Does gun control help reduce gun deaths? It's a crucially important question in light of the horrible news out of Texas, but even for PhDs, it's a tough one. There's been a mountain of research on the subject, but these dozens of studies conducted over many years and in many different countries reach a broad and sometimes contradictory range of conclusions. It's hard to know what it really tells us, taken together, about whether gun laws can reduce gun violence.

A 2016 study, published in the academic journal Epidemiologic Reviews, seeks to resolve this problem. It systematically reviewed the evidence from around the world on gun laws and gun violence, looking to see if the best studies come to similar conclusions. It was the first such study to look at the international research in this way.

The authors are careful to note that their findings do not conclusively prove that gun restrictions reduce gun deaths. However, they did find a compelling trend whereby new restrictions on gun purchasing and ownership tended to be followed by a decline in gun deaths.

"Across countries, instead of seeing an increase in the homicide rate, we saw a reduction," Julian Santaella-Tenorio, a doctoral student in epidemiology at Columbia University and the study's lead author, told me in an interview shortly after publication.

What the study found

Santaella-Tenorio's study (co-authored with Columbia professors Magdalena Cerdá and Sandro Galea, as well as the University of North Carolina's Andrés Villaveces) examined roughly 130 studies that had been conducted in 10 different countries. Each of those 130 studies had looked at some specific change in gun laws and its effect on homicide and/or suicide rates. Most of them looked at law changes in the developed world, such as the US, Australia, and Austria, while a few looked at gun laws in developing countries, specifically Brazil and South Africa.

This isn't, then, a study that compiled its own original data on one specific gun law. It's actually more valuable than that: It's telling us what all the different studies on individual laws say when you examine them put together.

So what do Santaella-Tenorio et al. conclude? First, and most importantly, that gun violence declined after countries pass a raft of gun laws at the same time: "The simultaneous implementation of laws targeting multiple firearms restrictions is associated with reductions in firearm deaths," the study finds.

This finding doesn't highlight one specific law, like an assault weapon ban, in isolation. There were "so many different kinds of laws," Santaella-Tenorio told me, that it was hard to make good international comparisons on every specific kind of gun restriction.

Rather, countries passed big packages of gun laws, which overhauled the nation's firearm code fairly broadly, which all tended to share similar features. According to Santaella-Tenorio, they generally included:

Banning powerful weapons, like automatic rifles.

Implementing a background check system.

Requiring people to get permits and licenses before buying a gun.

South Africa's comprehensive Firearm Control Act, passed in 2000, contained all these measures. One study found that firearm homicides in five major South African cities decreased by 13.6 percent per year for the next five years.

"Reductions in nonfirearm homicides were also observed," Santaella-Tenorio et al. note, "although not as pronounced as the ones observed for firearm homicides."

Austria's 1997 firearm law, similarly, required background checks, limited access to powerful firearms, and imposed rules about how gun owners had to store their guns. Santaella-Tenorio reviewed two studies on Austria's 1997 law, both of which found evidence that the law had reduced deaths. According to one of them, firearm homicides went down by 4.8 percent, while suicides went down by 9.9 percent.

Australia's 1996 National Firearms Agreement (which outright confiscated 650,000 guns, in addition to imposing background checks and licensing rules) is perhaps the best-studied of any of the international laws. Santaella-Tenorio et al. reviewed eight studies on it, most of which found clear and strong evidence of a reduction in firearm deaths after the law's passage.

One study, for example, compared the Australian state of Victoria to others around the country. Victoria had passed a raft of firearm restrictions in 1988, so the NFA didn't change policy there that much. But firearm deaths went down countrywide by an average of 14 percent in other states relative to Victoria, suggesting that the NFA provisions specifically had made the difference.

Santaella-Tenorio and his co-authors also found evidence that specific laws, such as background checks and rules on storage, reduced specific kinds of gun deaths.

