Bond-insurance is just a promise. When you buy it, you buy a promise. The bond is a promise in a sense, but it can also be thought of, like stock, as a part of the company’s stuff. That’s what you buy, a pound of steel, an employee’s MBA knowledge…. But insurance is a promise. Promises are performatives. So what? Shoshana Feldman might know.
A bank buys a bunch of corporate bonds. If it keeps them, it makes money in proportion to how well the companies fair. If all the companies do well and can pay, they make some interest money. If only some companies can pay and some default, the bank makes less money overall and may even lose money if a lot cannot pay. Instead of sticking with this old-fashioned notion, the bank can pool these bonds together and re-sell them. When it re-sells them, it sells them in risk-slices. Some slice-buyers will not get any money back if a proportion of the overall pool defaults. Instead of the bank getting lower return (or loss) in this situation, the secondary investor loses everything. So this is a way for banks to make higher profits. Li’s formula enabled these transactions to occur because it established a way to price the slices.
In other words, if I buy into a pool of bonds, I am giving money (via an investment bank) to a bunch of companies and expecting them to pay me back with interest. It’s possible that some or all of the companies won’t be able to pay back the money (they squander the loans). A bunch of people buy into this pool in order to spread the risk of non-payment around. Collectively, the companies agree to repay 10,000. If 10 investors are going to loan money and expect to get back 1000 each, we can split it up with the most risk-hungry person paying in only, say 250 (expecting 300% return) and the most risk-adverse paying 750 (expecting 33% return). The reason the first investor is higher-risk is that if the companies can only repay 9000, the first investor gets nothing and everyone else gets their 1000.
It is difficult to figure out the price spread (between 250 and 750 in my example) unless you know the relationship between the defaults of the various companies. Li’s formula defined this relationship and gave a number to price the slices. The model takes the current prices of all the individual bonds and gives a correlation number which implies the probability of everyone defaulting all at once. Now that this probability is “fixed” or “known,” it can be used to price the pool and different sections of the pool.
In addition to re-selling bonds it bought from companies, a bank can sell bond-insurance (called a credit-default swap, CDS). This means it collects money from investor-individuals and promises to pay if a particular bond defaults. The bank can also pool the bond-insurance and then slice up the pool in a way similar to the bond-pool case. It then passes out the money it collected for insuring the bonds but passes out more to those who will have to compensate for default first. Li’s formula was also used to create and trade the slices of insurance.
With pools of bond-insurance (called a synthetic collateralized debt obligation, CDO), I receive money and agree to pay when companies default to a certain degree. So in that pool above, when only 9000 of the 10,000 can be repaid, I have to make up the other 1000. If I take on this first part of the default obligation, I demand more money for my promise to pay. The payment each insurer (investor) receives varies and the Li’s formula was used to determine the payments (prices). This is (one of) the source(s) of the leverage people have been talking about. Investor-insurers, like hedge funds, were able to collect huge sums from these synthetic CDOs, pools of CDSs. In return for these huge sums, they had to promise to compensate others for company defaults, but they weren’t required to have anything to prove their ability to compensate. The 100 to 1 leveraged bank had gathered 100 in CDS premiums and had to show only 1 of ability to pay.