Following the Las Vegas mass shooting on Sunday, reporters asked White House press secretary Sarah Huckabee Sanders about gun control. She gave a typical conservative answer: “I think if you look to Chicago, where you had over 4,000 victims of gun-related crimes last year, they have the strictest gun laws in the country. That certainly hasn’t helped there.”

This is a typical argument in opposition to gun control: Chicago has fairly strict gun laws (although not the strictest), yet it still has a lot of crime and violence. That shows, the argument goes, that gun control doesn’t work.

The truth is much more complicated. While it’s true that Chicago has strict gun control laws, those measures can only go so far when its neighbors don’t. After all, people can simply cross the border to Indiana, buy a gun without ever going through even a basic background check, and bring the firearm back to Chicago.

The evidence suggests this happens fairly often — not just in Chicago, but in other places with strict gun laws as well. And the only way to stop it is through the one government that has jurisdiction across the city and state borders where these guns travel through: the federal government.

This is what Chicago Mayor Rahm Emanuel said in response to the White House’s latest comments. “If you really want a gun, you can just drive over the Indiana border, and get whatever you want,” he argued. “That is why you need national gun legislation that prevents gang members or criminals from getting their hands on an assault weapon that is not meant for the streets of any urban center.”

Trump, of course, is on the record opposing stricter gun control measures, often calling himself “a Second Amendment person.” But if he’s serious about stopping gun violence in Chicago, the research should lead him to reconsider his stance.

Chicago’s gun violence problem is partly Indiana’s fault

Trump has someone very close to him in his administration who should be intimately aware of Chicago’s gun problem: Vice President Mike Pence. As governor of Indiana, Pence helped set the laws for the state that, besides Illinois, contributes most to Chicago’s gun problem.

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 — making it the most common state of origin for guns besides Illinois.

Here’s how it works: Chicago requires a Firearm Owner Identification card, background check, three-day waiting period, and documentation for all firearm sales. But Indiana doesn’t require any of this for purchases between two private individuals — including those at gun shows and those who meet through the internet — allowing even someone with a criminal record to buy a firearm without passing a background check or submitting paperwork recording the sale.

So someone from Chicago can drive across the border — to Indiana or to other places with lax gun laws — and buy a gun without any of the big legal hurdles he would face at home. Then that person can resell or give guns to others in Chicago or keep them, leaving no paper trail behind. (This is illegal trafficking under federal law, but Indiana’s lax laws and enforcement — particularly the lack of a paper trail — make it impossible to catch someone until a gun is used in a crime.)

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. (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.”) And another 2016 report from the US Government Accountability Office found that most of the guns — as much 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.

This pipeline makes it impossible for states to stop the flow of guns used in crimes within their borders, since the root of the problem lies in other jurisdictions. The only way the pipeline could be stopped, then, is if all states individually strengthen their gun laws or if the federal government passes a law that enforces stricter rules, from universal background checks like Chicago’s to mandatory gun buyback programs like Australia’s, across all states.

The evidence suggests this could help: Time and time again, researchers have found that where there are more guns and more access to guns, there are more gun deaths.

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.)

What’s more, there appears to be a correlation between America’s high levels of gun violence and gun ownership, as this other chart from researcher Josh Tewksbury shows:

Research reviews by the Harvard School of Public Health’s Injury Control Research Center have concluded that more gun ownership leads to more gun violence. Other factors, such as socioeconomic issues, contribute to violence, but guns are the one issue that makes America unique relative to other developed countries in comparable socioeconomic circumstances.

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:

A more recent study from 2013, led by a Boston University School of Public Health researcher, reached similar conclusions: After controlling for multiple variables, the study found that a 1 percent increase in gun ownership correlated with a roughly 0.9 percent rise in the firearm homicide rate at the state level.

This holds up around the world. As Zack Beauchamp explained for Vox, a breakthrough analysis in 1999 by UC Berkeley researchers 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 crime — 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.”

These studies aren’t the only ones to reach such conclusions. Multiple reviews of the research, including the Harvard Injury Control Research Center’s aggregation of the evidence, have consistently found a link between gun ownership and gun deaths after controlling for other factors.

Guns are not the only factor that contributes to violence. (Other factors include, for example, 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.

That helps explain why the most rigorous reviews of gun policies have concluded that stricter gun laws can reduce gun violence and 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.

In short, more guns mean more gun deaths, and more restrictions on guns mean fewer guns and fewer gun deaths. That’s backed by the research, regardless of what talking points about Chicago from the White House and other conservatives may suggest.