The Industrial Organization of the US Residential Mortgage Market

We show that the US residential single-family mortgage-origination market is highly concentrated once account is taken of the contractual coordination that arises from the correspondent- and warehouse-funding channels. We represent these channels as a network, using the flow of loans through three strata of the loan origination market: origination, aggregation, and securitization. We develop a network representation of the origination market and demonstrate that it is a small world, in that most nodes are close in the network. We then rank-order the interlinked aggregators and securitizers using ex post mortgage foreclosure rates as a proxy for performance. Our findings suggest that these significant interlinkages in the mortgage-origination network represent a previously underappreciated source of systemic risk. Many apparently atomistic mortgage underwriters are, in fact, coordinated to act in parallel because of their funding relationships with the large, too-big-to-fail bank holding companies.