Securitisation — MBS, ABS, CLOs

Securitisation is the financial technology of pooling cash-producing assets — mortgages, auto loans, credit-card receivables, corporate loans — into a special-purpose vehicle and issuing tranches of debt against the pool's cash flows. Done well, securitisation expands credit availability and disperses risk to investors willing to bear it. Done badly, it played a central role in the 2008 crisis. The technology is neither virtuous nor evil; it is a power tool whose results depend on the discipline of the practitioner.

The basic structure

An originator (a bank, mortgage lender, or auto lender) sells a pool of assets to a special-purpose vehicle. The SPV issues tranches of debt — typically a senior AAA tranche, a mezzanine BBB-or-similar tranche, a junior B-or-similar tranche, and an equity tranche — and uses the proceeds to pay the originator for the assets. The pool's cash flows are then distributed to tranche holders according to a defined 'waterfall': senior gets paid first, equity absorbs the first losses, mezzanine sits in between.

Pool cash flows → Waterfall: Senior AAA → Mezzanine → Equity (residual)
Pool losses → Reverse waterfall: Equity first, then Mezzanine, then Senior

Mortgage-backed securities (MBS)

MBS are securitisations backed by residential or commercial mortgages. Agency MBS (Fannie Mae, Freddie Mac, Ginnie Mae) carry an explicit or implicit US government guarantee on the credit risk; the investor's main exposure is prepayment risk — borrowers refinancing when rates fall, returning principal early. Non-agency MBS carry credit risk and were the epicentre of the 2008 collapse.

Asset-backed securities (ABS)

ABS are securitisations backed by anything other than mortgages: auto loans, credit-card receivables, student loans, equipment leases, even royalty streams (Bowie Bonds, music-publishing securitisations). ABS markets are smaller than MBS but offer diversification across consumer credit categories and short, predictable cash flows.

Collateralised loan obligations (CLOs)

CLOs are securitisations backed by pools of leveraged loans (senior secured corporate loans). The CLO market has grown rapidly post-2010, with USD 1+ trillion outstanding by mid-decade. CLOs are the principal buyers of US leveraged loans, channelling demand from insurers and pension funds into corporate lending. Unlike subprime MBS, CLO collateral is closely monitored, actively managed by professional managers, and the historical loss experience of senior CLO tranches is excellent.

The 2008 lesson — correlation matters more than ratings

Pre-2008 ratings of senior subprime MBS tranches assumed mortgage defaults across regions and borrower types were weakly correlated. The historical data — drawn from the 1990s-early 2000s — supported that assumption. When the nationwide housing-price collapse arrived, however, defaults correlated almost completely. The 'thick' AAA cushion that had been computed under weak-correlation assumptions vanished. AAA tranches that were supposed to be protected by 30% of mezzanine cushion lost money. The lesson — that tail correlation can spike to one — has been repeated in every crisis since.

Post-crisis reforms

• Risk retention (Dodd-Frank, EU CRR): originators must retain 5% of the credit risk to align incentives.
• Standardised disclosure: deal-level loan-tape disclosure for ABS and MBS allows independent analysis.
• Conflict-of-interest rules: prohibitions on issuer-paid ratings shopping, designated independence requirements.
• Volcker Rule and similar: banks restricted from holding equity tranches of securitisations they originate.