The recurrent, violent phenomenon of mass shootings, including recent attacks in California and Oregon, has fueled Americans’ anxieties and reinvigorated a tense national debate over gun control. As the debate intensifies, candidates on both sides of the 2016 presidential race are staking out strong policy stands on gun laws—and yet, gun violence is far more of a mystery than most people realize.

Evidence and research that could be used to develop effective laws that might decrease deaths and injuries from firearms is severely lacking. Why? It’s partly the result of longstanding restrictions on federally funded gun-violence research. In the mid-1990s, Congress declared that funding at the Centers for Disease Control and Prevention shouldn’t be used to advocate for gun control, and it effectively blocked funding for the study of gun violence at the agency. It wasn’t an outright prohibition, but the action had a very real chilling effect on research.

“It’s actually kind of appalling,” said Sherry Towers, a professor at Arizona State University who has done research on mass shootings. “We’re one of the richest nations in the world, and we aren’t exactly forbidding scientists to look at this, but the federal government is strongly discouraging it.”

Many basic questions remain largely unanswered as a result. It’s difficult, for example, to pin down the precise impact of specific gun laws—like laws that allow people to openly carry firearms. Do open-carry laws make gun violence worse, or do they cut down on firearm injuries and deaths? Researchers can’t say with certainty. They also don’t know much about the path that guns take in order to fall into the hands of criminals, or how gun laws impact firearm sales on the black market. For that matter, the psychology of gun violence is not well understood. What motivates people to use guns to commit a crime or suicide, and what are the most effective ways to stop mass shootings, gun-related homicide, and suicide? Limited research makes it challenging to reach well-supported conclusions.

“I think people assume that we have a lot more information than we really do when it comes to guns, and that’s definitely not the case,” said Daniel Webster, the director of Johns Hopkins’s Center for Gun Policy and Research. “We have precious little data.”

Of course, gun violence is not the only area of research where politics and science clash. Climate change is a contentious subject that proves evidence doesn’t eliminate controversy. In the case of climate change, research has yielded a scientific consensus that man-made global warming is a real threat. But that hasn’t stopped a political debate over the existence of climate change from raging on Capitol Hill. Still, the ability to cite that consensus helps frame the debate, making it possible for the media, and anyone else, to more easily discredit the arguments of politicians who deny the science. Data acts as a check on rhetoric that has become untethered from reality, and evidence informs the creation of effective policy to deal with the threat.

In contrast, a scarcity of statistics on gun violence allows the political debate over gun control to take place in a realm that is often largely separate from actual fact. A lack of research makes it difficult to know which laws and regulations would reduce injuries or deaths from firearms and under what circumstances they might do so. When it comes to talking about gun laws, politicians are, to some extent, operating in the dark. “It’s very concerning that there’s been an attack on science and the ability to carry that out,” said Fred Rivara, a professor at the University of Washington who helped conduct a study on guns in the home in the early 1990s. “It’s resulted in a sort of stalemate in terms of being able to develop effective policy.”

To be clear, it’s not that there isn’t any research. Private foundations have stepped up to try and fill the void of federal gun-violence research. After the Sandy Hook Elementary School shooting in 2012, President Obama called for the CDC to conduct research on the causes of gun violence and how to prevent it. But Congress was unmoved and continued to withhold funding. Still, there are some signs that government agencies are daring to defy political pressure from lawmakers and the gun lobby. A year after Sandy Hook, for example, the National Institutes of Health put out a call for research on firearm violence in a direct response to the president’s plea.

Studies that do exist point to some potential conclusions. Research suggests that comprehensive background checks help keep guns out of the hands of criminals. Experts say that additional studies are needed to confirm that finding, however. Research indicates that safe gun storage can help reduce shootings. And evidence shows that firearms are often trafficked from states with fewer gun restrictions to states with tighter gun laws, which means that an uneven application of gun laws across the United States may fuel an illegal firearms market.

Researchers warn that a lack of data shouldn’t lead to paralysis. Each year, more than 30,000 Americans are killed by gun violence, a phenomenon that medical experts describe as a pressing public-health problem. “You don’t need lots of studies to realize that it’s probably better to make sure that everyone has background checks,” said David Hemenway, the director of Harvard’s Injury Control Research Center. “Various studies have found that background checks are effective, and it would be great to have lots more studies that could essentially prove that, but whether you have them or not, you can still take common-sense action.”

Firearms owners, and the gun lobby, however, fear that research ostensibly carried out to study gun violence will be used to promote gun control. (It’s worth noting, too, that there isn’t universal agreement over what constitutes “common-sense action.” Some gun owners believe background checks of any kind are a violation of constitutional rights, for example.) The NRA remains unyielding and has continued to press its case: “There is no shortage of biased, privately funded research that contorts the data to support gun control,” NRA lobbyist Chris Cox wrote in a recent Politico article titled “Why We Can’t Trust the CDC with Gun Research.”

That suspicion is part of what makes gun research so controversial in the first place. It also puts a strain on anyone who opts to study gun violence—even if they aren’t tapping federal funds. Some researchers say they actively discourage undergraduates from pursuing gun research, since the money simply isn’t there. Those who persist can face intense scrutiny and, in some cases, intimidation. “I’ve received death threats. It kind of comes with the territory,” said Garen Wintemute, the director of the violence-prevention research program at the University of California, Davis. “But … there is a tremendous social injustice here, and to acquiesce to that by not jumping into the fray is to be part of what makes those conditions possible in the first place. I’m not willing to accept that.”

Besides, Wintemute is all too aware of the possible repercussions: “I ask myself all the time: How many thousands of people have died as a result of our not having the answers to questions because we have not been allowed to do our work?”