"Laws restricting the purchase of (e.g., background checks) and access to (e.g., safer storage) firearms," they write, "are also associated with lower rates of intimate partner homicides and firearm unintentional deaths in children, respectively."

What they learned about guns in America

Santaella-Tenorio and his colleagues looked at a number of studies on gun control in the United States as part of their overall review. There was strong evidence that that restricting access to guns tends to reduce gun deaths.

One study, for example, looked at Missouri's 2007 repeal of its law requiring a permit to purchase a firearm, which in effect repealed the state's background check requirement. This study found that after 2007, Missouri's homicide rate jumped by 25 percent. No other changes in law or circumstance appear to be able to explain the increase.

By contrast, laws favored by the National Rifle Association (such as concealed carry or stand your ground), when implemented, either had no effect on gun deaths or increased gun violence. And Santaella-Tenorio found this by considering not just studies that reached this conclusion, but also studies that supported loosening gun laws.

Most of the studies that supported these laws were written by a handful of authors, like Florida State's Gary Kleck and Fox News columnist John Lott. Scholars who reexamined their conclusions, sometimes even using their own data, generally came to the opposite results.

For instance, a study by Lott and the University of Georgia's David Mustard found that laws permitting concealed carry reduced homicides in states and counties that passed them. However, a subsequent study that reexamined the same data found no effect, and that Lott and Mustard had used flawed statistical procedures in examining the data.

Another Lott study found that castle doctrine laws — laws that eliminate the legal duty to retreat before using deadly force, but only in your home — reduced homicides by 9 percent. But a second study came to the opposite conclusion, finding an uptick in homicides after states passed such laws.

A third study looking at stand-your-ground laws — which go beyond castle doctrine in eliminating retreat duties outside the home as well — also found an uptick in deaths. Santaella-Tenorio and his coauthors summarize: "Stand your ground laws were associated with a 6.8% increase in homicide rates, mainly driven by increments (14.7%) in homicide rates among white males."

The point, then, is that the pro-gun studies tended to be outliers in the literature, and were not supported by the most rigorous available analysis.

"Our goal was just to see what was out there, and identify the quality of the studies," Santaella-Tenorio told me. "We eventually found that many others had used Lott's data, and they have found different results after adjusting for other variables, or using more years of data, or using different models."

This isn't conclusive, but it's powerful evidence

In our conversation, Santaella-Tenorio was insistent that he and his colleagues have not "proven" that gun laws reduce violence. The data, he says, is simply too complicated, and the analyses too primitive, to come to such a hard conclusion.

"It's really hard. In epidemiological studies — for example, pharmaceutical ones — you can randomly select your groups, and then have one group exposed to the pill and the other group not exposed," he says. "However, in policy studies ... you can't have some states be exposed to legislation and other states not."

This helps explain some unusual results. For instance, some data from Quebec found that a Canadian law reducing access to firearms led to an increase in suicides by hanging — a large enough increase to offset the decline in suicides by firearm that followed the law. Other studies, from Australia and New Zealand, found a similar substitution effect.

However, there is very good evidence — some of it from the same countries — that reducing access to guns reduces overall suicides. Indeed, Santaella-Tenorio himself believes that despite those three studies, limiting access to guns is very likely to reduce suicide rates overall.

"There's some other evidence that we didn't include in this review," he says, that finds attempting suicide is an impulsive decision that people regret (if they fail) and thus don't repeat. Firearms, because they're much more effective than taking pills or slashing your wrists, don't give people that option. Thus, reducing access to guns should (and empirically generally does) reduce the overall suicide rate.

This illustrates, then, that the findings in this study don't end the academic debate over guns. It's limited by study design — it only reviewed studies on firearm policy changes, not firearm ownership in general — and the inherent limitations in studying the effect of complicated public policy issues.

Nonetheless, this is a very important contribution to the gun debate. About 130 studies, from 10 different countries, converged on the idea gun deaths declined after laws restricting access to firearms went into force.

While it's not conclusive, it is very, very suggestive.