Extension risk in mortgage-backed securities is the danger that principal comes back to you later than you expected, because rising interest rates have stopped homeowners from refinancing or selling. Your capital stays locked inside a pool earning yesterday’s coupon while new securities offer better yields, and the market price of what you hold falls further than a comparable Treasury would. As of the end of 2025, the Federal Reserve’s own portfolio of residential MBS carried an estimated weighted-average life of 8.0 years, up from the shorter durations that prevailed before 2022.1Federal Reserve. Combined Financial Statements 2025 – Federal Reserve Banks That number is extension risk showing up on a balance sheet.
Why Rising Rates Slow Prepayments
A homeowner with a 3.5% fixed-rate mortgage has no reason to refinance when new loans cost 6% or more. Monthly payments would go up, and lifetime interest would go up with them. Multiply that rational decision across millions of borrowers in a pool, and prepayment speeds collapse. The MBS investor who expected a steady stream of early payoffs instead receives only the scheduled monthly payments.
Rising rates also freeze the housing market. Giving up a cheap mortgage to buy a different house at a higher rate feels like a self-inflicted penalty, so people stay put. This lock-in effect has been unusually sharp since 2022, when the gap between existing mortgage rates and new-loan rates widened to historically large levels.2Federal Reserve. Locked In: Mobility, Market Tightness, and House Prices Fewer sales means fewer mortgages paid off at closing, which means less principal returning to MBS investors.
Even when rates eventually drop, prepayments don’t always snap back. Refinancing burnout describes borrowers who fail to refinance even when it would save them money, whether because of poor credit, inertia, or trouble qualifying.3Federal Reserve Board. Crowding Out Effects of Refinancing on New Purchase Mortgages After a long stretch of high rates, the borrowers most ready to act have already left the pool in earlier windows. The ones left behind respond slowly, which means prepayment recoveries undershoot what rate models alone would predict.
What the Slowdown Does to Your Cash Flow
Weighted Average Life (WAL) is how MBS investors talk about when their principal is actually coming back. An MBS doesn’t mature on a single date like a corporate bond; principal arrives in irregular chunks as borrowers pay down their loans and occasionally pay them off entirely. WAL averages those arrival times, weighted by dollars. A WAL of five years means that on average each dollar of principal comes back around the five-year mark.
Extension risk stretches WAL beyond the original estimate, sometimes by years. A security bought with an expected WAL of five years can extend to eight or nine years in a sharp rate rise. The Federal Reserve’s residential MBS portfolio carried a WAL of 8.0 years at the end of 2025 and 8.3 years at the end of 2024, numbers that reflect how far the high-rate environment has pushed durations across agency MBS.1Federal Reserve. Combined Financial Statements 2025 – Federal Reserve Banks
The practical problem is a mismatch. If a pension fund counted on principal arriving by year five to meet benefit payments and that principal won’t show up until year eight, the fund has to find liquidity elsewhere. That could mean selling other assets at an awkward time or borrowing to bridge the gap. The WAL extension is not just a number on a spreadsheet; it moves through the whole balance sheet.
Why MBS Prices Fall Further Than Treasuries
MBS have a property called negative convexity, and it works against the investor in both directions. In a standard bond, prices move roughly symmetrically with rates. MBS break that symmetry because every borrower holds an embedded prepayment option.
When rates rise, MBS prices fall for the same reason all bond prices fall: future cash flows get discounted at a higher rate. But MBS fall further than a comparable Treasury because those cash flows are also being pushed further into the future by slowing prepayments. The investor holds a lower-yielding asset for longer, and both effects compound the price decline. One industry analysis cited in the research found that for a 150-basis-point rise in rates, the duration of a broad MBS index extended from 4.54 years to 5.60 years, with total returns declining by more than 360 basis points as price losses overwhelmed coupon income.
When rates fall, you might expect a strong rally, but MBS underperform there too. Homeowners refinance, principal comes back early, and the investor has to reinvest at the new lower yields. That asymmetry is the defining feature of negative convexity, and it is the reason MBS normally offer a yield premium over Treasuries. The market demands payment for the risk.
Effective Duration Is the Only Honest Measurement
Macaulay and modified duration assume fixed, known cash flows. For MBS, where any borrower can refinance on any morning, that assumption falls apart. Effective duration measures price response to shifts in the benchmark yield curve rather than to changes in a security’s own yield-to-maturity, and for bonds with embedded options it is the only interest rate sensitivity measure that holds up.
Effective duration also moves. In a low-rate environment with fast prepayments, an MBS might show an effective duration of three years. If rates spike and prepayments vanish, that same security’s effective duration can jump to seven. The portfolio’s risk profile has changed without anyone buying or selling. Hedge ratios set last month may be stale this month, which is why MBS portfolio managers recalculate constantly. A duration jump can also feed through to collateral: the Treasury Market Practices Group recommends that counterparties exchange two-way variation margin on forward-settling agency MBS transactions, so a price decline driven by extension can trigger additional margin calls.4Federal Reserve Bank of New York. Frequently Asked Questions: Margining Agency MBS Transactions
Which MBS Structures Get Hit Hardest
Not every mortgage-backed security carries the same extension exposure. In a collateralized mortgage obligation (CMO), the structure decides who absorbs prepayment volatility and who is shielded from it.
Sequential-Pay Tranches
Sequential-pay structures send all principal to the first tranche until it is retired, then to the second, and so on. The tranches at the back of the line are acutely exposed to extension risk because they receive nothing until everything ahead of them is paid off. When prepayments slow, their wait grows by years. A final sequential tranche priced with an expected WAL of twelve years can stretch well beyond that if borrowers stay locked in.
