A common thread that runs throughout the history of humanity is the desire to build barriers that separate us from those we think aren’t like us. Historian David Frye joins us to talk about the purposes served by Hadrian’s Wall, the Great Wall of China and other structures – and about how a potential wall along the U.S.-Mexico border might affect the two modern-day cultures who live next to one another. His new book is called “Walls: A History of Civilization in Blood and Brick” (Scribner).