Since the late 1950s, economists have paid attention to “housing starts” — the number of times in a month that ground is broken to build a home. In recent years, however, economists have started to pay closer attention to something we might call “housing stops”: the thicket of laws and regulations that make it harder for communities to build. Since at least 1950, notes housing economist Joseph Gyourko, there has been a growing price divide between low-cost areas where housing is plentiful and cheap, and desirable areas where housing is scarce and expensive. In 1950, housing in the most expensive metropolitan areas cost twice what it did in an average market. By 2000, it was four times as expensive, and Gyourko expects that difference to keep growing. Some of the differences are just high demand meeting physical limits — many of those cities are hard up against one body of water or another. And their ability to spread outward in other directions is constrained by highway congestion, as well as the length of time that people are willing to commute to work each day.