Mexico into its economic regions:
Notice how the proportion of homicides rose in North Eastern Mexico, though it would be fairer to say it was more of a temporary dip. Also, there was a big rise in South Central Mexico long after the expiration of the assault weapon ban, this is the most populous region of Mexico. In the South East there was a big increase in the proportion a month before the ban went into effect.

Now just the municipalities that border the United States:
The chart shows that the proportion of homicides commited with a firearm did change around the time of the expiration of the assault weapons ban, but well within the normal range. The proportion didn't rise outside the normal range until 2008. But remember how only the North East had a rise in the proportion of homicides commited with a firearm?

Now, let's look at the big border cities:
Aha, now we see that in Nuevo Laredo the proportion of homicides commited with a firearm started rising at the same time the ban expired. It's also a good idea to look at the number of homicides by firearm instead of the proportion:
Wow, a big rise in homicides by firearm in Nuevo Laredo that coincided with the expiration. Plus, before the ban the number of monthly homicides was low, which would explain why the variance in the proportion chart was so much greater before the ban.

But before we conclude that it was due to the assault weapons ban we have to take into account that "La Barbie", working for the Sinaloa Cartel, was trying to take over the drug trafficking routes of Nuevo Laredo after the capture of Osiel Cárdenas, the leader of the Gulf Cartel, in March 2003. And that as a consequence of the violence Vicente Fox started Operation Secure Mexico in June 2005 and sent troops to Nuevo Laredo:
It looks like there was indeed an effect independent of the capture of Osiel Cardenas and the arrival of the army.

Now let's look at the proportion of homicides by firearm in each of the states of Mexico and the Federal District. I used the strucchange package to help visualize the breakpoints (even though it's not quite correct to run a linear regression on the data that's what I used):
And now the interesting states:
The drug trafficking states:
The changes in Guerrero and Durango are clearly not significant, and are in fact slight dips. Since the state of Tamaulipas shows no signs of the rise in violence in Nuevo Laredo, it must have been a localized phenomenon, at least in that state.

Of all the states I have to say that only Nuevo León and Chihuahua look especially interesting:
However in Chihuahua there's another big confounding variable in that the leader of the Sinaloa Cartel ended his alliance with the Juarez Cartel by killing Rodolfo Carrillo Fuentes, brother of the leader of the Juarez Cartel a couple of days before the ban expired.

Supposedly after "El Chapo" Guzmán escaped from prison he began to rebuild his drug empire and sometime thereafter entered into a non-aggression pact with the Juárez Cartel, but on Sep 11, 2004, he ended the alliance by killing the brother of the leader of the Juarez Cartel, since neither the proportion or the number of homicides committed with a firearm changed in Ciudad Juárez, my guess is that the breaking of the pact had more to do with the rise in violence than the assault weapon ban.

Nuevo León also experienced a rise in firearm violence:
Nuevo León is right next to Nuevo Laredo:
We can also look at the suicide and suicide with firearm rates for all of Mexico:
No effect. There's a very interesting seasonal pattern, and the fact that the suicide rate has been rising while suicide with firearm rate has been decreasing would go against the conventional wisdom that the availability of firearms increases suicides. This, of course, doesn't concern the effect of expiration of the ban on assault weapons so I'll leave it at that.

Before we conclude that the assault weapon ban affected Mexico we need a plausible mechanism for how assault weapons would benefit the drug cartels and clear up some confusions and misunderstandings regarding guns:
  • 90% of the guns in Mexico come from the US: Of those traced, about 90% came from the US. The keyword being traced. This isn't all guns that were confiscated in Mexico, not even a random sample, but a highly biased one. For example, according to GAO, 25% percent of traced guns were assault weapons, while according to the Mexican government 50% of seized guns were assault weapons. So it would be mistaken to draw inferences about where the total percentage of guns come from. However, given that thousands of guns have been traced to the US, it would be correct to say that the United States is one of the main suppliers of guns to Mexico.
  • Assault Weapons are "Military Grade": None of the assault weapons for sale to civilians in the United States are of military grade and with the exception of some collector items they are all semiautomatic. They deliver one round each time you pull the trigger. In a fully automatic weapon many bullets can be fired with one trigger press.
  • "Cop Killer" Guns: Some bullets can pass through body armor; however, they are not for sale to civilians. For some reason drug dealers took to calling the guns that can fire this kind of bullet "Cop Killers," even if they can only load them with regular bullets.

