Kevin Williamson of National Review Online offers readers “one weird trick” that can help cities thrive.

There are many variables in the success and failure of cities, but one stands out. It isn’t race — Philadelphia is a minority-majority city, as is New York. And it isn’t affluence, either: Rank U.S. metros by income and Los Angeles barely cracks the top 50. But each of those cities has enjoyed a measure of success in recent decades by improving the material conditions in poor neighborhoods through the sort of commercial development bitterly denounced as gentrification. The streets of West Philadelphia are not nearly so mean as they used to be. New York City’s transformation in the Giuliani years was dramatic not only for the well-off precincts of Manhattan but in the rest of the city, too, with development even in places such as the South Bronx, once written off as a total urban loss. Los Angeles, which experienced a much worse version of the Freddie Gray riots in 1992, is a different city today. Economic policy is of course a piece of that, though not so big a piece as the economic-policy wonks like to think — does anybody remember what Rudy Giuliani’s tax plan was?

These cities are now safe — that’s the difference. New York City may be backsliding under its new Sandinista regime, but there’s still not much of the old menace there. Downtown Los Angeles feels like Arlington with better weather, and it has a lower rate of violent crime than Portland, Ore. Philadelphia still has a high rate of violent crime compared with similar-sized cities such as Houston and Phoenix, but it is dramatically lower than the rates in the urban basket cases. Philadelphia’s murder rate, though still very high, is barely 60 percent of Baltimore’s. Baltimore’s violent-crime rate is twice New York City’s and three times Los Angeles’s.

There are two straightforward ways to improve the material conditions of people living in the poor parts of Baltimore: Move them out or move capital in.