Big caveat up front: Sussing out cause from effect in the policy realm is a complicated task. Most of the studies we have on gun violence are able to describe associations -- after you pass X law, Y effect happened. But that doesn't necessarily tell you that Y happened because of X. Researchers can control for various other factors that might also cause Y to happen, like changes in economic situation or demographics, but you can't eliminate uncertainty completely.
Still, by looking at the totality of available research, like this analysis, you can usually get a reasonably good picture of the associations between laws and outcomes. Its No. 1 conclusion was that comprehensive gun legislation packages -- which include an array of different policy changes -- seem to be associated with reductions in gun deaths.
"The simultaneous implementation of laws targeting multiple elements of firearms regulations reduced firearm-related deaths in certain countries," Santaella-Tenoria and his colleagues conclude.
The research suggests that a package of various gun law reforms is better than the sum of its parts. For instance, Australia's National Firearms Agreement (NFA) of 1996 banned the ownership of certain types of guns, implemented a mandatory buyback program for some weapons and included provisions on background checks, licensing and mental health requirements. Overall, the research Santaella-Tenoria and his colleagues surveyed found that "following the implementation of the NFA, a decline in firearm deaths and firearm suicides, as well as an absence of mass shootings, occurred."
Similar effects were found after multi-pronged gun reforms were implemented in Brazil and South Africa.
But results were more mixed when it came to individual pieces of legislation. The United States's famous assault weapons ban? "No association between the law and homicide rates in 15 states," one study found. The research on concealed-carry laws is all over the place -- some has shown that more guns lead to more gun crime, while other research shows precisely the opposite. Studies also showed that voluntary gun buyback programs also appear to have little effect on homicide rates.
On the other hand, one particular policy intervention seems to have a profound effect on reducing accidental gun deaths among children -- so-called "child access prevention" laws, which mandate safe storage of firearms and penalize gun owners who leave firearms out in the reach of children. "Most studies in the United States show that additional laws allowing for felony prosecution of offenders are associated with greater reductions in unintentional deaths among children," the authors found.
Another area where the research was largely in agreement was gun background checks -- specifically, background checks that included checks on domestic violence restraining orders. "Studies on background checks suggest that the quality of systems used to review applicants, in terms of the access to local and federal information on mental health conditions and criminal and domestic violence history, is a critical component of these laws," the researchers found.
One key point that Santaella-Tenorio's team stressed is that the existing body of gun violence research is still largely incomplete. "Improving data on non-fatal gun-related injuries will give us a much better understanding of the scope of the impact of these laws," he told me in an email. "Current research focuses mostly on the impact of firearm laws on gun-related deaths only, not on non-fatal injuries."
Most research focuses only on gun-related deaths, since those tend to leave a big paper trail that experts can analyze later. But firearm-related injuries are at least twice as prevalent as fatalities, meaning that most of the existing research is blind to one of the biggest effects of gun violence.