Subprime & the CDO Machine, 2008Medium

How a national bet on house prices was relabelled AAA thousands of times — and what happened when the one assumption underneath it failed.

4 min read · 671 words · Updated 4 Sep 2026

What happened

• 2003–06 — US mortgage lending expands into borrowers with thin documentation and low equity. Lenders sell nearly every loan onward, so underwriting quality stops being their problem.
• 2005–07 — pools of the riskiest mortgage bonds are re-packaged into CDOs, then into CDOs of CDOs. Rating agencies stamp large senior slices AAA.
• 2006–07 — US house prices stop rising, then fall. Delinquencies climb, and the bottom tranches begin to fail.
• 2007 — two Bear Stearns funds collapse; the commercial-paper market freezes in August. The crisis begins in funding markets, a year before the famous part.
• September 2008 — Lehman Brothers files for bankruptcy; AIG, having sold protection on hundreds of billions of notional without posting collateral against it, is rescued days later.

The mechanism

• Pooling only diversifies independent risks. Thousands of subprime mortgages across many states looked diversified. They were a single wager on the national house-price trend.
• The models used a correlation assumption of roughly 0.3; the world delivered something close to 1. At that correlation, the subordination protecting senior tranches is decoration.
• Re-securitisation multiplied labels, not safety — a CDO of BBB tranches is still BBB risk, however senior the new slice is called.
• Synthetic CDOs removed the natural limit: with credit default swaps, exposure to the same mortgages could be created without any mortgages, so losses exceeded the underlying market many times over.

What it teaches

• Ask what single variable would hit the whole pool at once. If there is one, the rating on the senior tranche is a statement about the model, not the risk (see securitisation).
• Incentives are part of the credit analysis. A lender who sells every loan has no reason to care whether it repays — which is why risk retention rules now exist.
• Complexity that grows with each layer is a warning, not a sign of sophistication.
• The machinery survived, chastened. Auto ABS and post-crisis CLOs came through both 2008 and 2020 with minimal senior losses. The lesson was about what was securitised, not that securitisation is inherently unsound.

The mechanisms behind this

Every case on this site is an instrument or a mechanism doing exactly what it was built to do, in a situation nobody had pictured. These are the pages that explain the machinery:

• Securitisation — how pooling and tranching turn a pile of loans into a rating.
• Mortgage-backed securities — the raw material, and what its cash flows actually depended on.
• CDOs & synthetic tranches — the second and third layer, where the correlation assumption did its damage.
• How to read a credit rating — why a AAA on a tranche was never the same thing as a AAA on a government.