What's really keeping urban home prices high? And how are some cities keeping prices comparatively affordable? In his new Kindle Single, The Gated City, Ryan Avent, the economics correspondent for The Economist, argues that high urban housing prices in cities like New York, San Francisco, and Washington, D.C. are the result of local regulations that place limits on new housing supply. Those limits help explain why Sun Belt cities like Houston and Phoenix are booming, and have managed to keep their housing prices comparatively low. Associate Editor Peter Suderman interviewed Avent this week about how zoning regulations and historical preservation rules make homes more expensive, why density is a good thing, and what San Francisco can learn from Houston.