In a pre-Super Bowl interview on Fox, President Donald Trump claimed sanctuary cities “breed crime.” But limited research on the effect of such policies has found no evidence that they lead to overall increases in crime rates.

U.S. Immigration and Customs Enforcement documented nearly 2,000 cases in which cities with sanctuary policies refused to honor ICE detainers, and unauthorized immigrants then went on to commit crimes. But some law enforcement officials say that in the big picture, sanctuary policies also can help to reduce crime.

In speeches, Trump has cited instances of crimes committed by immigrants living in the country illegally, including those committed in so-called sanctuary cities.

Sanctuary cities are those that limit the degree to which local police cooperate with requests from federal authorities to detain and turn over unauthorized immigrants. Although these policies have been a contentious political issue for years, the sanctuary city issue took on new prominence in July 2015 after the murder of Kathryn Steinle, who prosecutors allege was shot and killed in San Francisco by a Mexican national with a felony criminal record who had been deported several times.

The Steinle case sparked a national debate about sanctuary cities because of San Francisco’s policy of not honoring federal requests to detain people found to be in the country illegally.

During the campaign, Trump routinely cited the Steinle case and vowed to cut federal funding for cities with sanctuary policies — a promise he followed through on with an executive order on Jan. 25.

In a pre-Super Bowl interview on Feb. 5, Fox News’ Bill O’Reilly asked Trump about Californians voting on whether to become a sanctuary state. Trump said that “if we have to, we’ll defund” the state.

Trump, Feb. 5: Sanctuary cities, as you know, I’m very much opposed to sanctuary cities. They breed crime. There’s a lot of problems.

The authors of a recent study, however, contest Trump’s claim that sanctuary cities “breed crime.”

The study from researchers at the University of California, Riverside, and Highline College — which the authors wrote about for the Washington Post‘s Monkey Cage blog — concluded that there was no evidence of higher crime rates in cities with sanctuary policies.

“We find no statistically discernible difference in violent crime rate, rape, or property crime across the cities,” the researchers concluded. “Our findings provide evidence that sanctuary policies have no effect on crime rates, despite narratives to the contrary.”

One of the study’s co-authors, Ben Gonzalez O’Brien, a professor of political science at Highline College, told us via email that this study and others have found no support for Trump’s claim.

“In past statements, Trump has cited individual instances of crime, such as the Kathryn Steinle shooting in San Francisco, rather than any evidence that sanctuary cities ‘breed crime,'” Gonzalez O’Brien said. “If this was the case we would expect to see higher crime rates in sanctuary cities when compared to cities with similar demographic characteristics, or an increase in crime after a sanctuary declaration was made. In our research, we have found no support for either of these propositions. There is no generalizable or statistical evidence that crime increases after a city becomes a sanctuary.”

The authors first looked at whether there was a change in crime rates in cities before and after the implementation of a sanctuary policy — and they found that sanctuary city policies did not lead to an increase in crime.

Second, they compared sanctuary cities to otherwise similar cities without sanctuary policies. They controlled for variables such as population size, age, gender, education, income, ethnicity and citizenship status. In this test, they found violent crime was slightly higher in sanctuary cities, but not to a statistically significant degree.

“That is, a sanctuary policy itself has no statistically meaningful effect on crime,” the authors concluded.

Some critics, however, take issue with the study, which is currently under review for publication. For starters, sanctuary policies vary widely by city, so settling on a standard definition of what constitutes a sanctuary city can be troublesome.

Jessica M. Vaughan, director of policy studies at the Center for Immigration Studies, a think tank that describes itself as an advocate for “low immigration,” said the authors used a definition of sanctuary that is “not robust” and left out cities like “Chicago or Miami, both of which have long standing obstructive sanctuary policies.”

Vaughan said the study also fails to consider that crime rates are affected by other factors such as “the number of police officers, policing policies, incarceration rates, the economy, and even the weather” — all of which she argued “are likely to have a much more profound effect on crime rates.”

Vaughan told us she has “never seen a credible study” directly on the issue of whether sanctuary policies “breed crime,” as Trump put it.

However, Vaughan pointed to a report by U.S. Immigration and Customs Enforcement that found that detainers were declined by sanctuary cities for 8,145 people in 2014, between Jan. 1 and Aug. 31. Of those, 62 percent, or 5,132 people, were previously charged or convicted of a crime “or presented some other public safety concern.” Nearly 3,000 of them had a felony charge or conviction. Of the 8,145 people with declined detainers, 77 percent had no subsequent criminal arrests, but 23 percent — 1,867 people — did.

Vaughan argues that crimes committed by those 1,867 people were “entirely preventable” if they had been turned over to ICE and deported.

“Whether or not these additional and preventable crimes increased the overall crime rate is impossible to say,” Vaughan told us via email. “I’m sure the victims don’t care. I’m sure the Steinle family doesn’t care if Kate’s murder increased the overall murder rate in San Francisco that year – it would not have happened if Sheriff Mirkarimi had followed the law and turned her killer over to ICE.”

Others say that’s cherry-picking, and argue that sanctuary policies also have a net positive effect on crime — due to “better incorporation of the undocumented community and cooperation with police,” as the earlier study put it.

In research published by the left-leaning Center for American Progress in January, Tom K. Wong, a professor of political science at the University of California, San Diego, looked at the issue on a county basis, rather than by city, and concluded, “Crime is statistically significantly lower in sanctuary counties compared to nonsanctuary counties.”

Wong argues the findings confirm what many law enforcement officials have said for decades. “Assisting in federal immigration enforcement efforts can drive a wedge between local law enforcement officials and the communities they serve, which undermines public safety,” he said.

Indeed, in a statement expressing concern with Trump’s executive order, the U.S. Conference of Mayors argued that sanctuary policies can reduce crime.

“Cities that aim to build trusting and supportive relations with immigrant communities should not be punished because this is essential to reducing crime and helping victims, both stated goals of the new Administration in Washington,” the statement reads. “Local police departments work hard to build and preserve trust with all of the communities they serve, including immigrant communities. Immigrants residing in our cities must be able to trust the police and all of city government.”

The U.S. Conference of Mayors also is concerned that the order does not provide a clear definition of what constitutes a sanctuary jurisdiction.

Numerous studies have found that immigrants do not commit crimes at a higher rate than non-immigrants, or that higher concentrations of immigrants do not lead to higher rates of violent crime. A recap of the literature on this topic can be found here.

A 2013 study published in American Sociological Review confirmed that immigration is strongly associated with less violence in a neighborhood, and even more so in areas with sanctuary policies.

We reached out to the Trump administration for backup for the claim that sanctuary cities “breed crime,” but we did not hear back. As always, politicians are ultimately responsible to back up their claims. In this case, we could not find any research to corroborate Trump’s claim. To the contrary, the limited research so far has found no evidence that sanctuary policies led to an overall higher rate of crime in the cities or counties where they exist.