Background checks are supposed to be the main line of defense against a dangerous person looking to buy a gun. But a new study found that 22 percent of gun purchases in the US go through without a background check.

The study, published on Tuesday in the Annals of Internal Medicine, analyzed 2015 survey data of more than 1,600 adult gun owners in the US, gauging whether gun owners had gone through a background check if they purchased a firearm in the past two years.

The study is the first in decades to provide an estimate of the number of guns that are bought without a background check. A previous analysis, based on a 1994 survey, found that 40 percent of guns were purchased without a background check, suggesting that there has been progress in closing the gap — perhaps due to better enforcement — in the past several years.

The study also found that states with stricter gun laws were more likely to successfully force people through background checks. For example, state laws vary on whether they require background checks for private sales — those not at a store or pawnshop, but from friends or family, at a gun show, or online. In states that require a background check for such private sales, 26 percent of purchases reportedly went through without a background check. In states that didn’t require a background check for private sales, 57 percent went through without a background check — more than double the rate of stricter states.

There are two big limitations to the study: Since it’s based on a survey, it’s possible that some people misremembered whether they went through a background check. And it’s possible that some survey respondents lied about whether they went through a background check to avoid making themselves look bad.

Still, these are the best estimates we have for the number of guns out there that were purchased with a background check. They indicate that much of the US could stand to strengthen gun laws and their enforcement — to move toward universal background checks, which most Americans support. If states don’t act, they risk worsening gun crimes not just within their borders but in other places as well.

States with weak gun laws contribute to gun crime beyond their borders

A common critique of gun laws is that they supposedly don’t work to reduce gun crime. The argument goes something like this: Chicago has very strict gun laws, yet it still has high levels of gun crime. That suggests strict gun laws don’t have much of an effect on crime.

What this critique misses is that a big contributor to gun violence in Chicago is guns that come from outside the city and state — specifically, states that have much laxer gun laws. According to a 2014 report from the Chicago Police Department, nearly 60 percent of the guns in crime scenes that were recovered and traced between 2009 and 2013 came from outside the state. About 19 percent came from Indiana, which has relatively lax gun laws — making it the most common state of origin for guns besides Illinois. The report concluded that “Chicago’s violence problem is directly linked to the number of illegal guns available in the City.”

This isn’t a problem exclusive to Chicago or even the US. A 2016 report from the New York State Office of the Attorney General found that 74 percent of guns used in crimes in New York between 2010 and 2015 came from states with lax gun laws. And another 2016 report from the US Government Accountability Office found that most of the guns — as many as 70 percent — used in crimes in Mexico, which has strict gun laws, can be traced back to the US, which has generally weaker gun laws. (The gun trafficking chain from Southern states with weak gun laws to New York is so well-known it even has a name: “the Iron Pipeline.”)

The Annals of Internal Medicine study shows one reason this is possible: In states with weaker gun laws, people seem to have a much easier time purchasing a gun without going through a background check.

This is a problem because easier access to guns — especially by people with criminal records — can lead to more gun violence. In fact, the empirical evidence suggests that higher levels of gun ownership lead to more gun violence.

More guns mean more gun violence

The US has nearly six times the gun homicide rate of Canada, more than seven times Sweden’s, and nearly 16 times Germany’s, according to United Nations data compiled by the Guardian. (These gun deaths are a big reason America has a much higher overall homicide rate, which includes non-gun deaths, than other developed nations.)

And there appears to be a correlation between America’s high levels of gun violence and gun ownership, as this chart from Tewksbury Lab shows:

Research has consistently shown that more guns mean more gun violence. Researchers have found this to be true not just with homicides but also suicides, domestic violence, and violence against police.

Studies have found this at both the state and country level. Take, for instance, this chart, from a 2007 study by Harvard researchers, showing the correlation between statewide firearm homicide victimization rates and household gun ownership after controlling for robbery rates:

This holds up around the world. As Zack Beauchamp explained for Vox, a breakthrough analysis in the 1990s by UC Berkeley’s Franklin Zimring and Gordon Hawkins found that the US does not, contrary to the old conventional wisdom, have more crime in general than other Western industrial nations. Instead, the US appears to have more lethal violence — and that’s driven in large part by the prevalence of guns.

“A series of specific comparisons of the death rates from property crime and assault in New York City and London show how enormous differences in death risk can be explained even while general patterns are similar,” Zimring and Hawkins wrote. “A preference for crimes of personal force and the willingness and ability to use guns in robbery make similar levels of property crime 54 times as deadly in New York City as in London.”

Guns are not the only factor that contribute to violence. Other factors include, for example, concentrations of poverty, urbanization, and alcohol consumption. But when researchers control for other confounding variables, they have found time and time again that America’s high levels of gun ownership are a major reason the US is so much worse in terms of gun violence than its developed peers.

Given that, it should come as no surprise that the research has also found that tighter restrictions on guns can prevent deaths: A 2016 review of 130 studies in 10 countries, published in Epidemiologic Reviews, found that new legal restrictions on owning and purchasing guns tended to be followed by a drop in gun violence — a strong indicator that restricting access to guns can save lives.

MORE GUNS MEAN MORE GUN DEATHS, AND MORE RESTRICTIONS ON GUNS MEAN FEWER GUNS AND FEWER GUN DEATHS

Some countries can testify to the success of gun control measures. In Australia, after a mass shooting in 1996, lawmakers passed new restrictions on guns and imposed a mandatory buyback program that essentially confiscated people’s guns, seizing at least 650,000 firearms.

According to one review of the evidence by Harvard researchers, Australia’s firearm homicide rate dropped by about 42 percent in the seven years after the law passed, and its firearm suicide rate fell by 57 percent.

Although it’s hard to gauge how much of this was driven by the buyback program, researchers argue it likely played some role: “First, the drop in firearm deaths was largest among the type of firearms most affected by the buyback. Second, firearm deaths in states with higher buyback rates per capita fell proportionately more than in states with lower buyback rates.”

In short, more guns mean more gun deaths, and more restrictions on guns mean fewer guns and fewer gun deaths. So the fact that 22 percent of gun purchases appear to be slipping through without even a basic background check likely poses a threat to public safety.