Undocumented immigrants in the U.S. does not contribute to an increasing crime rate, according to a new study conducted by a UW-Madison professor. This is despite the fact that immigrants are struggling with many socioeconomic factors shared by people who are more likely to commit crimes.

The study from University of Wisconsin-Madison sociology professor Michael Light found that increases in undocumented immigration were associated with a heavy drop in violent crime.

“Since 1990, the undocumented immigrant population in the United States has tripled. It’s part of the largest wave of immigration the country has ever experienced,” Light told UW News. “In that same time, the violent crime rate has halved.”

Light and Purdue University sociology graduate student Ty Miller used state-level immigration data from the Center for Migration Studies and the Pew Research Center from 1990 to 2014, comparing undocumented immigration rates to an index of violent crimes – homicide, rape, robbery and aggravated assault – kept by the FBI. They also examined the impact of undocumented immigration on violent victimization rates from the National Crime Victimization Survey.

All told, the researchers examined annual rates of migration and crime-related factors such as unemployment, age, gun availability and drug activity for each of the 50 states and Washington, D.C., over the course of 24 years – more than 1,000 discreet time periods with which to weigh the effect of undocumented immigration.

A potential crime-confounding factor, Light says, is the selective nature of undocumented immigration.

“Immigrants are driven by pursuit of education and economic opportunities for themselves or their families,” Light told UW News. “Moreover, migration — especially undocumented migration — requires a lot of motivation and planning. Those are characteristics that aren’t highly correlated with a high crime-prone disposition.”