Congressional Democrats on Tuesday introduced a bill that would move toward universal background checks — legislation that, while unlikely to make it through the Senate, signals that Democrats are putting guns at the forefront of their agenda.

But a growing body of research suggests that comprehensive background checks alone won’t do much, if anything, to combat gun violence in America.

Under current federal law, licensed dealers are required to run a background check to make sure that a buyer doesn’t have a criminal record, history of mental illness, or another factor that legally bars him from purchasing a gun.

But the law has a big loophole: Private sellers — meaning unlicensed sellers — don’t have to run a background check. This means that someone who doesn’t run a gun shop but sells guns at a gun show, over the internet, or to friends and family is legally allowed to sell a gun without verifying that the buyer isn’t legally prohibited from purchasing a firearm.

The Democrats’ bill, HR 8, would close this loophole, although it would leave some exemptions for gun transfers among family and temporary transfers while hunting.

For years, the proposal — for universal or comprehensive background checks — has been the top item on gun control advocates’ wishlist. It polls extremely well among gun owners, people who don’t own guns, Democrats, Republicans — basically everyone. And it certainly makes sense: If there’s a loophole that potentially lets criminals get guns, why not close it?

But several studies released in the past year now suggest that enacting comprehensive background checks alone would have a very limited effect on US gun deaths, whether homicides or suicides.

Daniel Webster, director of the Johns Hopkins Center for Gun Policy and Research, said that comprehensive background checks are a “logical first step,” especially given the high levels of public support for the policy change. But the effects of such a policy, at least at the state level, have been discouraging; when states enact comprehensive background checks alone, Webster explained, “we’ve not seen reductions in homicides and suicides follow.”

The argument that Webster and other researchers are now putting forward is not that background checks don’t work at all. But the way these policies have traditionally been implemented aren’t working as well as supporters would hope. And even if background checks can act as a foundation for other changes, the evidence increasingly suggests that other policies — such as a system that requires a license to buy a gun — may be necessary to really tackle America’s gun violence problem.

Background checks alone don’t seem very effective

None of the researchers I talked to said they expected to find out that background checks aren’t effective on their own. But several studies in the past year produced similar findings:

• One study, by UC Davis and Johns Hopkins researchers, found that after California enacted comprehensive background checks, as well as misdemeanor violence prohibitions for guns, the policy changes were not associated with changes in the firearm homicide or suicide rates. (A caveat: It’s possible comprehensive background checks had some effect, but it was just too small to detect in a population-level study.)
• Another study, from UC Davis, UC Berkeley, and Johns Hopkins researchers, found that after Indiana and Tennessee repealed comprehensive background checks, the repeals were not associated with changes in the firearm homicide or suicide rates. (Again, it’s also possible that there was an effect, but it was simply too small for the study to detect.)
• A study, by Johns Hopkins researchers, found that comprehensive background checks alone in urban counties were actually associated with an increase in firearm homicides, although the authors cautioned that background checks very likely did not cause the increase so much as have little effect while homicides were already increasing.

Previously, the research base on background checks was limited but promising. A review of the evidence released by the RAND Corporation early last year looked at the best US-based studies for all sorts of gun policies, including background checks.

RAND found “limited” to “moderate” evidence that background checks in general reduce violent crime, including homicides, and suicides. But that was for all background check systems, not just the universal ones that these latest studies looked at, and it was before the newer studies were published. In fact, RAND cautioned that the research just on closing the background check loopholes, to make the system more universal, was “inconclusive” when it came to firearm homicides. The newer studies fill in that gap — and they don’t look good for comprehensive background checks.

In short: Establishing a background check system, as the US has already done on a national scale, likely has an effect. But making the system more comprehensive or universal doesn’t seem to have a significant effect on its own, at least at a population level.

That doesn’t mean background checks are useless. Policies that are proven to be far more effective, such as a gun licensing system, are built in part on background checks — to make sure that, say, someone’s criminal record doesn’t prohibit him from getting a license. To that end, comprehensive background checks may be worth enacting even if they don’t do much on their own.

As Cassandra Crifasi, a researcher (and gun owner) at the Johns Hopkins Center for Gun Policy and Research, put it to me, “There’s some challenges with background check systems as they currently stand, but you can’t have the other laws function without requiring a background check for every sale.”

Poor implementation and enforcement plague background checks

Another point made by researchers is it’s possible comprehensive background checks could be made more effective if they were simply implemented and enforced better than they are today, or at least at the time of the studies.

“What this new body of research is telling us is not that these policies don’t work,” Rose Kagawa, one of the UC Davis researchers involved in the new studies, told me, “but that they need to be designed and implemented in really rigorous and effective ways.”

For example, the California study primarily focused on data through the year 2000. But during the 1990s, the systems used to run background checks were antiquated by modern standards — with less complete records, particularly for mental health history. Since then, things have gotten better. So it’s possible that California’s background check system is more effective now, and perhaps it’s having an effect on gun deaths — just not an effect that a study looking at data before the 2000s would pick up. Similar issues may have applied to Indiana and Tennessee, which repealed their universal background check laws in 1998.

There are also huge problems with the enforcement of background checks. For one, the FBI’s background check system is notoriously underfunded and understaffed. Coupled with the limited maximum time that a background check can take — three days — this can lead to some problems slipping through the cracks.

