As the stock market soars to one record high after another, analysts
do not hesitate to tell us why. One popular explanation is that expectations of
higher interest rates are pushing up the stocks of banks and other financial
companies (example). Yet
not so long ago, the same analysts were telling us that Wall Street in general
and banks in particular were getting rich on the “free money” that the Fed was
supplying to them at historically low
rates (examples here
and here).
What gives?
To understand how interest rates affect bank profits, we turn
to a wonky concept of financial economics known as the duration gap. Setting the precise mathematics to one side (read
this if you really care), the duration gap refers to the difference between
the maturity of a bank’s assets and its liabilities. If a bank funds itself
with from short-term sources like deposits and uses those funds to make fixed
rate mortgage loans or buy long-term bonds, then it has a positive duration
gap. Interest rates tend to be higher on long-term financial instruments than
on those with short maturities, so is the way banks traditionally made a
profit.
The downside of the traditional banking model is that a positive
duration gap means that profits fall when interest rates rise. Suppose, for example, that your bank makes 30-year
fixed-rate mortgages and funds them with deposits that pay an interest equal to
the federal funds rate (the rate on overnight loans that the Fed uses at its
primary interest rate target). If the loans earn 4 percent and the fed funds
rate is 0.5 percent, you have a nice spread of 3.5 percent between return on assets and cost of funds, allowing
a good profit even after deducting operating
expenses. However, if short-term rates
went up, your bank would be in trouble. If the fed funds rate went up to 2
percent while your old fixed-rate mortgages still brought in just 4 percent
your spread would be cut to 1.5 percent and your profits, after operating
expenses, might evaporate altogether.






