The study looked at why homeowners with underwater mortgages decide to walk away from their homes. They found that most borrowers in Arizona, California, Florida, and Nevada defaulted when their home equity dropped to -62% of its value. This suggests that high costs play a role in these decisions. About 80% of defaults were due to income shocks and negative equity, but when equity fell below -50%, half of the defaults were solely because of negative equity. This supports the idea that borrowers default when it benefits them.