Along with Martin Hellwig, I think that the other bright light in the field of finance is Gary Gorton of Yale. He has published several very interesting papers on historical banking panic episodes; see here. He gives a detailed account of the subprime mortgage market and the financial innovations associated with it in his paper entitled "The Panic of 2007."
In this paper, he describes the nuances of subprime mortgages; in particular, how their particular design made them very sensitive to the underlying asset price (unlike conventional mortgages). Evidently, this was by design (there was no other way for creditors to make money servicing this particular demographic).
He goes on to describe how these subprime mortgages were packaged into mortgage backed securities (MBS). This is a common form of securitization (although, the design of these also differed in a subtle, but important manner, from standard securitizations). As with other securitizations, mortgages were pooled and then tranches were formed; e.g., a senior tranche, a mezzanine tranche, and an equity tranche.
This type of securitization has an economic rationale. Higher rated tranches can be sold to insurance companies and pension funds (whose liabilities are longer term in nature). In principle, the originator of the MBS should could then hold on to the junior (equity) tranche. This gives the originator the incentive to construct a sound MBS; as the originator is the first in line to potential losses.
What I do not understand is what followed. These MBS were then used as backing for new securities, called Collateralized Debt Obligations (CDOs). For example, the mezzanine tranche of the MBS (rated BBB) would then be divided into senior, mezzanine, and junior tranches. The mezzanine tranche of this CDO would then be divided again into senior, mezzanine, and junior tranches (a CDO squared). The senior tranches of the CDO squareds would be assigned AAA ratings!
I do not understand the economic rationale for this further subdivision of the original MBS. I presume that there must be one (perhaps to get around some government regulations?). Is there an expert in finance out there that can help me out? I have had little luck in finding anything that explains the motivation for why CDOs exist.