This story was updated on Feb. 18 at 11 a.m. ET.
BOSTON — Over the last 20 years, crime rates in the United States have plunged precipitously — and mysteriously.
Far from taking credit for the decline, criminologists have been scratching their heads over the reason for the drop in robberies, assaults and burglaries. But new research points to a combination of many small changes in recent decades, as well as the largely ignored contributions of private crime prevention efforts, as responsible.
"Over the course of a generation, we have had this extraordinary change in the crime picture," criminologist Philip J. Cook of Duke University said here Saturday (Feb. 16) at the annual meeting of the American Association for the Advancement of Science. "It is a mystery, because no criminologist can say with any confidence that they understand what's going on."
Cook and his colleagues studied the role of private security efforts in tackling the problem of crime.
"There are more private security guards than there are police in this country," Cook said. "I believe that private action, though it has been largely ignored, deserves part of the credit."
Business improvement districts
One reason for a recent increase in the number of private security guards is the rise of "business improvement districts" — nonprofit organizations of businesses that tax themselves to pay for extra measures to make their districts cleaner and safer, including private security guards and surveillance.
Cook and his colleagues studied 30 business improvement districts in Los Angeles between 1997 and 2008, and found that their efforts caused an average of 28 fewer serious crimes per neighborhood, which represents an 11 percent drop in crime in those neighborhoods.
The scientists looked at how much money was being put toward security in these districts, and they found that for every $10,000 spent, the average number of crimes per neighborhood went down by 3.4, meaning that business improvement districts that spend more money on private security see a greater reduction in crime. [ Q&A: A Psychiatrist's View from Inside Prison ]
"The bottom line here is we have a reduction in crime — a reduction that is closely associated with the expenditure of private security money," Cook said. "It seems like a terrific accomplishment."
Cook was quick to point out, though, that these effects were just a small part of the overall recent trends. He also pointed to other small-scale changes, such as the increased use of credit cards over cash, and the advent of immobilizer technology in cars that prevents their engines from running without the correct keys, as bearing some of the responsibility.
Another criminologist, Peter Reuter of the University of Maryland, who was not involved in Cook's research, said he agreed that private security efforts have played a role. He pointed to the rise of gated communities, and technology advances that prevent stolen cellphones from being reprogrammed, as additional factors in reducing crime.
Stop, Look and Listen
Jens Ludwig, director of the University of Chicago's Crime Lab, studied another small-scale project that could pay significant dividends in crime reduction.
Ludwig and his colleagues tested out a program called Stop, Look and Listen at a Chicago juvenile detention center. The program trained staff at the center to teach kids tactics used in cognitive behavioral therapy (CBT), a psychological treatment system that can help people override automatic behaviors, such as reacting violently. [ The 10 Most Controversial Psychiatric Disorders ]
"Part of crime is automatic behavior," Ludwig said. "In principle we can make a little bit of a dent in the problem if we can make kids slow down a little bit and act less automatically."
Though the program was relatively inexpensive, and not what Ludwig called "gold standard CBT" — after all, it wasn't trained psychologists working with the kids, but minimally trained detention center staff — it made a difference.
The researchers found that juvenile offenders at the center who were randomly assigned to the Stop, Look and Listen program were less likely to become repeat offenders than kids who weren't. Overall, the program reduced return rates to juvenile detention by about 5 percent.
"The costs of crime are so huge that you don't need to make very big changes in the problem to generate large dollar values," Ludwig said. "Social cost to society per homicide is on the order of $10 million. The marginal cost of the intervention is essentially just training the staff. We estimate the cost per kid is $100 to $150, in exchange for a 5 percent reduction in return rates to the facility."
Reuter, who was not involved in this study either, said it showed promising results.
"It turns out that short interventions can make a difference," he said.
Editor's Note: This story was updated to correct an error in identifying the criminologist Peter Reuter, who was originally identified as his co-author, Eric Sevigny of the University of South Carolina.