Companion (Support) Tranches
Companion tranches exist to absorb prepayment volatility so other classes can stay on schedule. When prepayments speed up, they soak up the excess. When prepayments slow down, they take the brunt of the extension. Philadelphia Fed research on CMO structures found that in one Freddie Mac multiclass series, sequential fixed-rate tranches showed WAL ranges running from 0.09 years at one prepayment speed to 10.18 years at another.5Federal Reserve Bank of Philadelphia. Understanding and Measuring Risks In Agency CMOs Companion tranches offer higher yields because buyers accept exactly that kind of uncertainty.
Planned Amortization Class (PAC) Bonds
PACs are the protected class. They operate inside a predetermined band of prepayment speeds, often called a collar, and as long as actual prepayments stay inside that band, the payment schedule holds. The protection works by diverting volatility into the companion tranches. The protection has limits. If prepayments stay far outside the band long enough and the companion tranches have already been exhausted, even PACs can extend. Treating a PAC as bulletproof in a severe rate shock is a mistake.
Interest-Only Strips
Interest-only (IO) strips are the one MBS structure that benefits from extension. An IO receives only the interest portion of the mortgage payments. When prepayments slow and the pool’s life stretches, the IO holder collects interest for longer, and the strip gains value. That is the opposite of how standard MBS behave.6Federal Reserve Bank of Kansas City. The Role of Stripped Securities In Portfolio Management
The reverse scenario is brutal. When rates fall and refinancing accelerates, the mortgages behind the strip pay off early, cutting the interest stream short, and the holder may not recover their initial investment. The Kansas City Fed has called the interest rate sensitivity of these instruments “extremely complicated” and “potentially dangerous in the hands of unsophisticated or unwary investors.” IOs can serve as a hedge against extension in another MBS position, but they require active management.
Extension Risk vs. Contraction Risk
Extension risk has a mirror image. Contraction risk, sometimes called prepayment risk, hits when rates fall and homeowners refinance in large numbers, returning principal early. The investor gets the cash back but can only reinvest at the new lower rates. Together, the two risks are what produces negative convexity: rates up, and you’re stuck with a low coupon for longer; rates down, and you get your money back at the worst time to redeploy it.
The two risks don’t weigh the same on every investor. A pension fund with long-dated liabilities may actually welcome extension if the coupon is attractive, because a longer-lived asset matches a longer-dated obligation. For that investor, contraction is the worse scenario. A portfolio manager who needs liquidity in a few years sees it the other way around. Figuring out where you sit is the first step toward choosing the right structure.
How Investors Measure and Hedge It
Pricing an MBS requires an assumption about how fast borrowers will prepay. The industry standard is the PSA prepayment model, originally developed by the Public Securities Association (now part of SIFMA). The 100% PSA benchmark assumes a conditional prepayment rate (CPR) of 0.2% in the first month after origination, rising by 0.2% each month until month 30, when it levels off at 6% CPR for the rest of the pool’s life. A 6% CPR means roughly 6% of the remaining balance is expected to prepay in a given year.
Faster or slower assumptions get expressed as multiples. At 200% PSA, prepayments run at twice the standard pace and plateau at 12% CPR. At 50% PSA, they crawl at half speed, topping out at 3%. Extension risk shows up in dollar terms as the gap between the PSA assumption baked into your purchase price and the speed you actually see. As of mid-2025, aggregate fixed-rate CPR speeds were running around 7.3% for Fannie Mae pools and 7.5% for Freddie Mac pools, a sluggish pace by historical standards that reflects the lock-in environment.7Ginnie Mae. Global Markets Analysis Report
The most common hedge for extension risk is selling Treasury bond or note futures to offset the rising duration of an MBS portfolio. If rising rates extend your MBS and make it behave like a longer-duration bond, a short Treasury futures position generates gains that compensate for the price decline. The hard part is that the hedge ratio is not static. As rates move, MBS duration shifts, and the number of futures contracts needed changes with it. Research in the Journal of Derivatives found that dynamically rebalanced hedges reduced residual portfolio volatility from 41 basis points to 24 basis points for weekly returns on GNMA securities, compared with 29 basis points for a static regression-based hedge. When rates are high and the prepayment option is deeply out of the money, MBS behave more like plain fixed-rate bonds and need a different hedge ratio than when rates are low and refinancing is active.
IO strips, discussed above, serve as a natural hedge because they gain value when extension occurs. Adding them to a portfolio heavy on pass-throughs can offset some of the duration extension and price decline that rising rates cause. The catch is that if rates reverse and drop sharply, the IO loses value at the same moment the rest of the portfolio faces contraction. That hedge needs frequent rebalancing.
Reading Prepayment Assumptions Before You Buy
The SEC requires issuers of asset-backed securities, including MBS, to provide static pool information on delinquencies, cumulative losses, and prepayments for prior securitized pools of the same asset type. The data must cover at least five years of history, or as long as the sponsor has been securitizing that asset type if shorter, and must include graphical illustration of prepayment trends.8eCFR. 17 CFR 229.1105 – (Item 1105) Static Pool Information
This disclosure is where you calibrate your own extension assumptions before you commit capital. If a sponsor’s prior pools consistently showed slower-than-expected prepayments during rising rate periods, that pattern sits in the static pool data, and it should feed directly into the PSA assumption you use to value the new offering. Buying an MBS without stress-testing its WAL against a realistic range of prepayment speeds is how extension risk turns from an abstract concept into a problem on your statement.