Some take comfort in pointing out that the national debt is "only" 60 percent of gross domestic product. That ranks the United States about 25th in the world. By comparison, Japan's public debt is 180 percent of its GDP. So we are at least better off than the Japanese, right? Wrong, says Robert D. Arnott, chairman of Research Affiliates, an institutional money management firm based in Newport Beach, Calif. Our debt burden only appears to be lighter than Japan's because we don't count all of it. State and local government debt is not included; neither is corporate debt nor household...