A new study says Illinois could reduce its prison by population by 25% over the next five years without contributing to an increase in crime.

Ending mass incarceration has been the stated goal of several Illinois lawmakers and advocates. Such efforts are frequently met with concerns that fewer people behind bars will mean more crime on the streets. This new report says those concerns are unfounded.

The paper, published Friday, points to states like New York, California and Maryland, which have significantly reduced their prison populations while also seeing a drop in crime. It also notes that Illinois itself has cut down on its prison population and still seen crime go down.

“If a person says, ‘Look, if we reduce the prison population, crime will go up.’ We have answered that question. The answer is, ‘No [it will not],’” said Todd Clear, a professor at Rutgers University.

Clear authored the study along with James Austin, a senior associate with the JFA Institute, which works with government agencies to evaluate criminal justice policies, and University of Missouri Professor Richard Rosenfeld. Their report is based on a model the trio created that predicts crime trends. Their research has found that long-term crime trends are driven by economic and demographic factors, like unemployment and teen pregnancy, and not by incarceration.

In Illinois, the prison population has decreased by about 10,000 — or 20% — since 2015, thanks largely to policy changes pushed by former Republican Gov. Bruce Rauner and Cook County bail reforms, according to the study.

“Changes in the Illinois prison, county jail, county probation and state parole populations have had no sizable impact on Illinois crime rates,” the study reads. “It is not apparent that more incarceration produces less crime, or the reverse.”

Rosenfeld acknowledged the perhaps bitter irony for people in Chicago suffering through a spike in gun violence to hear this somewhat-rosy outlook. He said the increase in shootings is happening in cities throughout the country and started around late May or early June.

“This goes to the point we make in our report,” Rosenfeld said. “What no statistical model can predict are unpredictable changes. And I dare say that the spike in violence that we saw coincides in time with the emergence of mass protest over police violence that was unpredictable.”

The average resident of the city of Chicago, and especially those in neighborhoods where the violence spike has been concentrated, should indeed be concerned.

He said those immediate concerns should not dissuade the state of Illinois from seeking to reduce its prison population.

And Rosenfeld stressed their research focuses on long term trends, rather than short-term spikes or drops in crime that are unpredictable and can be caused by a number of factors.

The study found that reducing incarceration leads to a slight increase in property crimes, but the impact is dwarfed by the effect of other factors. The researchers found that incarceration rates have no impact on violent crime.

“The question about what you’re going to do in the future, the best way to think about that is that Illinois has already engaged in that experiment,” Clear said. “And as in other populous states, the decline in the Illinois prison population occurred as crime rates also declined.”

The study recommends that Illinois seek to further cut its 38,000-person prison population by reducing prison sentences.

“The greatest potential lies in reducing lengths of stay, especially for people convicted of serious crimes who have already served a substantial portion of a very long sentence and no longer represent a significant risk of reoffending,” the study reads.

The study’s authors determined crime rate based upon crimes reported by police to the FBI’s Uniform Crime Reporting Program. The crimes reported to the FBI include murder, rape, burglary and car theft.

Using their predictive model, the study authors are forecasting that over the next two years crime in Illinois “will continue to decline in a fluctuating pattern, with moderate year-to-year changes.”

And the report contains warnings for leaders who might be tempted to create policies based on those year-to-year fluctuations.

“Crime policy reforms tend to be driven by short-term changes in crime rather than the long-term trends,” the study reads. “Yet in the short term, crime is relatively volatile.”

Patrick Smith is a reporter on WBEZ’s Criminal Justice Desk. Follow him @pksmid. Email him at psmith@wbez.org.