Negative Convexity Drag (MBS Prepayment Risk)
The Formal Definition
A structural risk characteristic common in mortgage-backed securities (MBS) and callable bonds where falling interest rates cap price appreciation due to homeowners refinancing their mortgages early, while rising interest rates cause duration to lengthen and prices to plummet.
Negative Convexity Payoff: As Yields Fall → Mortgage Prepayments Surge → Price Capped at Par | As Yields Rise → Prepayments Freeze → Duration Lengthens & Price Crashes
Cole Barrett's Reality Check
The Unvarnished Bottom Line"Negative convexity is heads the bank wins, tails you lose. When you hold an agency mortgage bond, you are lending money to homeowners. If interest rates drop to 2%, homeowners don't keep paying you high interest—they refinance and pay off the mortgage, handing you cash when bond yields are at the bottom. If interest rates surge to 7%, nobody moves, the mortgage stays alive for thirty years, and your bond drops like a rock."
Interactive Simulator: Test the Math
Real-World Example: Scenario Breakdown
Examining the real numbers for: $100,000 invested in an Agency Mortgage-Backed Security ETF (e.g., MBB) across a full interest rate cycle
| Execution Metric | Positive Convexity Treasury Investor (e.g., IEF) | Negative Convexity MBS Investor |
|---|---|---|
| Fee / Rate | $0.00 | 0.05% Fund TER |
| Spread / Buffer | Invested in non-callable US Treasuries with pure positive convexity | Held agency MBS portfolio; yields dropped 2% |
| Execution / Status | Rates dropped 2%: Treasury prices surged +18% without early redemption caps | Prepayment wave triggered; mortgages paid off early at par, capping capital appreciation at only +4% |
| Total Cost / Result | Maximized capital gains through positive convexity | Suffered asymmetric performance drag from negative convexity |
How Brokers Weaponize This Term
Advisors recommend mortgage bond funds as 'higher-yielding government alternatives' without warning clients that negative convexity caps their capital gains when interest rates decline.
Broker Evaluation Matrix
Cole Approves
Interactive Brokers / Charles Schwab: Provides institutional fixed-income screening tools displaying Effective Duration and Effective Convexity metrics across all secondary mortgage and agency debt.
Read Audit →Cole Flags / Avoids
Automated Balanced-Fund Desks: Allocates conservative retirement portfolios into mortgage-backed funds without disclosing prepayment capping and negative convexity risks.
View Trap Details →Frequently Asked Questions
What is 'extension risk' in mortgage bonds?
The risk that rising interest rates slow down mortgage refinancing and home sales, causing the effective maturity of the mortgage bond to lengthen right when bond prices are falling.
Why do mortgage-backed securities offer higher yields than Treasuries?
The yield markup (mortgage basis) compensates investors for taking on prepayment risk and structural negative convexity.