Why banks prefer your mortgage to fluctuate with the market
In short: Most people view a mortgage as a static debt, but for many banks, a fixed rate is a liability. By shifting interest rate risk onto you, lenders can protect themselves from the volatile costs of borrowing, creating a financial structure that favors their balance sheet over your long-term predictability.
Adjustable-rate mortgages (ARMs) function by periodically resetting the interest rate based on a market index, such as the 1-year constant-maturity Treasury (CMT) securities or the London Interbank Offered Rate (LIBOR). This mechanism allows the lender to pass on the cost of borrowing from credit markets directly to the borrower. While fixed-rate mortgages are shielded from inflation risk, ARMs are exposed to real interest rate fluctuations. Consequently, borrowers benefit when rates fall but face significant financial hardship if they rise. The primary driver for ARMs is the prevention of "asset-liability mismatches." Banks often fund long-term mortgages using short-term customer deposits. If a bank holds only long-term fixed-rate mortgages, it risks a scenario where the interest it earns is lower than what it must pay to depositors. Historical context highlights this danger: some argue the savings and loan crisis of the early 1980s was caused by this exact mismatch when interest rates were raised. ARMs solve this by ensuring the mortgage income stays aligned with the lender's funding costs. To manage the volatility of these loans, many ARMs include "caps"—legal limits on how much a rate can change during an adjustment period or over the life of the loan. However, many ARMs also feature "teaser periods," where the initial rate is substantially below the fully indexed rate for one year or less. These promotional discounts can make a loan appear to be a bargain, while actually predisposing the borrower to above-average payment increases once the adjustment period begins.
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Sources
- Summary by Intellect · source: Adjustable-rate mortgage
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