Key Takeaways
- Prepayment risk in an MBS is the uncertainty over when homeowners repay their mortgages early, which changes the timing of an investor's cash flows.
- When mortgage rates fall, refinancing tends to rise (contraction risk). When rates rise, prepayments slow and the bond's life stretches (extension risk).
- Because of this two-way pattern, MBS often show negative convexity, so they usually gain less than a plain bond when rates fall.
The Story: 30-Year Mortgage Rates Slip to 7.31%
In the latest daily reading, the average 30-year mortgage rate fell to 7.31%. For a home buyer, the headline is about borrowing costs. For an investor who holds mortgage-backed securities, the same move raises a different question: how many of the borrowers behind the bond now have a reason to refinance?
That question is the heart of prepayment risk MBS investors must price. A mortgage-backed security pools thousands of home loans and passes the borrowers' interest and principal payments through to bondholders. The investor does not receive a fixed schedule like a conventional bond. Instead, the cash flow depends on what homeowners choose to do, and what they choose to do depends heavily on where mortgage rates sit compared with the rate on their own loan.
The 7.31% figure does not say how much prepayment is happening. What it gives us is a reference point. A borrower whose loan carries a rate above the market rate has an incentive to refinance. A borrower whose loan carries a rate below it has almost none. The gap between those two groups is what drives the behaviour of an MBS pool.
What Prepayment Risk Actually Means
A homeowner can usually repay a mortgage early. They may refinance, sell the house, or simply pay extra principal. Each of these returns principal to the MBS investor sooner than the original schedule assumed. Early repayment is not a default. The investor gets the money back in full, but at a time the investor did not choose.
This matters because investors price an MBS using an assumed pace of prepayments. If actual prepayments differ from the assumption, the bond's true life and return differ too. The CFA curriculum treats this as a core feature of securitised products, and it is one of the main reasons an MBS cannot be analysed like a standard fixed-rate bond.
There are two sides to the risk. Contraction risk appears when rates fall and borrowers refinance in large numbers. The investor is repaid early and must reinvest at the new, lower yields. Extension risk appears when rates rise and borrowers hold on to their cheap loans. Prepayments slow, the bond lasts longer than expected, and the investor is stuck holding a below-market coupon for longer.
Now apply this to the latest rate move. A drop to 7.31% is a step toward the contraction side of the picture. If rates kept falling, more borrowers whose loans sit above the market rate could find refinancing worthwhile, and pools holding those loans would repay faster. A borrower who locked in a rate far below 7.31% has little reason to move, so that part of a pool would stay put. This is why two MBS with the same issuer and maturity can behave very differently: the mix of borrower coupons inside the pool matters.
Why Falling Rates Are Not Simple Good News for MBS Holders
When yields fall, a regular non-callable bond rises in price without limit from prepayment worries. An MBS is different. The homeowner effectively holds an option to repay the loan at par, and the investor has sold that option. As rates fall, the option becomes more valuable to the borrower and the MBS price gains get capped by the expectation of faster repayment.
This is called negative convexity. Its practical meaning is simple: the MBS holder gets less of the upside when rates fall, yet still faces the downside when rates rise and the bond extends. That is an uncomfortable asymmetry, and it is the reason MBS usually offer extra yield over comparable Treasuries. The extra spread is compensation for bearing prepayment uncertainty, not a free bonus.
The practical skill is to look at a rate move like this one and say which way prepayment speeds are likely to shift and what that does to an MBS investor.
What to Watch Next, and the Limits of One Data Point
The 7.31% reading is a single data point. Prepayment speeds respond to the level of rates over time, to how long rates stay there, and to practical factors such as home sales, borrower credit and the cost of refinancing. The source does not say how large the move was, so we should not assume a wave of early repayments.
Whichever way rates move from here, the investor's question stays the same: how does the pool's mix of loan rates compare with where new loans are being priced?
It also helps to remember that not all mortgage pools behave alike. Agency pools, and privately structured deals backed by other types of borrowers, carry different credit features on top of prepayment behaviour. This article only isolates the prepayment piece, which is the part that comes straight from the rate environment.
Turning the Headline Into Exam Skill
Ask yourself who gains and who loses when rates fall, why the MBS price reacts differently from a standard bond, and how contraction and extension risk fit together. If you hesitate on any of these, the gap is worth closing before exam day.
To see where you stand, try a free 30 question mini mock exam from EduFite and note how you do on the fixed income questions. The full EduFite course then builds the complete mechanics and structured practice behind prepayment risk, convexity and securitised products, so you can handle them calmly when they appear in the exam.
FAQ
What is prepayment risk in an MBS?
It is the risk that homeowners repay their mortgages earlier or later than expected, which changes the timing of the cash flows an MBS investor receives and, in turn, the bond's return.
Why do falling mortgage rates create contraction risk?
Lower market rates give borrowers with higher-rate loans a reason to refinance. Principal is returned early, and the investor has to reinvest at lower yields.
What is extension risk?
Extension risk arises when rates rise and borrowers prepay more slowly. The MBS lasts longer than expected, leaving the investor holding a below-market coupon for longer.
Why do MBS show negative convexity?
Homeowners hold an option to repay early. When rates fall, that option limits the MBS price rise. When rates rise, the bond lengthens. This asymmetry is negative convexity.