We contribute to the debate over the reform of the Basel Accord by developing risk-based capital requirements for mortgage loans held in portfolio by financial intermediaries. Our approach employs simulation of both economic variables that affect default incidence and conditional loss probability distributions. Results indicate that appropriate capital charges for credit risk vary substantially with loan characteristics and portfolio geographic diversification. Hence, rules that offer little risk differentiation, including the current Basel I regime and “standardized” approach proposed in Basel II result in significant divergence between regulatory and economic capital. These results highlight the incentive problems inherent in simplified methods of capital regulation.

We identify two issues in the work of Ouazad and Kahn (2022). Correcting either reverses the original result. The two changes are to use the correct FHFA conforming loan limits for each county and year and to compare the individual loan amount to that limit correctly. There is no evidence that lenders transfer climate risk by altering loan origination and securitization behavior. None of our results calls into question the value and importance of the O&K model as a test for adverse selection. The question addressed and the setup of the test are important and should be replicated over time.