What Is Prepayment Risk in Mortgage-Backed Securities?
Buying a mortgage-backed security means, in effect, buying a stream of homeowner payments, and homeowners don’t always stick to the original schedule.
The short answer
Prepayment risk in mortgage-backed securities is the risk that homeowners pay off their mortgages earlier than expected, whether through refinancing, selling their homes, or simply paying extra toward principal. This returns an investor’s principal sooner than anticipated and can reduce the total interest income the investment was originally expected to generate.
Why mortgage-backed securities behave differently than typical bonds
A traditional bond generally pays interest on a set schedule and returns principal at a defined maturity date. A mortgage-backed security is built from a pool of individual home loans, and each of those homeowners has the option to pay off their loan ahead of schedule at any time, subject to their loan’s terms. That flexibility at the homeowner level becomes uncertainty at the investor level, since the pace at which principal is returned isn’t fixed the way it is with most conventional bonds.
Why prepayment tends to spike when rates fall
When interest rates fall, refinancing into a lower rate becomes more attractive for many homeowners, and mortgage payoffs across a pool tend to pick up. For an investor holding the security, this means principal comes back faster than expected, but often at a time when reinvesting that money at a similarly attractive rate has gotten harder, since rates have moved lower in the meantime. This mismatch is essentially the reverse of extension risk, which shows up when rates move in the other direction.
Other things that can drive prepayments
- Home sales. A homeowner selling their home typically pays off the existing mortgage in full, regardless of where interest rates stand.
- Extra principal payments. Some homeowners pay more than their required amount, gradually accelerating mortgage amortization even without a full refinance.
- General economic conditions. Employment trends and housing market activity both influence how often people move or refinance across a given pool of loans.
A hypothetical illustration
Suppose an investor buys a mortgage-backed security expecting the underlying loans to pay down gradually over roughly a decade. If rates then fall sharply, a large share of those homeowners might refinance within a much shorter window, and the investor could see a large portion of the expected principal returned years ahead of schedule. The money comes back, but the steady stream of interest payments the investor had been counting on ends sooner than planned, and the reinvestment options available at that moment may look less attractive than the ones originally assumed.
How this differs from a plan built around a fixed schedule
Unlike a bond with a known maturity date, a mortgage-backed security’s expected timeline is really an estimate built from assumptions about homeowner behavior. A wave of refinancing activity can shorten that estimate considerably, which is part of why these securities are priced and evaluated somewhat differently than a bond with more conventional duration characteristics.
What to weigh
Prepayment risk is a reminder that mortgage-backed securities carry a layer of behavioral uncertainty that most other bonds don’t. Understanding that early principal return isn’t automatically a bonus, since it can arrive at an inconvenient time for reinvestment, helps clarify what makes this type of investment structurally different from a bond with a fixed schedule.