Negative convexity has significant implications for mortgage-backed securities (MBS). Mortgage-backed securities are financial instruments that represent an ownership interest in a pool of mortgage loans. These securities are created by pooling together individual mortgages and then selling them to investors as a single security. The cash flows generated from the underlying mortgage loans are passed through to the MBS holders.
Convexity refers to the relationship between the price of a bond or security and its yield. A positively convex security will have a price-yield relationship that is upward sloping, meaning that as yields decrease, the price of the security increases at an increasing rate. Conversely, a negatively convex security will have a price-yield relationship that is downward sloping, meaning that as yields decrease, the price of the security increases at a decreasing rate.
Negative convexity in mortgage-backed securities arises due to prepayment risk. Prepayment risk refers to the possibility that borrowers will pay off their mortgage loans earlier than expected, usually through refinancing or selling their homes. When interest rates decline, borrowers have an incentive to refinance their mortgages at lower rates, resulting in prepayments.
The prepayment feature of mortgage-backed securities introduces negative convexity because it affects the expected cash flows and duration of the security. As interest rates decline, the likelihood of prepayments increases, which reduces the expected maturity of the security. This reduction in maturity leads to a decrease in the duration of the MBS.
The implications of negative convexity for mortgage-backed securities can be summarized as follows:
1. Extension Risk: Negative convexity exposes MBS investors to extension risk. Extension risk refers to the risk that the expected cash flows from the MBS will extend over a longer period than anticipated. As interest rates decline, borrowers are less likely to refinance their mortgages, resulting in slower prepayment speeds and longer expected maturities. This extension of cash flows can lead to increased price
volatility and uncertainty for MBS investors.
2. Price Volatility: Negative convexity amplifies price volatility in MBS. When interest rates decline, the price of a negatively convex MBS increases at a decreasing rate. This means that as interest rates continue to fall, the price appreciation of the MBS slows down. Conversely, when interest rates rise, the price of the MBS can decline rapidly. This price volatility can make it challenging for investors to accurately assess the value and risk of MBS.
3. Yield Spreads: Negative convexity affects the yield spreads of MBS relative to
benchmark Treasury securities. As interest rates decline, the prepayment risk associated with MBS increases, leading to higher yields compared to Treasury securities. This is because investors require compensation for the extension risk they are exposed to. The widening of yield spreads can make MBS less attractive to investors, reducing their demand and potentially increasing their cost of funding.
4. Hedging Challenges: Negative convexity presents challenges for hedging strategies used by MBS investors. Traditional hedging techniques, such as using Treasury
futures or interest rate swaps, may not fully hedge the extension risk associated with MBS due to their negative convexity. This can result in imperfect hedges and potential losses for investors.
In conclusion, negative convexity has significant implications for mortgage-backed securities. The prepayment risk inherent in MBS introduces negative convexity, leading to extension risk, price volatility, widened yield spreads, and challenges in hedging strategies. Understanding and managing these implications is crucial for investors in mortgage-backed securities to effectively assess and mitigate the associated risks.