Christopher Palmer has posted Why Did So Many Subprime Borrowers Default During the Crisis: Loose Credit or Plummeting Prices? to SSRN. While this is a technical paper, it is clear from the title that it addresses an important question. If it can help us get to the root causes of the foreclosure crisis, it is worth considering. The abstract reads,

The surge in subprime mortgage defaults during the Great Recession triggered trillions of dollars of losses in the financial sector and accounted for more than 50% of foreclosures at the height of the crisis. In particular, subprime mortgages originated in 2006-2007 were three times more likely to default within three years than mortgages originated in 2003-2004.

In the ensuing years of debate, many have argued that this pattern across cohorts represents a deterioration in lending standards over time. I confirm this important channel empirically and quantify the relative importance of an alternative hypothesis: later cohorts defaulted at higher rates in large part because house price declines left them more likely to have negative equity.

Using comprehensive loan-level data that includes much of the recovery period, I find that changing borrower and loan characteristics can explain up to 40% of the difference in cohort default rates, with the remaining heterogeneity across cohorts caused by local house-price declines. To account for the endogeneity of prices — especially that price declines themselves could have been caused by subprime lending — I instrument for house price changes with long-run regional variation in house-price cyclicality.

Control-function results confirm that price declines unrelated to the credit expansion causally explain the majority of the disparity in cohort performance. Counterfactual simulations show that if 2006 borrowers had faced the price paths that the average 2003 borrower did, their annual default rate would have dropped from 12% to 5.6%.

Ok, ok — this is hyper-technical! The implications, however, are important: “These results imply that a) tighter subprime lending standards would have muted the increase in defaults, but b) even the relatively “responsible” subprime mortgages of 2003–2004 were sensitive to significant property value declines.” (40) It concludes that, “In reality, cohort outcomes are driven by both vintage effects (i.e. characteristics bottled into the contracts at origination) and path dependency in that exposure to economic conditions affect cohorts differently depending on their history.” (40)

So, the bottom line is that loose credit and plummeting prices were both causes of the defaults during the crisis. Mortgage underwriters and policymakers are on notice that they need to account for both of them in order to be prepared for the next crisis. This paper’s contribution is that it has quantified the relative impact of each of those causes.