Initially, there was little basis for re-pricing them rationally, because the event (a peacetime decline in US house prices) was unprecedented. Moreover, banks and other financial institutions, which held large volumes of RMBS, were ill-equipped to measure the risk, and in some cases would have been bankrupted had they been forced to sell their holdings at the fire-sale prices prevailing at the height of the crisis.
但2007年，房地产价格开始全面下跌，按揭损失率激增，这一假设也随之破产。结果，人们发现RMBS的风险比预计的高得多。一开始，人们找不到以什么为基础来对它们进行理性的重新估价，因为所发生的事件（美国房价在和平时期下跌）是前所未有的。此外，大量持有RMBS的银行和其他金融机构的风险测量手段有问题，在危机爆发高峰期，如果被迫以跳楼价甩卖手上的RMBS存货，可能导致它们破产。