Negative
convexity is a concept that plays a significant role in the
bond market, particularly in relation to mortgage-backed securities (MBS) and callable bonds. It refers to the non-linear relationship between changes in
interest rates and the price of a bond or security. In simple terms, negative convexity implies that the price of a bond does not increase proportionally with a decrease in interest rates, and it may even decrease.
To understand negative convexity, it is essential to grasp the concept of convexity itself. Convexity measures the curvature of the relationship between bond prices and yields. A positively convex bond exhibits a curved relationship, where the price increases at an increasing rate as yields decrease. Conversely, a negatively convex bond displays a curved relationship where the price increases at a decreasing rate as yields decrease or may even decrease.
Negative convexity is primarily associated with mortgage-backed securities, which are pools of mortgages packaged into bonds and sold to investors. These securities are subject to prepayment
risk, meaning that homeowners can
refinance their mortgages or sell their homes, resulting in the early repayment of the underlying loans. When interest rates decline, homeowners tend to refinance their mortgages to take advantage of lower rates, which accelerates prepayments.
The prepayment feature of mortgage-backed securities introduces negative convexity. As interest rates decrease, the likelihood of prepayments increases, causing the average life of the security to shorten. This shortening of the average life reduces the duration of the security, making it less sensitive to further
interest rate declines. Consequently, the price appreciation potential of the security diminishes.
The impact of negative convexity on mortgage-backed securities can be further understood by examining two key components: price
volatility and
yield volatility. Negative convexity amplifies price volatility, meaning that when interest rates change, the price of a negatively convex security will fluctuate more significantly compared to a positively convex security with similar duration and yield. This increased price volatility can introduce greater risk for investors.
Additionally, negative convexity affects yield volatility. When interest rates decline, the yield of a negatively convex security will decrease at a slower rate compared to a positively convex security. This reduced rate of yield decline can lead to a phenomenon known as "yield extension." Yield extension occurs when the expected cash flows from a bond are spread over a longer period due to slower prepayments, resulting in an extended duration. As a consequence, the bond becomes more exposed to potential interest rate increases, which can lead to greater losses if rates rise.
The impact of negative convexity is not limited to mortgage-backed securities. Callable bonds, which grant the issuer the right to redeem the bond before
maturity, also exhibit negative convexity. When interest rates decline, issuers are more likely to call their bonds and refinance at lower rates, leaving investors with reinvestment risk. This risk arises from the fact that investors may have to reinvest their
principal at lower rates, potentially reducing their overall return.
In conclusion, negative convexity is a crucial concept in the
bond market, particularly in relation to mortgage-backed securities and callable bonds. It describes the non-linear relationship between changes in interest rates and bond prices. Negative convexity introduces price volatility and yield extension, which can increase risk for investors. Understanding negative convexity is essential for market participants to effectively manage their bond portfolios and assess the potential impact of interest rate changes.