Items from 7 people Eapen thomas follows

Under the threat of a collapse of the entire system, the safety net - intended to help unfortunate individuals meet the exigencies of life - was generously extended to commercial banks, then to investment banks, insurance firms, auto companies, even car-loan companies. Never has so much money been transferred from so many to so few.

But mortgage pools are messier than most bonds. There's no guaranteed interest rate, since the amount of money homeowners collectively pay back every month is a function of how many have refinanced and how many have defaulted. There's certainly no fixed maturity date: Money shows up in irregular chunks as people pay down their mortgages at unpredictable times—for instance, when they decide to sell their house. And most problematic, there's no easy way to assign a single probability to the chance of default.

Wall Street solved many of these problems through a process called tranching, which divides a pool and allows for the creation of safe bonds with a risk-free triple-A credit rating. Investors in the first tranche, or slice, are first in line to be paid off. Those next in line might get only a double-A credit rating on their tranche of bonds but will be able to charge a higher interest rate for bearing the slightly higher chance of default. And so on.

<!-- pagebreak -->

The reason that ratings agencies and investors felt so safe with the triple-A tranches was that they believed there was no way hundreds of homeowners would all default on their loans at the same time. One person might lose his job, another might fall ill. But those are individual calamities that don't affect the mortgage pool much as a whole: Everybody else is still making their payments on time.

But not all calamities are individual, and tranching still hadn't solved all the problems of mortgage-pool risk. Some things, like falling house prices, affect a large number of people at once. If home values in your neighborhood decline and you lose some of your equity, there's a good chance your neighbors will lose theirs as well. If, as a result, you default on your mortgage, there's a higher probability they will default, too. That's called correlation—the degree to which one variable moves in line with another—and measuring it is an important part of determining how risk

mortgage bonds are.

But one of the key insights on optimal capital structures may relate not to regulation but to tax.  In Modiglaini and Miller’s analysis, the one factor that clearly makes it rational for either banks or corporates to increase their leverage is that in almost all corporate tax regimes across the world, returns to debt providers are tax deductible, but returns to equity providers not.  Even, therefore, if we had sufficient confidence in free market rationality to assume that both corporate and banks in an unbiased world would chose capital structures that optimally balance equity and debt, tax regimes have  introduced a huge bias towards sub-optimally high leverage.  Changing those tax regimes now may be impossibly difficult.  But we do at least need to understand that the bias exists and that our regulatory approach needs to lean against it.