Every time there is a mass shooting, the debate on gun control and gun rights sparks again in the United States. Numbers show that there are more guns out there than ever before, though fewer people actually own them. At the center of the debate is the question whether having a gun makes people safer or not. Normally, in order to answer questions about safety, people might turn to science, where researchers could study cases of gun violence and gun ownership to determine whether they make a person safer. It's not an easy question to answer, but due to legal restrictions, most scientists never even get to try.

For nearly 20 years, the Center for Disease Control and Prevention (CDC) has been subject to what some call a ban on funding to study gun violence. Originally including in the 1996 appropriations bill as a rider proposed by Arkansas Representative Jay Dickey, the amendment doesn’t explicitly ban gun research. Instead it says that "none of the funds made available for injury prevention and control at the Centers for Disease Control and Prevention may be used to advocate or promote gun control," reports Christine Jamieson for the American Psychological Association’s Psychological Science Agenda. She adds:

However, Congress also took $2.6 million from the CDC’s budget — the amount the CDC had invested in firearm injury research the previous year — and earmarked the funds for prevention of traumatic brain injury.

The vagueness of the Dickey amendment wording has kept the CDC from funding researchers that might study gun violence for fear that they would be financially punished. Producer Todd Zwillich and Editor T.J. Raphel report for The Takeaway:

There is other research that goes on at the CDC that does have to do with guns. There is a National Violent Death Reporting System, which does record the causes of all violent deaths, including in domestic abuse, youth violence, and child abuse. If a gun is the cause, that’s recorded — it’s not like they ignore it entirely. But gun deaths and gun injuries as a public health issue, as [Fred Rivara, a professor at the University of Washington and epidemiologist at University of Seattle Children’s Hospital] said, are still basically anathema to CDC researchers and anyone who gets CDC funding, which is potentially millions of dollars.

Rivara was part of a team that researched gun violence in the 1990s. They found that having a gun in the home increases the likelihood that a resident of that home will be injured. Studies like that lead to the National Rifle Association lobbying congress, accusing CDC researchers of promoting gun control using federal funds.

After the shooting at Sandy Hook Elementary in Newton, Connecticut, President Obama ordered the CDC to study the causes of gun violence. But the CDC still balked, citing difficulty securing dedicated funding, reported Todd C. Frankel for The Washington Post in January 2015. The Dickey rider was still in place. As it is today.

On June 24 of this year, just a week after the deadly shooting in Charleston, South Carolina, that killed nine people, the House Appropriations Committee voted to continue the rider.

Many basic questions still remain about gun violence. Some studies still happen — researchers recently concluded that shootings and mass killings really are contagious — but the CDC as a major source of funding (one available to other public health studies) is missing.

Right now, without good data on what is safer, a December 2014 Pew Research poll showed that public opinion favors gun rights over gun control.