One of President Obama's less-noticed actions on guns yesterday could actually prove to be one of the most significant in the long run. Among other things, the president signed an executive order directing the Centers on Disease Control to start studying "the causes of gun violence" once again.

Hard to study. (AP)

Why does this matter? Because one of the major reasons that gun-policy disputes are often so contentious and interminable is that there's remarkably little hard evidence to go on. And that ignorance is partly by design.

Back in 1996, Congress worked with the National Rifle Association to enact a law banning CDC funding for any research to "advocate or promote gun control." Technically speaking, that wasn't a ban on all gun research, but the law was vague enough that the centers shied away from the topic altogether. Funding for gun-violence research by the Centers for Disease Control dropped from $2.5 million per year in the early 1990s to a mere $100,000 per year today.

Since federal funding was the primary source of support for gun-violence research, the entire field withered as a result. Gun studies as a percentage of peer-reviewed research dropped 60 percent since 1996. Right now, there are only about a dozen researchers in the country whose primary focus is on preventing gun violence — despite the fact that more than 30,000 Americans were killed by guns in 2011.

The list of simple things that existing gun-violence research can't answer is quite striking. Over at Atlantic Cities, Emily Badger tallies up some very basic questions that we still don't know the answers to (below is just a partial list, and I've added a few from here and here, but you should read her full post):

*How many guns actually exist in the United States?

*How do guns get into the hands of people who commit crimes?

*What percentage of gun owners even commit gun crimes?

*Is there a relationship between gun ownership levels and crime?

*Are criminals deterred by guns?

*Do limits on high-capacity magazines reduce the number of deaths?

*Does firearm licensing and registration make people safer?

*How do gun thefts affect crime rates?

*Does gun ownership affect whether people commit suicide?

*What's the best way to restrict firearm access to those with severe mental illnesses?

*Why do gun accidents occur? Who's involved?

That helps explain why, when it comes to things like universal background checks or assault-weapons bans, policy experts are largely flying blind. Slate's Joel Shurkin recently attended a major conference on gun violence at Johns Hopkins and noticed a trend: "Most of the results are ambiguous, and the studies are retrospective — going back after the fact to look at possible predictors of gun violence and the best ways to prevent it — which is not the preferred way to prove anything in science."

Could this soon change? That's the idea, at least. In his executive order, Obama clarified that the Centers on Disease Control should go forward with research on "causes and prevention of gun violence." In other words, stop being so timid. Conduct research on anything that's not expressly forbidden by Congress.

Yet even so, some experts are skeptical that the order will massive a huge impact. "Now scientists will have one interpretation of the law from the executive branch and another from Congress." says Andrew Rosenberg of the Union of Concerned Scientists. Until Congress gives its explicit blessing to the CDC, federal gun research is likely to proceed only haltingly.

What's more, the freeze on CDC research is only part of the issue. Earlier this week, a  report from Michael Bloomberg's Mayors Against Illegal Guns* detailed all the different roadblocks to research on gun violence. The U.S. Justice Department, for instance, has stopped funding studies on firearms trafficking patterns. The National Institute of Justice (NIJ) used to be a "world leader" in research on gun ownership and trafficking. No longer:

And it's not just academic studies. Data is also sparse. The CDC's National Violent Death Reporting System has never been fully funded by Congress. The Bureau of Alcohol, Tobacco, Firearms, and Explosives is barred from keeping electronic records. Lawmakers have placed restrictions on how cities can share information about crime guns. That all directly impinges on research.

Back in the 1990s, the National Rifle Association charged that federal research centers were "putting out papers that were really political opinion masquerading as medical science." You can judge that for yourself by reading this story—the study that originally set the NRA's teeth on edge, by Art Kellerman, had found that a gun kept in the home was 43 times more likely to kill a member of the household than be used in self-defense. (For more criticism of public-health research on guns, see here.)

But by this point the pendulum has swung back the other way, to the point where very little research is being conducted at all. Now that the CDC is being prodded to move forward, that may begin to shift. But unless Congress goes along and makes some legislative changes as well, there's still a lot we won't know.

* Correction: The name of this group is Mayors Against Illegal Guns, not Mayors Against Gun Violence as originally written. Apologies for the error.

Further reading:

--NPR has a good narrative look at why gun research was barred in the 1990s.

--Emily Badger has a rundown of some basic gun-violence questions we still don't know the answers to.

--Meanwhile, over at Reason, Jacob Sullum is skeptical that more public-health research on guns will be useful.

--The full list of Obama's gun-control actions yesterday--and experts grade the proposals