Mortgage Backed Security (MBS)
Quick Definition
A mortgage backed security (MBS) is a bond that repays investors from the cash flow of a pool of underlying mortgages. Homeowners make monthly mortgage payments. Those payments flow through to MBS investors as principal and interest. The $9.5 trillion agency MBS market is one of the largest and most liquid fixed income markets in the world, second only to U.S. Treasuries.
What It Means
When you get a mortgage to buy a home, your lender often does not keep your loan. They sell it to a government-sponsored enterprise (GSE) like Fannie Mae or Freddie Mac, or to Ginnie Mae. These agencies bundle thousands of mortgages together into a pool and issue securities backed by those pools. Investors who buy the securities receive a share of the monthly mortgage payments from everyone in the pool.
This process, called securitization, transforms illiquid individual mortgages into tradable bonds. It lets lenders recycle their capital: instead of tying up money for 30 years on one mortgage, they sell it and make new loans. This is why mortgage credit is widely available in the United States. Without the MBS market, far fewer people could get home loans.
The agency MBS market totaled $9.5 trillion in outstanding securities as of mid-2026, according to AGNC Investment Corp. Average daily trading volume was approximately $350 billion, making it one of the most liquid markets in global finance. The Federal Reserve holds a significant portion of these securities, though its holdings fell below $2 trillion by the end of Q2 2026 as it continued reducing its balance sheet. Private investors, money managers, banks, and overseas buyers have absorbed the Fed's selling, keeping the market liquid.
MBS come in two main varieties. Agency MBS are guaranteed by Fannie Mae, Freddie Mac, or Ginnie Mae. The guarantee means investors receive principal and interest even if homeowners default. Non-agency MBS have no government guarantee. They carry higher yields to compensate for default risk. Non-agency MBS were at the center of the 2008 financial crisis, when pools of subprime mortgages defaulted at rates far higher than expected.
How It Works
Step 1: Origination
A borrower applies for a mortgage to buy a home. A lender (bank, credit union, or mortgage company) underwrites the loan, assessing the borrower's credit, income, and the property's value. The loan is funded and the borrower starts making monthly payments of principal and interest.
Step 2: Pooling
The lender sells the mortgage to Fannie Mae, Freddie Mac, or Ginnie Mae (for agency MBS) or to a private issuer (for non-agency MBS). The agency or issuer combines hundreds or thousands of similar mortgages into a pool. Mortgages in the same pool generally have similar loan sizes, interest rates, and terms.
Step 3: Securitization
The issuer creates securities backed by the mortgage pool and sells them to investors. In the simplest structure, called a pass-through, each investor receives a pro-rata share of all principal and interest payments from the pool. If you own 1% of a $100 million pool, you receive 1% of every payment that comes in.
Step 4: Monthly Payments
Homeowners make monthly mortgage payments. These payments include:
- Interest: Paid to MBS investors as yield
- Principal: Scheduled paydown of the loan, passed through to investors
- Prepayments: Extra payments from homeowners who refinance, sell, or pay down early, passed through to investors as early return of principal
The servicer (the company that collects payments from homeowners) deducts a small fee and forwards the rest to the MBS trustee, who distributes it to investors.
Step 5: Tranching (For Structured MBS)
More complex MBS, called collateralized mortgage obligations (CMOs), split the cash flow into pieces called tranches. Each tranche has different characteristics:
| Tranche Type | How It Works | Risk Profile |
|---|---|---|
| Sequential | Pays principal to the first tranche until it retires, then the next | First tranche has shortest average life |
| PAC (Planned Amortization Class) | Has a schedule of principal payments protected by a companion tranche | Most stable cash flow, lowest yield |
| Companion | Absorbs excess prepayments to protect PAC tranches | Most variable cash flow, higher yield |
| Z-tranche | Accrues interest until all other tranches retire, then pays | Longest duration, highest yield |
Real-World Examples
Example 1: Agency MBS Pass-Through
Fannie Mae pools 1,000 mortgages with an average balance of $300,000 and an average interest rate of 6.5%. The total pool value is $300 million. Fannie Mae issues $300 million in MBS with a 6.0% pass-through rate (the 0.5% difference covers guarantee and servicing fees).
An investor buys $1 million of these MBS. Each month, they receive their share of the interest (approximately $5,000 at 6.0% annualized) plus their share of principal paydowns. If homeowners in the pool prepay $2 million in a given month (through refinancing or selling), the investor receives about $6,667 of that prepayment (their 1/300th share), returning principal earlier than expected.
