When World War II ended, Washington, D.C.'s population was about 900,000; today it's about 700,000. In 1950, Baltimore's population was almost 950,000; today it's around 614,000. Detroit's 1950 population was close to 1.85 million; today it's down to 673,000. Camden, New Jersey's 1950 population was nearly 125,000; today it has fallen to 77,000. St. Louis' 1950 population was more than 856,000; today it's less than 309,000. A similar story of population decline can be found in most of our formerly large and prosperous cities. In some cities, population declines since 1950 are well over 50 percent. In addition to Detroit and St. Louis, those would include Cleveland and Pittsburgh.

During the 1960s and '70s, academic liberals, civil rights advocates and others blamed the exodus on racism -- "white flight" to the suburbs. However, since the '70s, blacks have been fleeing some cities at higher rates than whites. It turns out that blacks, like whites, want better and safer schools for their kids and don't like to be mugged or have their property vandalized. Just like white people, if they have the means, black people can't wait for moving companies to move them out.

At the heart of big-city exoduses is a process that I call accumulative decay. When schools are rotten and unsafe, neighborhoods become run-down and unsafe, and city services decline, the first people to leave are those who care the most about good schools and neighborhood amenities and have the resources to move. As a result, cities lose their best and ablest people first. Those who leave the city for greener pastures tend to be replaced by people who don't care so much about schools and neighborhood amenities or people who do care but don't have the means to move anywhere else. Because the "best" people -- those who put more into the city's coffer than they take out in services -- leave, politicians must raise taxes and/or permit city services to deteriorate. This sets up the conditions for the next round of people who can do better to leave. Businesses -- which depend on these people, either as employees or as customers -- also begin to leave. The typical political response to a declining tax base is to raise taxes even more and hence create incentives for more businesses and residents to leave. Of course, there's also mayoral begging for federal and state bailouts. Once started, there is little to stop the city's downward spiral.

Intelligent mayors could prevent, halt and perhaps reverse their city decline by paying more attention to efficiency than equity. That might be politically difficult. Regardless of any other goal, mayors must recognize that their first order of business is to retain what economists call net positive fiscal residue. That's a fancy term for keeping those people in the city who put more into the city's coffers, in the form of taxes, than they take out in services. To do that might require discrimination in the provision of city services -- e.g., providing better street lighting, greater safety, nicer libraries, better schools and other amenities in more affluent neighborhoods.

As one example, many middle-class families leave cities because of poor school quality. Mayors and others who care about the viability of a city should support school vouchers. That way, parents who stay -- and put a high premium on the education of their children -- wouldn't be faced with paying twice in order for their kids to get a good education, through property taxes and private school tuition. Some might protest that city service discrimination is unfair. I might agree, but it's even more unfair for cities, once the magnets of opportunities for low-income people, to become economic wastelands.

Big cities can be revitalized, but it's going to take mayors with guts to do what's necessary to reverse accumulative decay. They must ensure safe streets and safe schools. They must crack down on not only violent crimes but also petty crimes and misdemeanors, such as public urination, graffiti, vandalism, loitering and panhandling.