Mortgage-backed security - Valuation Risks and Applications of Mortgage‑Backed Securities

Valuation and Pricing of Mortgage-Backed Securities Understanding Key Valuation Metrics Before diving into how mortgage-backed securities are priced, you need to understand the three fundamental metrics that describe any mortgage pool at the time of issuance. Weighted-average maturity (WAM) is simply the balance-weighted average of all the loan maturities in the pool. Think of it this way: if you have a pool with many 30-year mortgages and a few 15-year mortgages, the WAM will be pulled toward 30 years, weighted by the dollar amount of each loan. This metric tells you roughly how long the mortgage cash flows will last before all loans are paid off (assuming no prepayments). Weighted-average coupon (WAC) is the balance-weighted average of the interest rates on all the underlying mortgages. If you have a pool where most mortgages have a 4% rate but some have 3.5%, the WAC will sit somewhere between these values, weighted by loan balances. The WAC represents the average yield the mortgage servicer collects from borrowers. Pass-through rate is the rate that investors actually receive after the servicer takes a fee for managing the pool. If the WAC is 4% and the servicer keeps 0.25% as a fee, the pass-through rate to investors would be approximately 3.75%. This is always lower than the WAC because the servicer extracts a fee for their work. The Three Sources of Risk in MBS Pricing Pricing mortgage-backed securities is fundamentally more complex than pricing bonds because investors face three distinct sources of risk simultaneously: Credit risk: the risk that borrowers fail to pay their scheduled principal and interest Interest-rate risk: the risk that changes in market rates affect the present value of future cash flows Prepayment risk: the risk that borrowers pay off their mortgages early, which creates a unique problem called negative convexity Most students first encounter credit and interest-rate risk in traditional bond courses. Prepayment risk is the new, challenging element specific to mortgages. Understanding Prepayment Risk and Negative Convexity Prepayment risk occurs because borrowers have the option to prepay their mortgages whenever they choose (unlike corporate bonds where prepayment is often forbidden or restricted). Borrowers behave rationally: when interest rates fall, refinancing becomes attractive, and many borrowers prepay their old high-rate mortgages to take out new low-rate mortgages. This creates negative convexity, which is the opposite of what bond investors typically experience: Bond positive convexity: When rates fall, bond prices rise more than expected. When rates rise, prices fall less than expected. MBS negative convexity: When rates fall, prepayments accelerate, so you get your principal back early and must reinvest at the new lower rates—making price appreciation less than a comparable non-prepayable bond. When rates rise, prepayments slow, so your capital is locked in longer at old rates while reinvestment rates are now higher. This is the crucial insight: MBS investors lose on both sides when rates change in certain directions. Higher-coupon securities are particularly vulnerable to prepayment losses. If a borrower has a 5% mortgage rate and market rates drop to 3%, they have strong incentive to refinance. Lower-coupon securities are less vulnerable because borrowers have less incentive to prepay. Market Pricing in Practice: CPR, PSA, and Loan Buckets Because closed-form pricing formulas don't exist for MBS (the prepayment option is too complex), the industry uses numerical methods like Monte Carlo simulation or modified binomial trees. These methods require assumptions about future prepayment behavior. Conditional prepayment rate (CPR) is the standard measure of prepayment speed. It represents the annual percentage of the remaining pool balance that is expected to be prepaid in a given year. A CPR of 6% means that 6% of the remaining balance is expected to prepay annually. The PSA (Public Securities Association) model provides a standard baseline for prepayment expectations. The 100% PSA model assumes a specific pattern of CPR that increases gradually during the first 30 months of the loan's life, then stabilizes. Market participants often quote multiples of PSA (e.g., "150% PSA" means 1.5 times the baseline prepayment speeds). Loan-balance buckets matter because smaller loans have different refinancing economics than larger loans. A $50,000 loan has much higher refinancing costs (in percentage terms) relative to the savings from a rate reduction compared to a $200,000 loan. Therefore, smaller-balance loans prepay more slowly, which affects their pricing and risk profile. Market participants often break pools into buckets like: <$85k, $85k–$110k, $110k–$150k, etc., and price each cohort separately. Risks Associated with Mortgage-Backed Securities Credit Risk: Agency vs. Non-Agency Not all MBS carry the same credit risk. The distinction between agency and non-agency securities is critical. Agency issuers (Ginnie Mae, Fannie Mae, and Freddie Mac) have fundamentally different credit profiles: Ginnie Mae is backed by the full faith and credit of the United States government. When you buy a Ginnie Mae MBS, the U.S. government guarantees that you will receive scheduled principal and interest payments, regardless of what borrowers do. Credit risk is essentially zero. Fannie Mae and Freddie Mac are government-sponsored enterprises (GSEs) that provide credit lines and insurance, but do not carry explicit government guarantees in the same way Ginnie Mae does. However, both historically required private mortgage insurance (PMI) for loans with less than 20% down payment, which provides an additional layer of protection against borrower default. Non-agency (private-label) MBS lack government backing entirely. Credit risk here is material and depends entirely on the creditworthiness of the borrowers in the pool and the reserve funds set aside by the issuer. The chart above shows how mortgage-backed securities issuance has shifted over time among these different categories, illustrating the varying appetite for different risk profiles. Interest-Rate Risk Interest-rate risk is straightforward: when market interest rates rise, the present value of the mortgage pool's future cash flows falls, and MBS prices decline. Conversely, when rates fall, MBS prices rise. However, prepayment risk complicates this story—recall the negative convexity discussion. The effective interest-rate sensitivity of an MBS is not as simple as calculating duration on a traditional bond, because the cash flows themselves change when rates move due to prepayments. Systemic Risk and Information Asymmetry One structural risk to consider: securitization can obscure the connection between original borrowers and final investors. When mortgages are pooled, bundled into securities, and sold to institutional investors worldwide, the lender no longer faces the direct credit risk of their borrower. Some economists and policymakers argue this structure can reduce lending discipline—a lender with no skin in the game may be less careful about underwriting standards. This information asymmetry between borrowers, originators, and ultimate investors has been identified as a contributing factor to systemic financial instability. <extrainfo> Uses of Mortgage-Backed Securities While not typically a focus of exam questions, understanding how institutions use MBS helps contextualize why they matter. Investors purchase MBS to gain exposure to real-estate-backed cash flows without owning physical property. Banks and insurance companies use MBS to match long-duration liabilities with long-duration assets. Sophisticated institutions use MBS for hedging—for example, using MBS to hedge prepayment risk or interest-rate exposure in other parts of their portfolio. </extrainfo> The diagram above illustrates how different MBS tranches with different risk levels are created from the same pool of mortgages, allowing different investors to choose their preferred risk-return tradeoff.