Negative Convexity
Negative Convexity describes bonds whose price appreciation is capped or curtailed in falling-rate environments because embedded options — such as call provisions or mortgage prepayment rights — allow the issuer or borrower to retire the debt early, limiting the bondholder's upside.
Standard positive-convexity bonds gain more in price when rates fall than they lose when rates rise by the same amount. Bonds with negative convexity behave oppositely in at least some rate scenarios: price gains are limited or reversed even as rates decline, because rising bond prices trigger the exercise of embedded options that work against the holder.
Callable corporate bonds are the most straightforward example. If a company issues a 10-year callable bond at a 5% coupon and market rates subsequently fall to 3%, the company will likely call the bond and refinance at the lower rate. The bondholder receives par value rather than the higher market price the bond would command if it could not be called. This capping of price appreciation is the essence of negative convexity from the investor's perspective.
Mortgage-backed securities (MBS) are the most prominent negatively convex instruments in the US fixed income market. When interest rates fall, homeowners refinance their mortgages en masse — the most prepayment-sensitive borrowers exit first, leaving the MBS holder with early return of principal at par precisely when they would prefer to hold a high-coupon asset. Conversely, when rates rise and prepayments slow, the MBS extends in duration, meaning the investor is stuck with a below-market-rate instrument for longer than expected. This dual dynamic — price compression in rallies, duration extension in selloffs — is why MBS analysis requires sophisticated models.
Investors in negatively convex securities are compensated for this unfavorable characteristic through higher yields. The OAS on MBS or callable bonds is generally higher than on equivalent maturity Treasuries even after stripping out option value, reflecting the additional uncertainty and the unfavorable convexity profile.
Prudent fixed income portfolio management explicitly accounts for negative convexity by measuring OAS-duration, tracking how much negatively convex exposure exists in a portfolio, and ensuring the higher spread income adequately compensates for the asymmetric return profile.