http://online.wsj.com/article/SB123897612802791281.html
But housing expenditures in the U.S. and most of the developed world have historically taken about 30% of household income. If housing prices more than double in a seven-year period without a commensurate increase in income, eventually something has to give. When subprime lending, the interest-only adjustable-rate mortgage (ARM), and the negative-equity option ARM were no longer able to sustain the flow of new buyers, the inevitable crash could no longer be delayed.
The price decline started in 2006. Then policies designed to promote the American dream instead produced a nightmare. Trillions of dollars of mortgages, written to buyers with slender equity, started a wave of delinquencies and defaults. Borrowers' losses were limited to their small down payments; hence, the lion's share of the losses was transmitted into the financial system and it collapsed.