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.