The crisis can be attributed to a number of factors pervasive in both housing and credit markets, factors which emerged over a number of years. Causes proposed include the inability of homeowners to make their mortgage payments, due primarily to adjustable rate mortgages resetting, borrowers overextending, predatory lending, speculation and overbuilding during the boom period, risky mortgage products, high personal and corporate debt levels, financial products that distributed and perhaps concealed the risk of mortgage default, monetary policy, international trade imbalances, and government regulation (or the lack thereof). Two important catalysts of the subprime crisis were the influx of moneys from the private sector and banks entering into the mortgage bond market and the predatory lending practices of mortgage brokers, specifically the adjustable rate mortgage, 2-28 loan.[34][35] Ultimately, though, specific to the bailout of Wall Street and the financial industry moral hazard lay at the core of many of the causes.

In its “Declaration of the Summit on Financial Markets and the World Economy,” dated 15 November 2008, leaders of the Group of 20 cited the following causes: During a period of strong global growth, growing capital flows, and prolonged stability earlier this decade, market participants sought higher yields without an adequate appreciation of the risks and failed to exercise proper due diligence. At the same time, weak underwriting standards, unsound risk management practices, increasingly complex and opaque financial products, and consequent excessive leverage combined to create vulnerabilities in the system. Policy-makers, regulators and supervisors, in some advanced countries, did not adequately appreciate and address the risks building up in financial markets, keep pace with financial innovation, or take into account the systemic ramifications of domestic regulatory actions.

Boom and bust in the housing market

Main articles: United States housing bubble and United States housing market correction Existing homes sales, inventory, and months supply, by quarter.
Vicious Cycles in the Housing & Financial Markets Low interest rates and large inflows of foreign funds created easy credit conditions for a number of years prior to the crisis, fueling a housing market boom and encouraging debt-financed consumption. The USA home ownership rate increased from 64% in 1994 (about where it had been since 1980) to an all-time high of 69.2% in 2004. Subprime lending was a major contributor to this increase in home ownership rates and in the overall demand for housing, which drove prices higher.

Main articles: Subprime crisis background information and Subprime crisis impact timeline Number of U.S. residential properties subject to foreclosure actions by quarter (2007-2009). Subprime borrowers typically have weakened credit histories and reduced repayment capacity. Subprime loans have a higher risk of default than loans to prime borrowers. If a borrower is delinquent in making timely mortgage payments to the loan servicer (a bank or other financial firm), the lender may take possession of the property, in a process called foreclosure.

The value of USA subprime mortgages was estimated at $1.3 trillion as of March 2007,  with over 7.5 million first-lien subprime mortgages outstanding. Between 2004-2006 the share of subprime mortgages relative to total originations ranged from 18%-21%, versus less than 10% in 2001-2003 and during 2007. In the third quarter of 2007, subprime ARMs making up only 6.8% of USA mortgages outstanding also accounted for 43% of the foreclosures which began during that quarter. By October 2007, approximately 16% of subprime adjustable rate mortgages (ARM) were either 90-days delinquent or the lender had begun foreclosure proceedings, roughly triple the rate of 2005. By January 2008, the delinquency rate had risen to 21% and by May 2008 it was 25%.

The value of all outstanding residential mortgages, owed by USA households to purchase residences housing at most four families, was US$9.9 trillion as of year-end 2006, and US$10.6 trillion as of midyear 2008. During 2007, lenders had begun foreclosure proceedings on nearly 1.3 million properties, a 79% increase over 2006. This increased to 2.3 million in 2008, an 81% increase vs. 2007. As of August 2008, 9.2% of all mortgages outstanding were either delinquent or in foreclosure Between August 2007 and October 2008, 936,439 USA residences completed foreclosure Foreclosures are concentrated in particular states both in terms of the number and rate of foreclosure filings. Ten states accounted for 74% of the foreclosure filings during 2008; the top two (California and Florida) represented 41%. Nine states were above the national foreclosure rate average of 1.84% of households.