Strategic mortgage defaults can spread like a contagious virus, according to a study on U.S. mortgages from 2005 to 2009. Borrowers near others who default strategically are more likely to do the same, even if they could afford to keep paying. This "contagion effect" is strongest when the delinquency rate in the area goes up by just 1%. This means that people might choose to stop paying their mortgages not because they can't, but because their neighbors are doing it too.