In February, House Democrats passed legislation that would require all gun buyers go through a background check, regardless of whether they buy a weapon from a licensed dealer, collector at a gun show, or stranger in a parking lot.

Universal background checks are popular and enjoy political momentum. Poll after poll shows they win near-universal approval. According to a Quinnipiac University poll conducted immediately after the school shooting at a high school in Parkland, Florida, in February 2018, 97 percent of American voters said they approved of the policy, with identical levels of support among respondents living in households with a gun.

But it’s worth asking how effective universal background checks are at reducing gun violence. And the real-world evidence that they reduce crime is more complicated than the political momentum might suggest.

“The direct evidence on effectiveness is limited,” said Duke University public policy expert Philip Cook, who has studied background checks. “It’s very plausible that a state that tries to close this huge loophole and devotes resources to enforcing it will have good results. But not every program is going to be equally effective. It takes a real, concerted effort to get people to change the way they have been doing things for so long.”

Why expand the federal gun background check system?

Current federal law subjects a gun sale to a background check only if the seller is a licensed dealer “engaged in the business” of selling firearms. That means collectors who occasionally set up booths at gun shows or individuals who advertise a couple of weapons on a classifieds website like Armslist don’t have to vet their customers.

A number of states have tried to close this loophole by requiring all gun transactions to go through a check, regardless of whether the seller is a licensed dealer or a private individual. California was the first to create a universal background check law, in 1991. Today, 21 states and the District of Columbia regulate private sales, either by mandating checks at the point of every sale or by requiring permits to purchase a firearm.

With private sales unmonitored in so much of the country, it won’t come as a surprise that a survey published by Harvard and Northeastern universities in 2017 found that approximately one out of five guns is sold in an unregulated transaction.

There’s good reason to suspect that the unregulated gun market fuels gun crime. Interviews with people convicted of gun crimes have found relatively few get their weapons directly from retailers. Instead they rely on informal networks and underground sellers who take advantage of the legal loophole to sell guns free of oversight. Advocates believe that eliminating the private sale loophole could make it harder for criminals to get guns and thus reduce shootings.

How effective are expanded gun background checks?

Academic research suggests that background checks have the potential to reduce crime, but in practice the record has been mixed.

A 2001 study by Garen Wintemute of the University of California, Davis, found that Californians convicted of violent misdemeanors who had been denied a gun sale by the state’s background check system were less likely to be re-arrested for a violent or gun crime than people with similar criminal histories who were not denied. That suggests that where the system works as intended, background checks can reduce gun crime.

Yet other research suggests implementation has been spotty. In a recent study, Wintemute and colleague Rose Kagawa found that in California’s first decade with a universal background check law on the books, the policy had no significant effect on the number of gun homicides or suicides. Wintemute and Kagawa co-authored another study that found no evidence that Tennessee and Indiana’s passage, and later repeal, of comprehensive background check requirements affected homicide or suicide rates.

“In individual cases, people who are denied have lower arrest rates. So there, you’re seeing an effect,” Kagawa said in an interview. “But at the state level, you’re not seeing the impact.” In other words, the policy doesn’t seem to scale.

Reviews of broad swaths of research have reached differing conclusions. One 2012 meta analysis found little statistically significant evidence that checks reduce shootings. Two later reviews of academic literature from 2016 and 2017 strongly endorsed the policies. A landmark 2017 review of various gun policies by the RAND Corporation found only moderate evidence that checks reduce suicide and violent crime.

Why is the research all over the place?

The ambivalence of all this research may be partly due to study methodology, and partly due to shortfalls in implementation.

Wintemute and Kagawa’s study of California only looked at the first 10 years that the state’s system was in operation, when it still had serious kinks to work out. Most criminal records were incomplete. “That’s just not true anymore because the data has gotten so much better,” Wintemute said. A future study that looked at the state’s more recent experience could conceivably find that as operations improved, so did the impact on gun crime rates.

It’s also difficult to isolate the effect of one policy on gun violence. The past 30 years have seen violent crime reach historic highs and lows. Wintemute stressed that such volatility is almost certainly the result of multiple interacting factors like the economy and changes to the criminal justice system. That makes it hard to parse the effect of any single policy.

