Saturday, May 15, 2010

Mortgage math

Matthew Yglesias discusses a simplified model of mortgage based structured financial products (CDOs and CDOs2) posted by Alex Tabarrok taken from a book by Robert Pozen. The model shows how structured finance can transform a collection of moderately risky mortgages into securities some of which are intended to be quite safe and others of which are intended to be quite risky by assigning defaults to the risky securities first. Not surprisingly if defaults turn out to be higher than expected some of the "safe" securities can prove risky.

Yglesias says "Importantly, this is not a scam. The math really checks out. ...". The math may check out but I think it is largely besides the point and these products were essentially scams. The main incentive for making financial products complicated is to make them hard to value and thus easier to sell for more than they are worth. Here the optimistic model assumptions which led to these securities being overvalued were not some unfortunate accident but a necessary part of the scheme. There is in fact no compelling reason to reassign the risk of defaults in this way. So if the complicated structured finance securities were valued correctly they would not be worth more than simple pools of their component mortgages meaning there would be no incentive to create them.

1 comment:

  1. I find it plausible that one incentive for people who create these things is to sow confusion. But as to another of your points - is it really true that there's never extra value created?

    Maybe I've just bought into the hype, but I was under the impression that, at least in some cases, the complex, derivative whole can be more than the sum of its simple, nonderivative parts, because different investors, due to their different circumstances, differ in their valuation of the derivative pieces.

    About here is where you sometimes step in with a wonderfully clarifying reductio ad absurdum thought experiment. I am glad to play straight man once again should you once again have one handy:-)