Book excerpt: Takes a skeptical look at a leading argument about what is causing the foreclosure crisis and what should be done to stop it. The authors focus on two key decisions: the borrower's choice to default on a mortgage and the lender's subsequent choice whether to renegotiate or modify the loan. Unaffordable loans, defined as those with high mortgage payments relative to income at origination, are unlikely to be the main reason that borrowers decide to default. The efficiency of foreclosure for investors is a more plausible explanation for the low number of modifications to date. Policies designed to reduce foreclosures should focus on ameliorating the effects of job loss rather than modifying loans to make them more affordable on a long-term basis. Illustrations.