One prominent example is Dylann Roof, who killed nine people at a predominantly black church in Charleston, South Carolina, in 2015. Roof should have failed a background check for a handgun purchase after admitting to illegally possessing controlled substances in the past, but the FBI examiner did not obtain the shooter’s record in time. With more time and resources, this shooting could have been prevented.

There are also issues with reporting, meaning the data that background checks actually check. As one example, the gunman who killed 26 churchgoers in Sutherland Springs, Texas, should have been denied from obtaining a firearm after he was convicted of two counts for assaulting his spouse and their child while he was in the Air Force. But the Air Force never reported the convictions to the national background check system. Similar problems happen at all levels of background checks, from states failing to report mental health histories to some previous crimes going unreported.

There are also huge gaps in federal oversight of licensed gun dealers, with the Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF) frequently letting violators get away with it. As Ali Watkins reported for the New York Times, “Of about 11,000 inspections of licensed firearm dealers in the year starting in October 2016, more than half were cited for violations. Less than 1 percent of all inspections resulted in the loss of a license.” Some violations were shocking:

One store was cited for failing to conduct background checks before selling a gun. Another store owner told investigators he actively tried to circumvent gun laws. One threatened an A.T.F. officer, and another sold a gun to a customer who identified as a felon. All were previously cited by the A.T.F. In each instance, supervisors downgraded recommendations that the stores’ licenses be revoked and instead let them stay open.

These are stores that are breaking the law, and federal officials know it. Yet they’re allowed to stay open.

Part of this comes down to insufficient resources, which can force officials to prioritize less difficult cases or simply fail to complete an investigation. Some of it is caused by a lack of universal background checks, since it’s much harder to investigate people if some guns are being sold and transferred without any checks or records whatsoever.

But Webster and Crifasi argued that poor enforcement also reflects a lack of social and cultural emphasis on good enforcement. In their view, there needs to be a broader social and cultural campaign that encourages stronger enforcement of existing gun laws.

They both drew a comparison to the work of Mothers Against Drunk Driving in the 1980s, which focused not just on getting new laws passed but also persuading the public and the people implementing the policy changes to take drunk driving seriously. A similar movement, they said, needs to happen with the enforcement of gun laws.

“We’re lacking a social norm that selling a gun without a background check is a bad thing,” Crifasi said. “I would like to see some social pressure changing about the importance of making sure you don’t sell to someone who’s prohibited. And that social pressure may change the perspective of law enforcement — as to whether the laws are important to enforce.”

Other gun policies could have stronger effects

The disappointing results for background checks don’t mean that no gun policies can have an effect. On the contrary, there’s mounting evidence for at least one approach: a licensing system, which requires people to obtain a permit, typically via a police department, before they can purchase a gun from a licensed or private seller.

The big studies so far come out of Connecticut and Missouri. In Connecticut, researchers looked at what happened after the state passed a permit-to-purchase law for handguns — finding a 40 percent drop in gun homicides and 15 percent reduction in handgun suicides. In Missouri, researchers looked at the aftermath of the state repealing its handgun permit-to-purchase law — finding a 23 percent increase in firearm homicides but no significant increase in non-firearm homicides, as well as 16 percent higher handgun suicides.

In the past, advocates pointed to these studies as evidence that comprehensive background checks work, because the licensing systems in the states were paired with comprehensive background checks. But the evidence increasingly suggests that it’s the licensing system, not the comprehensive background checks, that’s key.

For example, the Johns Hopkins study, which found that comprehensive background checks alone correlated with more firearm homicides in urban counties, found that licensing systems were the one policy associated with fewer firearm homicides.

As Webster put it, “Ironically, advocacy organizations and politicians trying to promote background checks used studies on licensing that we had done to say background checks work — when it is a combination of background check requirement as part of a permitting process [that works].”

One possible explanation for why gun licensing works better than background checks: It’s a more straightforward system.

“It’s far easier as a seller to say, ‘Okay, I need to see your permit,’” Crifasi suggested. “And there’s accountability. If my gun is recovered in the hands of someone else, and I’m in a state with licensing, and that person doesn’t have a license, it’s easier to hold me accountable. I can’t say, ‘Oh, sure, I asked for a license,’ because they don’t have a license.”

That’s not to say gun licensing systems are the end-all-be-all. RAND’s report found some evidence that child-access prevention laws reduce suicides and unintentional shootings, and prohibitions associated with mental illness reduce suicides and violent crime. There’s also some evidence that Australia’s gun reforms in the 1990s, including a mandatory buyback of certain types of firearms, led to fewer gun deaths.

And some experts aren’t giving up on background checks. Garen Wintemute, who leads gun research at UC Davis, said that he would like to “give fixing standard comprehensive background checks a chance,” and intends to figure out how to do that with future studies.

The best approach may even be a combination of policy changes. A 2016 review of 130 studies in 10 countries, published in Epidemiologic Reviews, found that gun control laws lead to fewer gun deaths, but it likely came down not to one policy but the “potential synergistic effects, or the aggregated individual effects of multiple laws, when they are simultaneously implemented within a narrow time window.” Indeed, the Johns Hopkins researchers emphasized that a licensing system can’t work well without a comprehensive background check system, and some form of database or record keeping to track gun sales and transfers benefits both licensing and background checks.

Still, the research increasingly indicates that comprehensive background checks alone, at least as they’ve long worked in the US, don’t do much about gun violence. So if lawmakers really want to tackle this issue, they’re going to have to think bigger than they have so far.