America and Greece have lately been running large budget deficits, roughly comparable as a percentage of G.D.P., notes Paul Krugman. Yet markets treat the countries very differently: “The interest rate on Greek government bonds is more than twice the rate on U.S. bonds, because investors see a high risk that Greece will eventually default on its debt, while seeing virtually no risk that America will do the same. Why? One answer is that we have a much lower level of debt — the amount we already owe, as opposed to new borrowing — relative to G.D.P.”