The investor's yield depends on how fast homeowners prepay. If prepayments are slow, the investor receives interest for longer, earning more total interest. If prepayments are fast (like when rates drop and everyone refinances), the investor gets their principal back sooner but must reinvest it at lower current rates. This is called prepayment risk.
Example 2: The Federal Reserve and MBS
The Federal Reserve bought approximately $2.7 trillion in agency MBS between 2020 and 2022 to support the housing market during the pandemic. By Q2 2026, the Fed's MBS holdings had fallen below $2 trillion through a process called quantitative tightening, where the Fed lets mortgage paydowns and prepayments reduce its portfolio without reinvesting.
This matters for MBS investors because the Fed was the largest single buyer of agency MBS for years. As the Fed steps back, private buyers must absorb the supply. In 2026, money managers and overseas investors became the marginal buyers. The transition has been orderly so far, but it affects MBS spreads (the yield premium over Treasuries). Agency MBS spreads to swaps were approximately 142 basis points in Q2 2026, providing attractive yield for fixed income investors.
Example 3: Non-Agency MBS and the 2008 Crisis
Before the 2008 financial crisis, private-label (non-agency) MBS issuance reached $1.2 trillion in 2006. These securities were backed by subprime mortgages with low credit standards: no income verification, no down payment, teaser rates that reset higher after two years. Rating agencies gave many of these securities AAA ratings based on models that assumed housing prices would keep rising.
When home prices fell starting in 2006, defaults surged. Some subprime pools saw default rates above 50%. The AAA-rated tranches of these securities lost 50% to 80% of their value. Investors worldwide, including banks, pension funds, and foreign governments, held these securities and took massive losses. The crisis led to the failure of Bear Stearns and Lehman Brothers and triggered the global financial crisis.
Post-crisis reforms changed the market. Non-agency MBS issuance collapsed and has only partially recovered. Agency MBS now dominate the market, with government guarantees providing credit protection. The Consumer Financial Protection Bureau established ability-to-repay rules that require lenders to verify borrowers can afford their loans.
Example 4: MBS in a Fixed Income Portfolio
A retiree holds a $500,000 fixed income portfolio split between Treasuries and agency MBS:
| Holding | Amount | Yield | Duration | Monthly Income |
|---|---|---|---|---|
| 10-year Treasury | $250,000 | 4.3% | 8.5 years | $0 (semi-annual) |
| Agency MBS (Fannie Mae) | $250,000 | 5.8% | 4.5 years | ~$1,208 |
The MBS pays monthly income, which helps with retirement cash flow. The yield is 1.5 percentage points higher than the Treasury, compensating for prepayment risk. The shorter duration provides some protection against rising rates. However, if rates fall and homeowners refinance, the MBS principal returns early and the retiree must reinvest at lower rates.
Key Points to Remember
- The agency MBS market is $9.5 trillion in size with $350 billion in daily trading volume as of 2026, making it the second largest U.S. fixed income market after Treasuries
- Agency MBS are guaranteed by Fannie Mae, Freddie Mac, or Ginnie Mae, protecting investors from default risk
- MBS investors face prepayment risk: when rates fall, homeowners refinance and return principal early, forcing reinvestment at lower rates
- The Federal Reserve's MBS holdings fell below $2 trillion in 2026 as it reduced its balance sheet
- Non-agency MBS carry no government guarantee and pay higher yields to compensate for default risk
- MBS pay monthly, unlike most bonds that pay semi-annually, which helps investors needing regular income
- Tranching in CMOs lets investors choose their risk and return profile, from stable PAC tranches to high-yield companion tranches
- MBS spreads to swaps were approximately 142 basis points in Q2 2026, offering attractive yield relative to Treasuries
Common Mistakes to Avoid
Mistake 1: Ignoring prepayment risk. When interest rates drop, homeowners refinance their mortgages. This sends principal back to MBS investors earlier than expected. The investor now has cash to reinvest, but current rates are lower. This cuts the investor's yield and total return. Prepayment risk is the defining feature of MBS and the main reason they yield more than Treasuries. Do not buy MBS thinking they behave like regular bonds.
Mistake 2: Assuming MBS duration is fixed. A 30-year mortgage pool does not have a 30-year duration. Because homeowners make principal payments and prepay over time, the average life of an MBS is much shorter, typically 5 to 12 years. But this average life changes with interest rates. When rates fall, prepayments speed up and the average life shortens. When rates rise, prepayments slow and the average life lengthens. This is called negative convexity, and it means MBS underperform regular bonds in both rising and falling rate environments.
