Extension Risk
Extension risk is the risk, specific to mortgage-backed securities (MBS) and other prepayable fixed income instruments, that rising interest rates cause borrowers to slow or stop prepayments—because refinancing is no longer economically beneficial—thereby extending the effective duration of the security beyond initial expectations and exposing investors to longer-than-anticipated interest rate risk. Extension risk is the opposite of prepayment risk (contraction risk).
Key takeaways
- Extension risk arises because MBS investors are short the prepayment option: borrowers can repay early at par, benefiting them when rates fall (contraction risk) or they extend at a disadvantage when rates rise (extension risk).
- When rates rise and prepayments slow, MBS duration extends, causing prices to fall more than initially expected under a parallel shift.
- Average life—the weighted average time to principal receipt—can extend from 5 years to 15+ years as prepayment speeds drop from 20 PSA to near zero.
- Planned Amortization Class (PAC) bonds in CMO structures are specifically designed to provide extension protection by transferring prepayment variability to 'support' or 'companion' tranches.
- OAS (Option-Adjusted Spread) analysis accounts for extension and contraction risk by simulating interest rate paths and averaging spreads across scenarios.
Explanation
Extension risk is one of the two faces of prepayment risk in mortgage-backed securities, representing the negative convexity that makes MBS fundamentally different from standard fixed-rate bonds. When an investor purchases a mortgage-backed security, they are effectively selling a call option to the underlying borrowers: each homeowner retains the right to prepay their mortgage at par at any time. This embedded option is valuable to borrowers—they exercise it by refinancing when rates fall—and costly to MBS investors, who receive par back just when reinvesting at lower rates is least attractive (contraction risk). The opposite scenario—rising rates and minimal prepayment—creates extension risk.
The PSA (Public Securities Association, now SIFMA) prepayment model provides a standard framework for describing prepayment speeds. 100 PSA assumes a schedule of prepayments starting at 0.2% annualized conditional prepayment rate (CPR) in month 1 and rising to a plateau of 6% CPR by month 30. Actual prepayments in a rising rate environment might fall to 5% PSA (very slow) or even 0% CPR, dramatically extending the expected average life of the security. A 30-year mortgage pool with 200 PSA prepayment speeds has an average life of approximately 7–8 years; at 50 PSA, the average life extends to 15+ years.
The duration extension caused by slower prepayments compounds the price decline of MBS in rising rate environments. A standard fixed-rate bond has positive convexity—as rates rise, its duration shortens modestly (because the present value of near-term cash flows becomes more important). An MBS has negative convexity: as rates rise, prepayments slow, duration extends, and the security acts like a longer bond precisely when holding longer duration is most painful. This negative convexity is the primary reason MBS typically trade at a spread above equivalent-duration Treasuries—the 'MBS spread' compensates investors for the option they have sold.
Collateralized Mortgage Obligations (CMOs) were developed in part to address extension risk through structured segmentation of cash flows. Planned Amortization Class (PAC) bonds define a principal payment schedule that is maintained across a range of prepayment speeds (the 'PAC band'). As long as actual prepayment speeds remain within the band, PAC bond investors receive stable, predictable cash flows. The variability that PAC holders avoid is absorbed by companion (or support) tranches, which receive excess principal when prepayments are fast and defer principal when prepayments are slow—experiencing the full volatility of prepayment uncertainty on behalf of PAC bond holders.
For fixed income portfolio managers, extension risk must be managed through careful duration targeting and convexity hedging. Managers who are overweight MBS relative to Treasury benchmarks must hedge the negative convexity by purchasing receiver swaptions or long-dated Treasury options—instruments that gain value as rates rise and MBS duration extends. The cost of this convexity hedge reduces the net yield advantage of MBS over Treasuries, making the net carry of hedged MBS positions a critical determinant of relative value.
Formula
Modified Duration (MBS) = Duration at current PSA speed; Price ≈ -Modified Duration × ΔYield; Negative Convexity: ∂²Price/∂Yield² < 0 for MBS
Example
An MBS investor purchases $10 million face value of agency MBS at a coupon of 5.5%, expecting a 150 PSA prepayment speed and a 9-year average life. The investor's modified duration is 6.5 years. Rates rise 100 basis points, causing prepayments to slow to 50 PSA. The new average life extends to 16 years, and the modified duration rises to 11.0 years. The 100 bps rate increase now causes a price decline of approximately 11.0% (instead of the initially expected 6.5%)—a 4.5 percentage point larger loss due to extension risk. If the investor had bought a standard 9-year Treasury bond at purchase, a 100 bps rate rise would cause approximately a 6.5% loss (roughly constant duration, positive convexity). The 4.5% additional loss represents the realized cost of extension risk.
Related terms
Basis Bond Call Option Clean Price Convexity Day Count Convention Duration Effective Duration Face Value Flat Yield Curve Hedging Interest Rate