Summary: <p>Say you're a country with a decent-size national debt. Everything's going fine: Investors are willing to lend you money at a low interest rate, andyou can pay your bills without too much trouble.</p> <p>But then investors get nervous and start demanding higher interest rates. All of a sudden, you have to devote more and more of your money just to pay off your debt.<br><br>Your economy starts to falter, and investors demand still higher interest rates. Now you're really in trouble.<br><br>What causes this to happen? Is there some magic threshold that countries cross before they get into trouble?<br><br>On today's Planet Money, we put that question to Ken Rogoff — a Harvard economist and an expert on the history of sovereign debt crises. We talk to Rogoff about three countries in particular: Greece, Italy and the U.S.<br><br><br></p>
