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It’s been less than two years since a gunman shot 48 people dead at a nightclub in Orlando, and already that’s been eclipsed as the deadliest mass shooting in recent American history by an equally senseless, preventable, tragic mass shooting in Las Vegas that killed 59 people. In that time, and in the years before it—with mass shootings in Sandy Hook, Charleston, Aurora, and elsewhere—Congress has done nothing to make guns safer. In the wake of Las Vegas, Republicans are sticking to a similar script, offering their thoughts and prayers while they silently work to pass laws that make guns more dangerous and more readily accessible. However, there’s another issue holding back progress on gun control: it’s hard to know what reforms will be effective in curbing gun-related fatalities because, for the past two decades, the federal government has actively stifled research on gun violence.

In 1996, after a CDC-backed study linked gun ownership with a higher risk of homicide, the NRA accused the organization of promoting gun control. NRA-supporting Republicans in Congress threatened the Centers for Disease Control and Prevention’s funding, diverting $2.6 million away from the CDC’s gun research. Congress then passed the Dickey Amendment, which states that “none of the funds made available for injury prevention and control at the Centers for Disease Control and Prevention (CDC) may be used to advocate or promote gun control.” While the amendment isn’t an outright ban on funding research on gun violence, it serves as a powerful, government-backed deterrent against researching gun violence.

STATNews reports that the NIH has “awarded far fewer grants to gun violence research than to the study of rare diseases in the U.S. like cholera, polio, and rabies during that period, according to researchers.” And in late 2016, the Journal of the American Medical Association published a letter by Stanford University researchers that noted only $22 million in federal funds was devoted to gun violence research between 2004 and 2015 and concluded that, “In relation to mortality rates, gun violence research was the least-researched cause of death.”

After the Sandy Hook massacre, in 2013, President Obama signed an executive order asking health agencies to “conduct or sponsor” research on curbing gun violence, but bills seeking to fund the CDC’s gun violence research failed and the organization refused to pursue the studies without designated funding. And while the NIH pumped $18 million into 22 gun violence-related projects since then, the projects have come to a halt under the Trump administration. Their renewal is still “under consideration,” according to an NIH spokesperson.

“If research on cancer were stopped for a single day, there would be a huge protest. But this research has been stopped for 20 years,” Dr. Mark Rosenberg, who used to lead the CDC’s gun violence research, told the Los Angeles Times last June. And, so long as research is stifled, the debate on gun violence can’t move forward. In an interview with the Washington Post in 2015—which profiles Rosenberg’s unlikely relationship with Dickey Amendment author, former Rep. Jay Dickey—Rosenberg described the consequence, “In the area of what works to prevent shootings, we know almost nothing.”