A risk management team at a large institutional investor is analyzing its portfolio of mortgage-backed securities. One significant holding is a support tranche from a Collateralized Mortgage Obligation (CMO) backed by a pool of 30-year fixed-rate mortgages with a weighted average coupon (WAC) of 4.5% and a current aggregate balance of $500 million. The support tranche is specifically designed to absorb excess prepayment and extension risk to stabilize the cash flows of the senior Planned Amortization Class (PAC) tranches. If market interest rates experience a sudden and significant increase of 150 basis points, what is the most likely impact on the effective duration of this support tranche, assuming no change in credit spreads?
A.Its effective duration will significantly increase as slower prepayments extend its expected maturity and cash flow stream.
B.Its effective duration will decrease due to the higher discount rate applied to its future cash flows.
C.Its effective duration will remain largely unchanged, as support tranches are designed to absorb volatility but not experience significant duration shifts themselves.
D.Its effective duration will decrease, but only marginally, as the increased discount rate is partially offset by a slight extension of its cash flows.
Rationale:
When market interest rates rise significantly, homeowners are less likely to refinance their mortgages, leading to a slowdown in prepayments. A support tranche in a CMO is designed to absorb the prepayment and extension risk from the more stable PAC tranches. In a rising rate environment, as prepayments slow, the PAC tranches are protected by receiving their scheduled principal payments, while the support tranche's principal payments are consequently delayed. This delay in principal payments extends the weighted average life of the support tranche, which directly leads to a significant increase in its effective duration. This phenomenon is known as extension risk, and support tranches are specifically exposed to it. The generalized decrease in duration due to higher discount rates on future cash flows, while technically present, is typically dominated by the extension effect for mortgage-backed securities, especially for support tranches which bear the brunt of this risk.
The option suggesting a decrease due to higher discount rates misapplies the general duration rule without accounting for the embedded option (prepayment behavior) that is dominant for MBS. The idea that duration remains largely unchanged misunderstands the core function of a support tranche; its purpose is to absorb risk, meaning its own cash flow profile and duration are highly sensitive and volatile. The option suggesting a marginal decrease with slight extension incorrectly prioritizes the discount rate effect and understates the magnitude of the extension risk inherent in support tranches, which makes their duration highly sensitive to rate increases.
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