What is really behind the mushrooming rate of mortgage foreclosures since 2007? The evidence from a huge national database containing millions of individual loans strongly suggests that the single most important factor is whether the homeowner has negative equity in a house -- that is, the balance of the mortgage is greater than the value of the house. This means that most government policies being discussed to remedy woes in the housing market are misdirected.

Many policy makers and ordinary people blame the rise of foreclosures squarely on subprime mortgage lenders who presumably misled borrowers into taking out complex loans at low initial interest rates. Those hapless individuals were then supposedly unable to make the higher monthly payments when their mortgage rates reset upwards.

But the focus on subprimes ignores the widely available industry facts (reported by the Mortgage Bankers Association) that 51% of all foreclosed homes had prime loans, not subprime, and that the foreclosure rate for prime loans grew by 488% compared to a growth rate of 200% for subprime foreclosures. (These percentages are based on the period since the steep ascent in foreclosures began -- the third quarter of 2006 -- during which more than 4.3 million homes went into foreclosure.)

Sharing the blame in the popular imagination are other loans where lenders were largely at fault -- such as "liar loans," where lenders never attempted to validate a borrower's income or assets.

This common narrative also appears to be wrong, a conclusion that is based on my analysis of loan-level data from McDash Analytics, a component of Lender Processing Services Inc. It is the largest loan-level data source available, covering more than 30 million mortgages.