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.”

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.