Between 1970 and 1990, the population of Philadelphia shrank by a quarter, dropping from 1.95 to 1.59 million. Like many American cities, it seemed caught in a downward spiral. Since then – like many American cities – Philadelphia has stabilized. The population now appears to have bottomed out at the millennium, and has been regaining residents over the past decade. But as it rebounds, Philly is becoming a different kind of city. In the two most recent decades, which comprise the bounce of the city’s population curve, owner-occupied housing dropped even more steeply than in the ’70s and ’80s. Between...