A report from real estate consultant First American CoreLogic states that 10.7 million, or 23 percent of all residential properties with mortgages, were in negative equity at the end of September 2009. Another 2.3 million mortgages have less than 5 percent equity. 

The distribution of negative equity is heavily concentrated in five states: Nevada (65 percent), Arizona (48 percent), Florida (45 percent), Michigan (37 percent) and California (35 percent). California and Florida accounted for more than 40 percent of the national total of negative equity mortgages. 

A paper by Luigi Guiso, Paolo Sapienza and Luigi Zingales argues that there is a correlation between borrowers' willingness to "strategically default" and the percentage of foreclosures in the same area. They argue that a "contagion effect" reduces the social stigma of foreclosure.

• Academic paper