The crime rate puzzle: did incarceration reduce the crime rate, or did it get in the way?

• Author: Balko, Radley
• Position: Column

"CRIME KEEPS on Falling, But Prisons Keep on Filling" Conservative pundits have been poking fun at that headline ever since it appeared in The New York Times in 1997. For the law-and-order right, it typifies the clueless mind-set of elite liberals. Can they not comprehend that America's soaring incarceration rate and the historic two-decade drop in crime that began in the mid-1990s might be connected?

The idea sounds straightforward enough: As we have put more people in jail, the violent crime rate has indeed dropped, from 758 victims per 100,000 people in 1991 to 429 in 2009. It's intuitive to say that putting more murderers and rapists behind bars is the reason why. But on closer inspection, the causal link is far from clear.

In a series of studies published in 2009, the University of Missouri-St. Louis criminologist Richard Rosenfeld and the SUNY-Albany sociologist Steven Messner found that during the last 15 years, states with lower incarceration rates saw bigger drops in crime, on average, than those with lock-'em-up policies. Moreover, the historic increase in the prison population began in the early 1980S, a decade after the crime rate began to rise and a decade before it started to fall. The incarceration rate increased by more than 100 percent in the 1980S, but violent crime still increased that decade, by 22 percent.

If it wasn't incarceration, what caused the drop? There is no shortage of theories: Scholars have pointed to everything from the legalization of abortion to the prohibition of lead-based paints. Other theories credit America's aging population (the vast majority of criminals are under 30), President Bill Clinton's program to put more cops on the street, and either stronger gun control laws or an increase in gun carrying by law-abiding Americans.

The studies behind all these theories claim to produce statistically significant results. Could they all be right?

"I don't think any of them are right" says Sam Walker, an emeritus professor of criminal justice at the University of Nebraska. Walker has studied crime for 35 years and has written 13 books on criminal justice. "You can alter variables to make them say whatever you want them to say," he says. "Conservatives say the crime drop was because of incarceration. Liberals say it was programs like community policing. I don't think there's much convincing evidence for either"

There is an academic consensus about just two factors: the ebb in the crack trade after its peak...