(The Wall Street Journal) - In 2000, Standard & Poor's made a decision about an arcane corner of the mortgage market. It said a type of mortgage that involves a "piggyback," where borrowers simultaneously take out a second loan for the down payment, was no more likely to default than a standard mortgage.

While its pronouncement went unnoticed outside the mortgage world, piggybacks soon were part of a movement that transformed America's home-loan industry: a boom in "subprime" mortgages taken out by buyers with weak credit.