One of the last junk-bond-market innovations was the collateralized bond obligation (CBO). CBOs are diversified investment pools of junk bonds that issue their own securities with the underlying junk bonds as collateral. Several tranches of securities with different seniorities are usually created, each with risk and return characteristics that differ from those of the underlying junk bonds themselves.
What attracted underwriters as well as investors to junkbond CBOs was that the rating agencies, in a very accommodating decision, gave the senior tranche, usually about 75 percent of the total issue, an investment-grade rating. This means that an issuer could assemble a portfolio of junk bonds yielding 14 percent and sell to investors a senior tranche of securities backed by those bonds at a yield of, say, 10 percent, with proceeds equal to perhaps 75 percent of the cost of the portfolio. The issuer could then sell riskier junior tranches by offering much higher yields to investors.
The existence of CBOs was predicated on the receipt of this investment-grade credit rating on the senior tranche. Greedy institutional buyers of the senior tranche earned a handful of basis points above the yield available on other investment grade securities. As usual these yield pigs sacrificed credit quality for additional current return. The rating agencies performed studies showing that the investment-grade rating was warranted. Predictably these studies used a historical default-rate analysis and neglected to consider the implications of either a prolonged economic downturn or a credit crunch that might virtually eliminate refinancings. Under such circumstances, a great many junk bonds would default; even the senior tranche of a CBO could experience significant capital losses. In other words, a pile of junk is still junk no matter how you stack it.