The proximate cause of the financial market turbulence, of course, is the home mortgages made from late 2005 through early 2007, near the end of long U.S. housing boom that began in 1995...
It will take years of research to untangle the quantitative contribution of various causal factors to the decade-long housing boom, the accompanying rise in subprime mortgage lending, and the subsequent increase in mortgage losses. A definitive assessment is too much to ask at this point, but a list of the most plausible suspects can easily be discerned. One candidate that is often overlooked is the significant increase in productivity growth, and thus growth in real household income, which began around 1995 and lasted until some time earlier in this decade...
Another plausible contributing factor was the wave of technological innovation in retail credit delivery, which allowed lenders to make finer distinctions between potential borrowers. This facilitated lower interest rates for some borrowers and an expansion of lending to borrowers formerly viewed as unqualified for credit...
The regulatory and supervisory regime surrounding U.S. housing finance also seems likely to have contributed to the boom in housing and housing finance. Here, several factors deserve mention...
Another key causal suspect is the relatively low path of interest rates after the recession earlier this decade, especially in 2003 and 2004. Some economists have argued, with the benefit of hindsight, that tighter monetary policy during that period would have led to better outcomes by preventing core inflation from rising, thus limiting the housing boom and mitigating the subsequent bust. This view strikes me as quite plausible, but again, further research will be required to substantiate this hypothesis.