Mortgage-Backed Securities (MBS): Prepayment and Risk

Key Takeaways

• Agency MBS carry a Ginnie Mae, Fannie Mae, or Freddie Mac guarantee; non-agency MBS bear direct credit risk.
• Negative convexity means MBS underperform plain bonds in both rallies (prepayments accelerate) and sell-offs (extension risk).
• Option-adjusted spread strips out the prepayment option cost, allowing fair comparison with non-prepayable bonds.
• Prepayment speeds depend on mortgage rates, home prices, and seasonality, making model error a significant source of risk.

What It Is

MBS are debt obligations that represent claims on the cash flows from pools of mortgages, most commonly on one-to-four family residential property. A bank or mortgage originator sells a pool of loans to an issuer. The issuer puts those loans into a trust and sells securities backed by the trust's cash flows. Each month, as homeowners make their mortgage payments, the servicer collects the cash and passes through the principal and interest to MBS holders.

MBS fall into two broad groups. Agency MBS are issued or guaranteed by Ginnie Mae, Fannie Mae, or Freddie Mac. Non-agency MBS, also called private-label MBS, are issued by banks, broker dealers, and other private entities without a government or GSE guarantee.

Agency MBS is one of the largest and most liquid fixed-income markets in the world after US Treasuries. Fannie Mae alone had more than three trillion dollars of outstanding MBS in recent years.

The Intuition

A single mortgage is small, illiquid, and credit-risky. A pool of thousands of mortgages is large, diversified, and easier to sell. Securitization turns an unwieldy set of loans into a tradable bond, which lets pension funds, insurance companies, and banks fund home lending at lower cost. The homeowner gets a cheaper mortgage. The investor gets a bond with a predictable coupon backed by real property.

The catch is that the homeowner can prepay the mortgage at any time, typically by refinancing or selling the house. That optionality is the defining feature of an MBS and the reason the asset class behaves differently from a plain vanilla bond.

How It Works

Most agency MBS are pass-through securities. The servicer collects scheduled principal and interest plus any prepayments and passes the cash to investors monthly. The principal balance amortizes over time rather than paying all at once at maturity.

Three cash-flow streams matter:

• Scheduled principal and interest. The regular amortization from on-time payments.
• Prepayments. Homeowners who refinance, sell, or otherwise pay early.
• Defaults. Handled by the guarantor in agency deals and absorbed by investors in non-agency deals.

The guarantee structure splits agency MBS into two tiers. Ginnie Mae carries the full faith and credit guarantee of the United States, making it effectively credit risk free. Fannie Mae and Freddie Mac carry an implicit guarantee. Both GSEs have been in federal conservatorship since September 2008, which strengthened market confidence in the implicit backstop but did not make it explicit.

Prepayments produce the defining MBS risk: negative convexity. When rates fall, homeowners refinance and investors get their principal back early, precisely when reinvestment options are unattractive. When rates rise, homeowners hold on to their low-rate mortgages longer and the MBS's effective maturity extends, precisely when investors would prefer their cash back. Investors get the worst of both directions, so agency MBS yields trade at a spread above Treasuries to compensate.

A simplified yield breakdown looks like:

MBS yield = Treasury yield + option-adjusted spread + option cost

The option cost is the value given up to the homeowner because of the prepayment right. Institutional investors use option-adjusted spread (OAS) to compare MBS against other bonds on a like-for-like basis.

Worked Example

An investor buys a 30-year Fannie Mae pass-through with a 5.5 percent coupon at par. The 10-year Treasury yields 4.2 percent. The nominal spread looks like 130 basis points.

Six months later, the 10-year falls to 3.5 percent. Homeowners refinance aggressively. Monthly prepayment speeds, often measured in CPR (conditional prepayment rate), jump from 6 percent to 25 percent. The investor's effective duration shortens, and the bond barely rises in price even though Treasuries have rallied hard.

Twelve months after that, rates climb to 5.5 percent. Prepayments collapse. Now the bond behaves like a much longer-dated security because homeowners keep paying on their old mortgages. The MBS falls more than expected as rates rise. That is negative convexity in action, captured in the same security within two years.

