Banks found it profitable to invest in the AAA rated tranches because the promised return was significantly higher than the cost of funds and capital requirements were low !In 2007 the bubble burst. Some borrowers could not afford their payments when the teaser rates ended. Others had negative equity and recognized that it was optimal for them to exercise their put options. !U.S. real estate prices fell and products, created from the mortgages, that were previously thought to be safe began to be viewed as risky !There was a “flight to quality” and credit spreads increased to very high levels