What motivates a subprime borrower to default?

This paper uses a real options approach to analyse the exercise of the default option embedded in mortgages. In particular, it examines a subprime household who borrows at a premium, but hopes to refinance at prime rates if their house appreciates, We show how these optimal default decisions can be used to calculate probabilities of default - an important input for risk management and pricing purposes. Numerical examples are provided, calibrated to US data. In a low interest rate environment, the credit-upgrade potential may discourage subprime borrowers from defaulting. However, default probabilities are highly sensitive to changes in interest rates and house prices. This provides a rational explanation for the prevalence of adjustable rate mortgages among subprime borrowers, and the subsequent large numbers of defaults, when interest rates rose and house prices declined. (C) 2008 Elsevier B.V. All rights reserved.