As the U.S. continues to grapple with gun control policy after the deadliest mass shooting in its modern history, research in this area has never been more critical.

There's just one problem: For almost two decades, pro-gun representatives in Congress have prevented the Centers for Disease Control and Prevention from funding gun violence research.

The history

In the early '90s, the CDC funded gun violence research by pediatrics and epidemiology professor Fred Rivara. Rivara's research suggested having a gun in the home increased the risk of homicide and suicide threefold.

The study got a lot of attention, particularly from the National Rifle Association.

Pro-gun congressmen and -women wanted to defund the National Center for Injury Prevention and Control at the CDC, according to an essay published in the Journal of the American Medical Association. While they couldn't quite defund it, they did manage to get $2.6 million removed from the CDC's budget — which, as the essay noted, was "precisely the amount the agency had spent on firearm injury research the previous year."

If the intention wasn't clear enough, the final budget for the National Center for Injury Prevention came with the stipulation that none of the funding be used to "advocate or promote gun control." The change is known as the Dickey Amendment, named after former Rep. Jay Dickey, who sponsored it.

The ban's effect was immediate

Researchers quickly backed away from gun research over fear of losing their jobs. The CDC alsostopped funding grants for gun studies.

"It's such a controversial area, the money has been effectively dried up," said Dan Hemenway, professor of health policy at Harvard, in a phone interview. "There are so many public health issues, why choose to study one where you're going to get attacked and vilified?"

Today, most gun research comes out of a handful of universities that can afford it. One researcher at the University of California spent $1 million of his own money to keep various gun research projects going.

"Today, less than $5 million is spent each year on gun studies," according to the Trace. "A single HIV or cancer study can cost twice as much."

Meanwhile, we desperately need more gun-violence research

"Data and research are crucial for showing that there really is a problem and what can be done about it," Hemenway said.

Need proof? Look no further than America's licensing program for drivers.

Research showed that 16- and 17-year-olds were at a high risk for having fatal car crashes, and there were certain times and conditions when they appeared to be most at risk: at night, or when there are only other teens in the car.

The first four states to restrict the time newly licensed teens could drive — as well as who they could have in the car with them — saw the number of car crash fatalities fall 30 to 40%,Hemenway said.

"So now all 50 states have these graduated license programs," Hemenway said. "Without the data and the research, it wouldn't have happened."

Even Jay Dickey, sponsor of the Dickey Amendment, is onboard. Today, Dickey has changed his mind about the congressional ban. He and Mark Rosenberg, a former CDC official, wrote anop-ed in 2012 explaining why.

Antique hand guns are seen during the East Coast Fine Arms Show in Stamford, Connecticut, Jan. 5, 2013. Reuters/Carlo Allegri

"We were on opposite sides of the heated battle 16 years ago, but we are in strong agreement now that scientific research should be conducted into preventing firearm injuries and that ways to prevent firearm deaths can be found without encroaching on the rights of legitimate gun owners," they wrote. "The same evidence-based approach that is saving millions of lives from motor-vehicle crashes, as well as from smoking, cancer and HIV/AIDS, can help reduce the toll of deaths and injuries from gun violence."

The pair also recently wrote a joint statement following the Orlando shooting, explaining that no one is going to create new gun control policies unless there's research to demonstrate those policies will work.

It's ridiculous, they wrote, "for any state to expect its legislators to vote on gun violence legislation if they do not know that it will be effective in both protecting the rights of law-abiding gun owners and reducing gun violence."

Here's what we need to research about guns

AR-15 rifles line a shelf in the gun library at the U.S. Bureau of Alcohol, Tobacco and Firearms National Tracing Center in Martinsburg, West Virginia Dec. 15, 2015. Reuters/Jonathan Ernst

It's a lot. "It's incredible to me to be in an area where we know so little about such a big problem," Hemenway said.

We don't know enough about gun accidents, gun suicides, gun homicides or how guns get into the wrong hands. "Is it because the initial background check isn't good enough or is it that guns are stolen or people are buying guns illegally for somebody else?" he said.

We need research on smart guns, childproof weaponry and guns with ballistic fingerprints.

"What about gun training?" Hemenway added. "What's being taught there, and is it even effective?"

We simply do not know. There's little to no research.

Where does it leave us?

Despite recent mass shootings, there's still no funding for gun violence research.

Last year, after the mass shooting at a church in South Carolina, Congress again struck down an amendment that would have allowed the CDC to fund gun violence research.

"The CDC is there to look at diseases that need to be dealt with to protect public health," former House Speaker John Boehner (R-Ohio) said at a press conference following the decision. "I'm sorry, but a gun is not a disease. Guns don't kill people — people do. And when people use weapons in a horrible way, we should condemn the actions of the individual and not blame the action on some weapon."

And on June 20, in the wake of the Orlando shooting, the Senate again failed to pass gun reforms that would have barred suspected terrorists from purchasing guns and expanded background-check requirements for people wishing to purchase firearms.

But there's still hope: California recently voted to create a firearm violence research center at the University of California.

"Acts of firearm violence like Sunday's horrific mass shooting in Orlando leave us searching for answers," Senator Lois Wolk said in a statement. "California made finding those answers a priority, taking leadership once again where Congress has failed."

If we have some meaningful research to guide us, perhaps the next bill won't fail.