The study showed just a slight increase in lending rates when countries lose their AAA rating. In May 1998, S&P knocked Belgium, Italy and Spain from AAA to AA. A week later, 10-year rates had barely budged. In some cases, rates actually fell. A week after S&P took Ireland's AAA rating away in March 2009, 10-year rates in that country fell 0.18 percentage points.
Analysts and bond traders are not convinced rates will rise much if the U.S. loses its AAA rating. Caughey-Forrest and others note the recent high demand and the resulting falling yield for Treasurys.
Global investors still consider U.S. debt one of the safest investments. Many mutual funds, money market funds and banks find U.S. debt so safe they hold Treasurys as a proxy for cash. And they've continued to do so, despite the threat of a debt default and a downgrade.