During the period in which the Assault Weapon Ban was in effect, the sale of any newly manufactured gun that met any of the following characteristics was prohibited:
  • Folding stock
  • Pistol grip
  • Bayonet mount
  • Flash suppressor
  • Guns specifically mentioned in the ban
The Assault Weapon Ban also prohibited magazines that held more than ten rounds. The law did not ban the possession or sale of pre-existing assault weapons or magazines. As a consequence pre-ban assault weapons and magazines rose in price until the ban ended.

In response to the ban, firearm manufacturers quickly retooled their guns so that they didn't meet the law's definition of assault weapon, and most of the banned features don't strike as particularly important. For example, I bet most guns smuggled to Mexico are disassembled and so a stock of any kind would be removed. And I've never heard of a cartel shootout that involved bayonet mounts.

However a recent analysis by the Texas Ranger's Border Security Operations Center highlights the importance of high capacity magazines.Unlike assault weapons, high-capacity magazines, which are used with many guns, have been selling briskly since the ban ended because prices have dropped considerably.
—The New York Times
It is very common to find live ammunition rounds strewn on the ground after drug cartel shootouts (examples: 1 2 3), reloading a gun when other people are shooting at you is not an easy task and cheap large capacity magazines are an advantage during long shootouts, like the ones that occured in Nuevo Laredo when "La Barbie" was trying to take it over from the Gulf Cartel. This would be one plausible mechanism through which drug cartels could have benefited from the expiration of the ban.

It's also a good idea to put the size of the effect (if indeed there was one) in the context of the drug war. Clearly they are separate processes:
Sadly for Nuevo Leon I have no recent data on the number of homicides by firearm, so instead I'm showing the overall number of homicides. The relatively small rise at the beginning of 2007 coincided with the start of the joint operation Tamaulipas-Nuevo León:
And for Nuevo Laredo I have no recent homicide data, which is a real pity since it wouldn't look ridiculously low, here's a chart comparing it to other border cities, you can see how it stands out in 2005 and 2006. During 2010, in Tamaulipas there were 338 drug related homicides during the first half of the year according to the Reforma newspaper, during all of 2005 there were a total of 351 homicides (all kinds). I can only surmise that the whole state of Tamaulipas and Nuevo Laredo are much more violent now than in 2005, even if the difference is not quite as stark as in other regions of Mexico.

I modeled the effect of the assault ban as a Zero-Inflated Poisson regression model predicting the number of homicides that occurred daily in Nuevo Laredo from the time Osiel Cárdenas was captured to the time the army arrived. The model with a breakpoint ocurring at the time the assault weapon ban expired proved superior to one without a breakpoint.

I think it is much easier to visualize the size of the effect with weekly data:
Certainly the assault weapon ban wasn't the only variable that contributed to the rise in violence: A weak and inept government, a large Mexican-American population willing to smuggle guns (Edgar Valdez Villareal "La Barbie" was born an raised in Texas), the corruption of the public security forces of Mexico, and the weakening of the Gulf Cartel with the capture of Osiel Cásrdenas also played a big part.

But the capture of Osiel Cardenas occured more than a year before the violence started, the army arrived more than six months after the violence started, there's a plausible advantage to having cheap high capacity magazines, and the rise in homicides coinicided with the expiration of the ban. I can only conclude that the expiration of the Assault Weapon Ban contributed to the violence in Nuevo Laredo with possible spillover effects in Nuevo León, and I've certainly updated my beliefs about the effect of the Assault Weapon Ban.

Clearly much work remains to be done, more complex models could be fitted to the data and I didn't answer all the questions of interest: I didn't perform a detailed analysis of the change in proportion of homicides by firearm in Nuevo Laredo or an analysis of what happened in Nuevo León, but given that "La Barbie", the guy in charge of taking over Nuevo Laredo, was caught on Monday, finding out the whether he took advantage of the expiration of the assault weapon ban should be a simple matter of asking him.

The code is here. (I used the excellent package Project Template to build the skeleton)

To leave a comment for the author, please follow the link and comment on his blog: Diego Valle's Blog.

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