There is an alternative, more behavioral hypothesis for the fragility of the securitization market that does not rely on a predominance of short-term debt financing. This alternative hypothesis begins with the observation that a large proportion of ABS tranches–both in the traditional and subprime sectors–were rated AAA. The AAA rating may have encouraged investors such as pension funds or insurance companies to think of these securities as essentially riskless, and therefore to treat them as being equivalent to Treasury bonds when constructing their portfolios. When the problems in the subprime area became apparent, this premise was utterly destroyed, and investors who were determined to allocate a fraction of their portfolios to safe assets realized that they had to dump their holdings of AAA-rated ABS, and buy actual Treasuries instead. Thus instead of a short-term-debt-driven bank run, we have what might be called a widespread buyer’s strike. In this account, the mechanism of contagion from the subprime market to the traditional consumer ABS market is that the failures of the rating agencies with respect to subprime called into question their credibility more generally, so that any AAA-rated tranche of an ABS, be it linked to subprime or credit cards, was no longer considered to be a virtually riskless asset.
The full essay is here, interesting throughout, via David Warsh's very good column. Stein also makes the simple yet neglected point that higher capital requirements may simply shift more financial activity into the less regulated shadow banking sector.