We document the emergence of a disconnect between mortgage and Treasury interest rates in the summer of 2003. Following the end of the Federal Reserve expansionary cycle in June 2003, mortgage rates failed to rise according to their historical relationship with Treasury yields, leading to significantly and persistently easier mortgage credit conditions. We uncover this phenomenon by analyzing a large dataset with millions of loan-level observations, which allows us to control for the impact of varying loan, borrower and geographic characteristics. These detailed data also reveal that delinquency rates started to rise for loans originated after mid 2003, exactly when mortgage rates disconnected from Treasury yields and credit became relatively cheaper.

An increase in credit supply driven by looser lending constraints in the mortgage market is the key force behind four empirical features of the housing boom before the Great Recession: the unprecedented rise in home prices, the surge in household debt, the stability of debt relative to house values, and the fall in mortgage rates. These facts are more difficult to reconcile with the popular view that attributes the housing boom only to looser borrowing constraints associated with lower collateral requirements, because they shift the demand for credit.