But it’s clear that in many cases, the policy isn’t working as hoped.

A background check system is only as good as the records available, as was demonstrated in the Sutherland Springs, Charleston, and Virginia Tech mass shootings. In each of those cases, the gunmen were able to pass background checks despite convictions for domestic violence, a history of drug abuse, or involuntary psychiatric hospitalization, respectively. The relevant records either weren’t entered into the appropriate database or weren’t clearly labeled as prohibiting. In the Charleston case, the background check was also hampered by a provision that allows sales to go through after three business days, whether or not federal examiners have cleared the buyer.

Additionally, many people just ignore the law. In a study conducted last October, Wintemute reviewed survey data that found large numbers of California gun owners disregard background check requirements nearly three decades after the policy was first implemented. One in four gun owners said they had purchased a gun without going through a check. That’s roughly the same proportion as gun owners in the rest of the country, according to the Harvard/Northeastern survey.

In another study, Wintemute, Kagawa, and others found that in three states that passed universal background check laws — Colorado, Washington and Delaware — only one saw the number of checks increase. Logically, if gun owners complied with the law and got checked for sales that would have previously been unregulated, the number of checks should have risen substantially.

What’s the point if checks might not reduce gun violence?

Shootings themselves are only one kind of criminal activity that policy makers seek to reduce with background checks. Research by Daniel Webster of the Johns Hopkins University Bloomberg School of Public Health suggests that universal background checks effectively constrain the illegal gun market.

Webster found in a 2009 analysis of gun trace data from the Bureau of Alcohol, Tobacco, Firearms and Explosives that universal background check laws reduced trafficking. States with universal background checks, along with more stringent dealer inspections, reduced the rate at which criminals committed crimes with guns they didn’t originally purchase at retail within the state, a common proxy measure of trafficking.

Webster said in an interview that this research shows “there is less in-state diversion of guns from legal owners to criminals when private sales are regulated.”

The same goes for flows of guns between states. In a book on the effectiveness of gun policy that Webster edited in 2013, he reviewed a wealth of economic literature that found that states with weak gun laws that allowed unregulated private sales routinely exported guns to states with stronger laws — but not the other way around.

What type of screening does reduce gun violence?

So-called permit-to-purchase systems have shown a lot of promise. In such a system, all gun buyers must apply for a gun license from local law enforcement. Licenses are only granted to residents who clear a complete background check. Five states (Connecticut, Hawaii, Massachusetts, Illinois, and New Jersey) require a permit for all gun purchases. Five more (Iowa, Michigan, Nebraska, New York, and North Carolina) require permits to buy or own a handgun.

Research by Webster strongly suggests that permit-to-purchase laws reduce gun deaths. That may be because they are straightforward to comply with and enforce: Before a private dealer completes a sale, he just has to ask to see the buyer’s permit, rather than initiate a background check through a computerized system and wait for the result. When a police officer apprehends someone with a firearm, the permit, or lack of one, provides strong indication of whether the person is a legal gun owner.

What’s more, after Missouri repealed its license law in 2007, Webster and several colleagues found sharp increases in the percent of guns recovered at crime scenes shortly after initial retail sale, a measure known as “time-to-crime” and considered an indication of trafficking. The percentage of guns recovered at crime scenes within three months of their original sale nearly tripled from 2006 to 2011.

In a 2015 paper, Webster and others compared Missouri with Connecticut, which implemented a licensing requirement in the early 1990s. In Missouri, firearm suicides increased 16 percent after the repeal of the permit requirement. In Connecticut, the firearm suicide rate fell by 15 percent after the state imposed its permit law.

If states are addressing this, why does the federal government need to pass its own law?

Even if a state could effectively screen all gun sales within its borders, people banned from owning firearms could still travel to a neighboring state with looser laws to arrange a private sale. That’s the case in many of the states with policies that seek to vet all gun buyers. Tough-on-guns California borders Arizona, where firearm regulations are thin. New York borders Pennsylvania, one of the National Rifle Association’s strongest bases of influence outside the South and West. Illinois borders Indiana, and so on.

A federal universal background check law would replace the current legal patchwork with one strict standard.

As Webster put it: “We’re so far from a comprehensive regulation in sales. There’s no way to tackle this without universal background checks.”