The Democrat-controlled House of Representatives on Wednesday passed a universal background check bill — which, if the Senate were interested in passing it, would be the most significant gun control legislation in a generation.

Under current federal law, licensed dealers are required to run a background check to make sure a buyer doesn’t have a criminal record, history of mental illness, or any other 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. So someone who doesn’t run a licensed gun shop can sell or gift a firearm at a gun show, over the internet, or to friends and family without verifying through a background check that the buyer isn’t legally prohibited from purchasing the weapon.

The new bill, HR 8, would close this loophole, although it would leave some exemptions for gun transfers among family and temporary transfers (like lending a gun) while hunting.

For years, the proposal — for universal or comprehensive background checks — has been the top item on gun control advocates’ wish list. 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?

Yet the House proposal faces tough odds in the Senate, which is controlled by Republican lawmakers who have been much more reluctant to approve any gun control bills — despite widespread support for universal background checks among GOP constituents. President Donald Trump’s administration has also come out against the bill.

And some research suggests a universal background check bill alone may not do much to reduce gun violence in the US. Several studies published in the past year found background checks would have a limited effect on gun deaths, whether homicides or suicides.

Still, advocates and experts argue that comprehensive background checks are a much-needed foundation. Daniel Webster, director of the Johns Hopkins Center for Gun Policy and Research, previously told me that they’re a “logical first step.” Based on the research, though, more will be necessary.

Universal background checks may have a limited effect on gun violence

The universal background check bill would help close a major loophole under the current law. About one in five gun transfers (sales or otherwise) are done without any background check at all, based on recent research. The bill would aim to close this loophole and, in doing so, attempt to ensure that fewer potentially dangerous people are obtaining firearms.

But several studies in the past year have found that universal background checks, enacted at the state level, have a limited effect:

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 and instead had 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 RAND cautioned that the research just on closing the background check loopholes, to go from having a system at all to making 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 universal 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 ensure 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.

“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,” Cassandra Crifasi, a researcher (and gun owner) at the Johns Hopkins Center for Gun Policy and Research, told me.

The problem with universal background checks may come down to poor implementation and enforcement. For example, the California study looked at the policy during a period when the state’s background check system was less sophisticated and comprehensive — missing some records, particularly for mental health history. Perhaps background checks are now better by virtue of having fuller data to work with.

“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.”

There are also some practical questions. Even if a system is built well, how can officials possibly track every single gun transfer, especially when it happens between two friends or family members in a private home or vehicle? Governments can encourage individuals to record and report these transfers, but it will be a constant challenge to ensure this always happens.

Other problems can arise. Maybe a system is underresourced or understaffed, limiting just how thorough the checks, even under a supposedly universal system, can be. Perhaps some law enforcement officials just aren’t taking warning signs seriously enough, as has been found to be true in the past with the US Bureau of Alcohol, Tobacco, Firearms, and Explosives (ATF). Some experts, like Crifasi, argue that there needs to be a cultural change too, creating “a social norm that selling a gun without a background check is a bad thing.”

All these limitations lead to a limited effect in the evidence so far.

Other policies could have stronger effects on gun violence

The disappointing results for background checks don’t mean gun control policies are totally ineffective. In fact, there’s growing 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.

“Ironically, advocacy organizations and politicians trying to promote background checks used studies on licensing that we had done to say background checks work,” Webster said, “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 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.