The article explores why many people defaulted on their subprime mortgages from 2000 to 2007. The researchers used a model to see why borrowers chose to default, finding that falling home prices and poor loan quality were major factors. When home values dropped, some borrowers owed more than their homes were worth, making defaulting a way to save money. Additionally, more people with bad credit and high debt struggled to make payments, leading to more defaults. The study suggests that problems in how mortgages were bundled and sold contributed to the crisis, and offers ideas for preventing future defaults.