Barkley Rosser,

I haven't studied the issue, but I suspect that the "objective" determination is that the US Govt. is less likely to default on bonds than the Japanese govt. Really. Japan had regime change as recently as the 1940s. And as a country, it probably faces greater threats than does the United States, particularly from China and North Korea. However, Tbills and Japanese govt. bonds are most likely in an elite class, in terms being seen as relatively low risk places to invest money.