Researchers say they believe this is the manifestation of decades of American gun policy debate, in which gun rights proponents are better organized, better funded and more active in the political process than gun control advocates.

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“Ultimately, the research suggests that mass shootings do, tragically, create policy windows where change is possible and likely,” study co-author Deepak Malhotra said via email. “The evidence to date suggests that Republicans who want fewer restrictions on guns have been the ones taking advantage of these situations.”

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Researchers compiled data of mass shootings in the United States from 1989 to 2014. The analysis defined a mass shooting as “an incident in which four or more people, other than the perpetrator(s), are unlawfully killed with a firearm in a single, continuous incident that is not related to gangs, drugs, or other criminal activity.” They restricted the sample to include only shootings in which “at least three of the fatalities were individuals unrelated to, and not romantically involved with, the shooter(s).”

They also collected data on the 20,409 gun bills introduced in legislatures during period, and on the 3,199 proposals that eventually became law. They classified each piece of legislation according to whether it restricted gun access (for instance, by implementing stricter background checks or banning certain types of weapons) or loosened access (such as eliminating concealed-carry requirements or allowing guns to be brought into places where they were previously prohibited). Laws that did both, or which were otherwise neutral or unclear, were excluded from the analysis.

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They controlled for external factors that could affect the introduction of gun bills, like the legislative calendar (bills are more likely to be introduced in the first year of a two-year session), as well as economic and demographic factors like unemployment and divorce rates.

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The researchers found “the annual number of laws that loosen gun restrictions doubles in the year following a mass shooting in states with Republican-controlled legislatures.” Mass shootings in states where Democrats held the majority, on the other hand, didn’t appear to elicit any significant policy response in either direction.

The researchers explain these findings by noting that “Republican voters: (a) tend to be in favor of expanding gun rights and access to guns; (b) often argue that such actions reduce gun crime, and (c) are more likely than Democratic voters (during our sample period) to mobilize for political action on this issue.”

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Another explanation for this asymmetry can be found in recent political science literature showing that lawmakers have a poor understanding of what their constituents want when it comes to gun policy. Several studies have shown both Democrats and Republicans believe their constituents favor much more conservative policies than they actually do. One possible reason is the disproportionate influence of business-oriented lobbying groups on the lawmaking agenda.

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There are some indications that the lopsided policymaking landscape of gun control is changing. In recent years, gun-control advocates have had considerable success in mobilizing their own supporters and closing some of the intensity gap with gun rights advocates. Much of that mobilization has roots in the reaction to the mass shooting at Marjory Stoneman Douglas High School in Parkland, Fla., when survivors of the violence made a sustained push for policy changes.

Political concerns aside, there’s the separate issue of the consensus among gun-violence researchers that loosening gun restrictions leads to more violence. That was made clear in a 2018 RAND Corporation analysis of the balance of evidence uncovered by gun policy research done in recent decades.

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That analysis found evidence that a number of the permissive policies favored by the National Rifle Association and its allies, like stand-your-ground laws and permitless concealed-carry regulations, actually increase homicides and violent crime. Policies like strict background checks and minimum age requirements, meanwhile, appear to reduce gun violence.