Mass incarceration not major driver of US falling crime rates, finds study

America’s crime rate has been falling steadily ever since the 1990s, and social scientists have spent years trying to figure out why. A new study from New York University Law School’s Brennan Center for Justice offers no definitive answers, but it does claim to rule out one explanation as the driving factor: mass incarceration.

Between 1991 and 2013, the violent crime rate declined by more than 50 percent, according to FBI figures. During the same time frame, property crime fell by 46 percent. Meanwhile from 1990 to 2009, the number of people behind bars in America more than doubled, from 771,243 to 1,615,500. But the increased prison community has had only a modest impact on the crime rate during that period, and only in the first decade of its decline, write the Brennan Center’s researchers. Since then, “incarceration in the U.S. has reached a level where it no longer provides a meaningful crime reduction benefit,” their report adds.

During a Thursday conference call, researchers involved in the drafting of the report were careful to stress that they weren’t saying incarceration had no deterrent effect. Instead, they argued that imprisoning more offenders eventually reached a point of diminishing returns.

“The shorthand is putting the first million people in prison is probably a lot more effective than the second million,” said economist Oliver Roeder, one of the authors of the report.

The researchers suggest a confluence of other factors better explain the fall in crime, including “increased numbers of police officers, deploying data-driven policing techniques such as CompStat, changes in income, decreased alcohol consumption, and an aging population.”

Inimai Chettiar, director of the Brennan Center’s Justice Program, said she hopes the report will “provide a wakeup call for the country regarding our incarceration policy."

“Mass incarceration is a poor investment,” she said. “It costs $252 billion a year and it isn’t helping to bring down crime."

The report’s release comes amid a nationwide debate over “Broken Windows” policing, a strategy that involves aggressively policing low-level crimes in an attempt to eliminate the conditions that theoretically lead to more severe illegal acts. The architects of the Broken Windows theory have argued that its implementation played a key role in reducing crime within New York through the 1990s. The Brennan Center report says it is difficult to tell whether such policing strategies have had an effect nationally “because each city and department defines and applies these tactics differently."

The report looks at the impact of 14 potential factors in the downward trajectory of the U.S.’s crime rate.

Among those investigated were the belief by some that brain damage from lead exposure was correlated with criminality, and another that argues that legalized abortion has helped bring down crime. Brennan Center researchers say that it is “possible” that legalized abortion and reduced lead exposure both had an effect on the crime rate, but that insufficient evidence exists to say for sure.

According to Chettiar, researchers were only able to account for the reasons behind roughly one-third of the crime drop with any degree of certainty.

But the results give further ammunition to those calling for penal reform at a time when U.S. prisons struggle are suffering from overcrowding.

One in every 110 American adults was incarcerated on Dec. 31, 2013, according to the most recent figures from the federal Bureau of Justice Statistics. While the incarceration rate went up by more than 400 percent between 1970 and 2010, within the past five years it has begun to decline slightly even as crime has continued to fall.