Negative
convexity refers to a characteristic of certain financial instruments or portfolios where the
price sensitivity to changes in
interest rates is asymmetric. In other words, as interest rates change, the price of the instrument or portfolio does not move in a linear fashion. Instead, it exhibits a non-linear relationship, resulting in a convex shape when plotted on a graph.
To understand negative convexity, it is essential to first grasp the concept of convexity itself. Convexity measures the curvature of the relationship between
bond prices and their yields. It quantifies how the price of a bond changes in response to fluctuations in interest rates. Positive convexity implies that as interest rates decrease, bond prices increase at an increasing rate, and as interest rates rise, bond prices decrease at a decreasing rate. This relationship creates a convex curve on a graph.
Negative convexity, on the other hand, occurs when the relationship between bond prices and interest rates is concave. This means that as interest rates decrease, bond prices increase at a decreasing rate, and as interest rates rise, bond prices decrease at an increasing rate. Consequently, the graph of negative convexity appears concave.
The primary reason for negative convexity is embedded call options or prepayment options associated with certain fixed-income securities. These options allow the issuer or borrower to redeem or prepay the debt before its
maturity date. Mortgage-backed securities (MBS) and callable bonds are common examples of securities with negative convexity.
In the case of MBS, homeowners have the option to
refinance their mortgages when interest rates decline. This leads to higher prepayment rates, as homeowners take advantage of lower borrowing costs. As a result, the cash flows from the underlying mortgages are returned to investors sooner than expected. This early return of
principal reduces the duration of the MBS and increases its price sensitivity to
interest rate changes. Consequently, MBS exhibit negative convexity.
Callable bonds also exhibit negative convexity due to the embedded
call option. When interest rates decline, issuers may choose to call back the bonds and reissue them at a lower interest rate, reducing their borrowing costs. This deprives bondholders of future interest payments and the potential for capital appreciation if interest rates continue to fall. As a result, callable bonds have negative convexity.
The key difference between positive and negative convexity lies in the price-yield relationship. Positive convexity implies that as yields decrease, prices increase at an increasing rate, while negative convexity suggests that as yields decrease, prices increase at a decreasing rate. Similarly, as yields increase, prices decrease at a decreasing rate for positive convexity and at an increasing rate for negative convexity.
In summary, negative convexity is a characteristic of certain financial instruments or portfolios where the price sensitivity to changes in interest rates is asymmetric and exhibits a concave shape on a graph. It arises due to embedded call options or prepayment options in fixed-income securities such as MBS and callable bonds. Understanding the differences between positive and negative convexity is crucial for investors and financial professionals to effectively manage interest rate
risk and make informed investment decisions.