Common Mistakes

1. Assuming all MBS are low risk. Agency MBS carry a government or GSE guarantee. Non-agency MBS do not. Subprime residential MBS were at the center of the 2008 crisis, and many senior tranches of private-label deals suffered large losses despite originally triple-A ratings.

2. Ignoring negative convexity. A standard bond rallies more when rates fall than it loses when rates rise. An MBS does the opposite. Retail investors who buy MBS funds expecting symmetric behavior are often disappointed in both large rate rallies and large sell-offs.

3. Underestimating prepayment-model error. Prepayment speeds depend on mortgage rates, home prices, borrower credit quality, and seasonality. Models that worked well for decades can break when the housing market changes regime, as they did during both 2008 and 2020.

4. Treating agency MBS as Treasury-equivalent. Even Ginnie Mae trades at a spread to Treasuries because of the embedded prepayment option. That spread is not free yield. It is the price paid by the market for bearing optionality the Treasury market does not have.

5. Overlooking institutional capital charges. For banks and insurers, MBS have different regulatory capital treatment than Treasuries or GSE debentures. Changes in those rules can shift demand, and with it MBS spreads, in ways that have nothing to do with mortgage fundamentals.

Frequently Asked Questions

What is a conditional prepayment rate (CPR) and why does it matter? CPR is an annualized rate expressing what percentage of a mortgage pool's outstanding principal is expected to prepay each year. If a pool has a CPR of 10 percent, roughly 10 percent of the remaining balance is projected to be paid off during the year through refinancings, home sales, or lump-sum payments. Faster prepayments shorten the effective life of an MBS and return principal when reinvestment opportunities may be less attractive.

How does the option-adjusted spread differ from the nominal yield spread? Nominal yield spread is the simple difference between the MBS yield and the Treasury yield at similar maturity. Option-adjusted spread removes the value of the embedded prepayment option, representing the spread you would receive if the bond had no optionality. OAS provides a cleaner comparison of MBS against other bonds because it isolates the pure credit and liquidity compensation from the prepayment risk premium.

Why were non-agency MBS at the center of the 2008 financial crisis? Private-label MBS issued in the mid-2000s were backed by pools of subprime and alt-A mortgages with weak underwriting standards. When home prices fell, default rates on the underlying loans far exceeded model expectations. Many senior tranches initially rated AAA suffered severe principal losses. The crisis exposed how reliant the ratings and pricing models were on continued home price appreciation.

What is extension risk in MBS? Extension risk is the possibility that rising interest rates will cause homeowners to prepay more slowly than expected, extending the effective maturity of the MBS well beyond what was initially modeled. Investors who planned for a 7-year average life may find their MBS has stretched to 12 or 15 years, increasing duration and exposure to further rate increases.

How do investors access MBS without buying individual pools? Most retail investors use agency MBS ETFs or mutual funds that provide daily liquidity and broad diversification across thousands of loan pools. Institutional investors purchase specific TBA (to-be-announced) contracts or identified pools with known characteristics. The TBA market, where Fannie and Freddie MBS are traded for future delivery, is one of the most liquid bond markets in the world.

Sources

1. SEC / Investor.gov. "Mortgage-Backed Securities and Collateralized Mortgage Obligations." https://sec.gov/fast-answers/answersmortgagesecuritieshtm.html
2. Fannie Mae. "Mortgage Backed Securities." https://capitalmarkets.fanniemae.com/mortgage-backed-securities
3. Ginnie Mae. "Foreign Ownership of Agency MBS." https://www.ginniemae.gov/newsroom/publications/Documents/foreign_ownership_agency_mbs2021.pdf
4. Fuster, A., Lucca, D., Vickery, J. "Mortgage-Backed Securities." Federal Reserve Bank of New York Staff Reports. https://www.newyorkfed.org/medialibrary/media/research/staff_reports/sr1001.pdf
5. Federal Reserve. "L.211 Agency- and GSE-Backed Securities." https://www.federalreserve.gov/releases/z1/preview/html/l211.htm

Disclaimer

This article is educational content only and is not financial advice. Nothing here is a recommendation to buy, sell, or hold any security. Consult a licensed advisor before making investment decisions.