Mistake 3: Confusing agency and non-agency MBS. Agency MBS have government backing and minimal default risk. Non-agency MBS do not, and their performance depends on the credit quality of the underlying borrowers. During the 2008 crisis, non-agency MBS lost enormous value while agency MBS continued paying. Know which type you are buying. Most retail investors access MBS through mutual funds or ETFs that hold agency MBS.
Mistake 4: Overconcentrating in MBS for the yield. Agency MBS yield 1 to 1.5 percentage points more than Treasuries, which is attractive. But MBS have unique risks (prepayment, extension, negative convexity) that Treasuries do not. A fixed income portfolio with 80% in MBS is not well diversified. Most advisors recommend MBS as one component of a diversified bond portfolio, not the whole thing.
Mistake 5: Not understanding extension risk. When rates rise, homeowners stop refinancing. Prepayments slow to a trickle. The MBS average life extends from 7 years to 12 or 15 years. The investor is locked into a below-market yield for longer than expected. This is extension risk, the opposite of prepayment risk. In 2022 and 2023, as rates rose from 3% to 7%, MBS investors experienced significant extension as homeowners refused to give up their low-rate mortgages.
Mistake 6: Buying individual MBS without understanding the collateral. Individual MBS can have very different characteristics depending on the underlying mortgages. A pool of recently originated 6.5% mortgages will prepay slowly because no one refinances to a higher rate. A pool of 3% mortgages from 2021 will prepay slowly for the same reason. But a pool of 7% mortgages from the 1990s will prepay rapidly because every borrower wants to refinance. Read the prospectus or pool supplement before buying.
Related Concepts
MBS are part of the broader bond and fixed income market, sitting alongside Treasuries and corporate bonds. They are connected to real estate because their cash flow comes from individual home mortgages. The Federal Reserve influences MBS prices through its balance sheet policy and interest rate decisions. MBS are related to derivatives because complex MBS structures like CMOs use tranching to create different risk profiles. The amortization of underlying mortgages determines the MBS cash flow schedule. The debt-to-income ratio of borrowers in the pool affects default and prepayment rates. Collateralized debt obligations (CDOs) are a related but broader structure that can include MBS alongside other debt. For investors, MBS fit into a diversification strategy alongside other fixed income. Read our guide on bonds explained and consider a bond ladder strategy. For official data, visit Fannie Mae's MBS disclosure page.
Frequently Asked Questions
Q: What is the difference between an MBS and a regular bond?
A: A regular bond pays fixed interest on set dates and returns principal at maturity. An MBS pays monthly (not semi-annually), and the principal returns gradually as homeowners pay down their mortgages. MBS also have prepayment risk: principal can return early if homeowners refinance or sell. This makes MBS cash flows less predictable than regular bonds.
Q: Are MBS safe after the 2008 crisis?
A: Agency MBS (those guaranteed by Fannie Mae, Freddie Mac, or Ginnie Mae) are considered very safe because the government guarantees payment even if homeowners default. Non-agency MBS carry default risk and require careful analysis. Post-crisis regulations, including ability-to-repay rules and risk retention requirements, have improved underwriting standards. Most retail investors access MBS through funds holding agency MBS.
Q: How do rising interest rates affect MBS?
A: Rising rates cause MBS prices to fall, like all bonds. But the effect is amplified by extension risk: when rates rise, homeowners stop refinancing, so the MBS average life lengthens. The investor is stuck holding a below-market yield for longer. This is why MBS have negative convexity and tend to underperform regular bonds when rates rise.
Q: Can I buy MBS directly?
A: Individual agency MBS can be purchased through some brokerages with minimum investments of $25,000 or more. Most retail investors access MBS through mutual funds or ETFs like the Vanguard Mortgage-Backed Securities ETF (VMBS) or iShares MBS ETF (MBB), which have low minimums and provide diversification across many pools.
Q: Why does the Federal Reserve buy and sell MBS?
A: The Fed buys MBS to lower mortgage rates and support housing market activity, as it did during the 2008 crisis and the 2020 pandemic. When the Fed sells or lets MBS roll off its balance sheet (quantitative tightening), it reduces demand for MBS, which can widen spreads and slightly raise mortgage rates. The Fed's MBS holdings fell below $2 trillion by mid-2026.
