Mortgage-Backed Securities (MBS) are financial instruments created by pooling together multiple home loans or mortgages and selling interests in the resulting pool to investors. The cash flows from borrowers’ mortgage payments, including principal and interest, are passed on to MBS investors.
This article explores how Mortgage-Backed Securities (MBS) works.
Key Takeaways
- Mortgage-Backed Securities bundle individual home loans into a single tradable debt instrument yielding regular monthly income.
- Returns comprise the principal and interest paid by underlying borrowers every month.
- Lenders pool mortgages to free up balance sheet capital and maintain liquidity for new lending.
- Pass-through certificates and collateralised mortgage obligations represent the two core structural models.
- Prepayment speed and default rates drive the primary risk profile of Mortgage-Backed Securities investments.
What Does Mortgage-Backed Securities Mean?
Mortgage-Backed Securities are investment products backed by a pool of home loans (mortgages). Banks bundle together thousands of individual mortgages and sell them as a single security to investors.
Mortgage-backed securities (MBS) are associated with the US financial market, where they are a significant part of the housing finance system. While the concept of securitising mortgage loans also exists in other markets, including India, the following discussion of Indian regulatory references should be understood within the context of India’s own regulatory framework.
Issuers of Mortgage-Backed Securities
The safety and credit risk profile of a Mortgage-Backed Security depend heavily on the financial institution or entity that originates and packages the underlying loans:
- Agency MBS: Issued or guaranteed by government-sponsored enterprises (such as Fannie Mae or Freddie Mac) or government agencies (such as Ginnie Mae). Because of this institutional or sovereign backing, these securities carry very low default risk and offer higher secondary market liquidity.
- Non-Agency MBS: Issued by private financial institutions, commercial banks, or investment banks. These lack government backing, often feature non-conforming or subprime loans, and carry significantly higher yields alongside elevated credit and default risks.
How does Mortgage-Backed Securities Work?
- Origination: A bank lends money to homeowners to buy houses (mortgages).
- Pooling: Instead of holding these loans on its books for 15-30 years, the bank bundles hundreds or thousands of similar mortgages together.
- Securitisation: This pool is sold to investors as a bond-like security.
- Cash Flow: As homeowners pay their monthly mortgage (principal + interest), that money flows through to the investors who bought the Mortgage-Backed Securities.
Worked Example of an MBS Pool
To understand cash flow mechanics, consider a structured pool:
- The Pool: A housing finance company packages 1,000 separate home loans into one asset pool.
- Total Value: Each loan averages ₹10,00,000, creating a total pool value of ₹100 crore.
- Institutional/Fractional Share: An investor acquires a unit worth ₹10,00,000 (representing a 1 per cent slice of the pool).
- Monthly Collection: Borrowers collectively pay ₹1.2 crore in principal and interest EMIs during the month.
- Your Payout: The 1% share yields ₹1,20,000 for the month before management and servicing fees.
Note: The example is for illustration purpose only.
Types of Mortgage-Backed Securities
- Pass-Through Certificates (PTCs): Pass-through certificates distribute borrowers' EMIs directly to investors on a pro rata basis. If homeowners prepay their loans ahead of schedule, capital is returned early, exposing investors to reinvestment risk.
- Collateralised Mortgage Obligations (CMOs): CMOs divide the loan pool into distinct payout tiers known as tranches. Senior tranches enjoy higher payment priority and credit protection, while junior tranches absorb initial default losses in exchange for higher yields.
| Feature | Pass-Through Certificates | Collateralised Mortgage Obligations |
| Payment Flow | Direct pro-rata distribution | Sequential or prioritised tranches |
| Risk Distribution | Shared evenly across all holders | Divided by seniority and credit rating |
| Prepayment Exposure | Borne equally by all investors | Shielded or concentrated by tranche |
| Complexity | Simple structure | Advanced multi-layered structure |
Step-By-Step Valuation Process for Secondary Market Trading
Secondary market trading for Mortgage-Backed Securities (MBS) is different from buying a standard fixed deposit or a straightforward corporate bond. It is equally important to understand how the process works here:
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Find the Average Interest Rate (WAC): Calculate the average interest rate of all the mortgages in the pool, weighted by how much principal is left on each loan.
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Find the Average Time Left (WAM): Figure out the average number of months remaining before the underlying mortgages are fully paid off.
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Guess Early Payoffs (Prepayment Speed): Estimate how many borrowers will pay off their loans early or refinance, which typically happens when interest rates drop.
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Apply a Discount Rate: Adjust future cash flows back to today's money by using a benchmark interest rate plus a risk spread.
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Calculate the Final Price: Add up all those discounted future cash flows to find out what the MBS is worth today.
Risks of Investing in Mortgage-Backed Securities
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Ignoring prepayment velocity: Falling interest rates trigger borrower refinancing, returning principal early and forcing reinvestment at lower yields.
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Chasing unrated high yields: Buying unrated or junior tranches for superior returns exposes capital to severe default risks.
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Liquidity constraints: Secondary debt markets feature lower trading volumes than equities, potentially leading to steep price discounts during market stress.
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Neglecting diversification: Concentrating capital in a single housing finance issuer heightens credit exposure.
Regulatory Framework and Compliance
In India, securitisation transactions fall under strict guidelines issued by the Reserve Bank of India (RBI) and the Securities and Exchange Board of India (SEBI).
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Minimum Holding Period (MHP): Originating lenders must hold loans on their books for a mandatory period before securitisation.
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Minimum Retention Requirement (MRR): Lenders must retain a specified portion of the securitised portfolio on their balance sheets to ensure "skin in the game."
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Listing norms: Securitised Debt Instruments (SDIs) intended for broader market participation must comply with SEBI (Issue and Listing of Non-Convertible Securities) Regulations.
Retail Accessibility and Taxation
Direct over-the-counter (OTC) trading in raw Mortgage-Backed Securities pools is generally restricted to institutional investors due to high minimum ticket sizes. However, retail investors can access securitised debt indirectly through debt mutual funds or exchange-traded Securitised Debt Instruments (SDIs) via a verified demat and trading account.
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Tax Treatment: Interest income earned from debt instruments and mutual funds is added to the investor's taxable income and taxed according to their applicable income tax slab rates, while capital gains are subject to prevailing SEBI and Income Tax Act guidelines.
Conclusion
Secondary market trading of Mortgage-Backed Securities (MBS) relies heavily on rigorous cash flow modelling rather than static yield calculations. Because underlying borrowers can prepay their loans at any time, especially when interest rates fluctuate, investors must dynamically balance metrics like WAC, WAM, and CPR to accurately project future cash flows before applying a market discount rate to determine fair value.
