How immigrants, especially illegal immigrants, affect crime has been the most salient immigration-related debate topic in America for many years. Trump began his 2015 quest for the Republican nomination by warning of Mexican illegal immigrant criminals. The 2015 killing of Kate Steinle by an illegal immigrant and subsequent other tragic high-profile murders have focused public and private scrutiny on how immigrants affect crime, lending seeming credence to Trump’s worries about an illegal immigrant crime wave.

Most central to this debate is whether sanctuary cities, shorthand for jurisdictions in the United States that limit their cooperation with Immigration and Customs Enforcement (ICE), result in higher crime rates. After all, the murder of Kate Steinle occurred in a sanctuary city and most illegal immigrants live in jurisdictions that limit their cooperation with ICE.

Empirically examining how sanctuary cities affect crime has been a tough nut to crack. The quality and completeness of crime data varies tremendously across jurisdictions, categorizing what counts as a sanctuary city is not as straight-forward as it seems, there are many different types of sanctuary policies that limit cooperation with ICE to different degrees, endogeneity can make causal claims difficult, the number of illegal immigrants within sanctuary jurisdictions is important but estimating their numbers in places with few people produces uncertain results, and many other problems bedevil this type of research.

As a result, my colleagues and I have recently focused on simpler ways to measure how immigrants affects crime and whether sanctuary policies have an impact on deportations and crime in particular areas.

A new paper by political scientist David K. Hausman closes the loop and convincingly shows that sanctuary city policies reduce deportations and don’t have an impact on crime. Using a difference in differences methodology, Hausman first shows that sanctuary policies reduce deportations from the jurisdictions that enact them. This is an important first step because if sanctuary policies had no effect on deportations then they couldn’t have any impact on crime. Hausman found that sanctuary policies reduce deportations by about one-third in the counties that adopted them. Interestingly, sanctuary policies reduced deportations of people without criminal convictions by about half.

Hausman’s second step was to see how sanctuary policies then affected crime. Using the same methods, he found no statistically significant effect on property crime rates, violent crime rates, or police clearance rates. In other words, sanctuary policies don’t reduce crime as some pro-immigration advocates claim nor do they increase crime as many immigration restrictionists claim.

Hausman’s work is the best so far on how sanctuary policies affect crime rates. More work should be done on this topic that extends over a longer period of time, but I doubt that any further studies will make such a large improvement in our understanding of sanctuary cities and crime.

Well done.