This paper explores the causes and consequences of cross-country variation in mortgage market structure. It draws on insights from several fields: urban economics, asset pricing, behavioral finance, financial intermediation, and macroeconomics. It argues that the US has much to learn from the experience of other countries, and calls for deliberate experimentation with mortgage market design. Residential mortgages are of first-order importance for households, for financial institutions, and for macroeconomic stability. The typical household in a developed economy has one dominant asset a house and one dominant liability a mortgage. Mortgages are a major fraction of bank assets, despite financial innovations that allow banks to securitize mortgage pools. And the financial crisis that began in 2007 has made it abundantly clear that problems in mortgage lending have the potential to destabilize the  financial system and the economy.

Despite their importance, mortgages have traditionally been a specialty topic in finance, and most mortgage research has been published in real estate and housing finance journals, not general-interest finance or economics journals. In this paper I argue that to understand mortgage markets we need a much broader perspective that integrates insights from across our discipline: not only from fields within finance such as asset pricing, behavioral finance, and financial intermediation, but also from urban economics and macroeconomics. Each of these fields can be compared to the proverbial blind man groping an elephant, accurately recording one aspect of the phenomenon but unable to perceive the whole. Here I attempt a sketch of the whole elephant.