IF YOU MEET HOMO ECONOMICUS IN THE ROAD, KILL HIM:

Surprise: 'Toxic' Mortgages Are the Best: A new study from professors at Columbia and NYU finds that the "optimal" mortgage in a perfect world is an option ARM (Peter Coy, 99/21/07, Business Week)

[A]ccording to a new study by professors from Columbia and New York universities, the "optimal" mortgage in a perfect world is precisely that kind of loan—an adjustable-rate mortgage with an option for negative amortization and a ban (or at least severe restriction) on prepayment.

Crazy? Not as crazy as you might think. The key, according to professors Tomasz Piskorski of Columbia Business School and Alexei Tchistyi of New York University's Stern School of Business, is that this kind of mortgage is optimal only in a perfect world—namely, one in which borrowers are fully rational and always do what's in their own best interest. [...]

[H]ere's a quick, intuitive feel for the three parts of the concept:

• The option to pay less than the minimum monthly interest owed on the loan is valuable for people with good self-control whose income fluctuates a lot. They can pay just a little in lean months and catch up in fat months. It's good for lenders, too, because they don't have to foreclose on people who fall behind, which is an expensive process. People with steady incomes don't need this feature, but having it doesn't hurt them.

• The fact that the loan is an ARM—namely, its rate fluctuates with market interest rates—is especially valuable to lenders. This is a subtler notion, but the idea is that if there are going to be a certain number of defaults in a pool of mortgages because of random bits of bad luck like a job loss or a divorce, the lender would prefer that they be concentrated during periods of high interest rates. Why? Because when market interest rates are high, the lender that forecloses and gets back (most of) its money can redeploy the cash in high-yielding alternatives. The lender would prefer not to foreclose and get its money back when rates are low and other options are unattractive. An ARM loan achieves what the lender wants. Borrowers, meanwhile, are neutral about whether they default in periods of high or low market interest rates.

• Finally, the economists say the optimal loan contract would outright ban getting a new loan from a different lender. There are no such bans. But they say that the prepayment penalties that are common in subprime loans are a good second best. How could that be? Because lenders will offer more favorable terms if they know that they'll be able to hang onto the loan long enough for it to be profitable. If they fear that the borrower will refinance at the drop of a hat, they'll give less favorable terms.

One neat twist is that the paper demonstrates that an interest-only loan coupled with a home equity line of credit is also optimal because it's the functional equivalent of an option ARM. Think about why: Someone with an IO loan and a line of credit is equally able to tap into home equity (i.e., add to the principal owed) from month to month.