Understanding Mortgage-Backed Securities and Their Role in the Housing Market
Mortgage-backed securities (MBS) are financial instruments that play a central role in the U.S. Housing finance system. These securities are created when pools of residential mortgages are bundled together and sold to investors, who receive periodic payments derived from the underlying loan repayments. MBS allow lenders to free up capital, enabling them to originate more home loans while transferring credit risk to investors.
The two primary government-sponsored enterprises (GSEs) involved in issuing MBS are Fannie Mae and Freddie Mac. Both agencies guarantee the timely payment of principal and interest on their MBS, whereas these securities are not backed by the full faith and credit of the United States government.
How Mortgage-Backed Securities Work
When a homeowner makes a monthly mortgage payment, that cash flow passes through a series of intermediaries before reaching MBS investors. The process begins with the origination of a mortgage loan by a bank or lender. These loans are then sold to aggregators, who package them into pools that back MBS issuances.
Fannie Mae and Freddie Mac issue various types of MBS, including traditional pass-through securities and more complex structures. One notable product is the “Super” MBS, which represents a single-class resecuritization of Uniform Mortgage-Backed Securities (UMBS) or other eligible collateral. Supers are designed to be TBA-eligible with a 55-day payment delay, meaning investors receive payments 55 days after interest begins to accrue on the underlying mortgages.
Supers can be backed by a combination of UMBS, other Supers issued by either Fannie Mae or Freddie Mac, existing TBA-eligible MBS, or certain types of mortgage certificates that have undergone exchange processes. This flexibility allows for efficient pooling and enhances liquidity in the secondary mortgage market.
Key Differences Between MBS and Treasury Securities
While mortgage-backed securities share some similarities with U.S. Treasury bonds, they differ in two important ways:
- Prepayment Risk: Unlike Treasuries, MBS are subject to prepayment risk due to the fact that homeowners can pay off their mortgages early through refinancing, home sales, or other means. When interest rates fall, borrowers are more likely to refinance, causing MBS to pay down faster than expected. This behavior means MBS investors may receive their principal sooner than anticipated, often when reinvestment opportunities are less favorable.
- Credit Backing: U.S. Treasury securities are backed by the full faith and credit of the federal government, making them virtually risk-free in terms of default. In contrast, MBS issued by Fannie Mae and Freddie Mac carry only the guarantee of those agencies, not the U.S. Government. Although the GSEs have strong implicit government support, their MBS do not benefit from an explicit sovereign guarantee.
The Relationship Between MBS Prices and Mortgage Rates
There is an inverse relationship between MBS prices and mortgage interest rates. When demand for MBS increases, their prices rise, which tends to push mortgage rates lower. Conversely, when MBS prices fall, mortgage rates typically increase.
This dynamic occurs because MBS compete with other fixed-income investments for investor capital. Higher MBS prices indicate lower yields, which translates to lower borrowing costs for homebuyers. The 10-year Treasury yield is often used as a benchmark for mortgage rates, as the two have historically moved in close correlation due to similar duration profiles and sensitivity to inflation and monetary policy expectations.
Types of Mortgage-Backed Securities Offered by the GSEs
Fannie Mae and Freddie Mac offer a range of MBS products to meet diverse investor needs:
- Traditional Fixed-Rate MBS: Available in 10-, 15-, 20- and 30-year maturities, these are the most common form of agency MBS.
- Adjustable-Rate MBS (ARMs): These securities are backed by mortgages with interest rates that adjust periodically based on market indices.
- Structured MBS: Includes collateralized mortgage obligations (CMOs) and other tranched securities that redistribute cash flows and risk among different investor classes.
- Supers: As previously noted, Supers are resecuritized UMBS or eligible MBS designed for enhanced tradability and standardization in the TBA (To Be Announced) market.
The Role of UMBS in the MBS Market
Uniform Mortgage-Backed Securities (UMBS) were introduced to increase fungibility and transparency in the MBS market. By standardizing key characteristics such as loan size, geographic limits, and underwriting standards across both Fannie Mae and Freddie Mac pools, UMBS allow for greater interchangeability between securities issued by the two GSEs.
This standardization supports the TBA market, where investors agree to buy or sell MBS pools with specified characteristics before the exact pool composition is known. The UMBS framework has contributed to improved liquidity and efficiency in agency MBS trading.
Factors That Influence MBS Performance
Several factors affect the performance and pricing of mortgage-backed securities:
- Interest Rate Environment: Changes in prevailing interest rates directly impact prepayment speeds and MBS valuation.
- Economic Conditions: Employment trends, housing market activity, and consumer confidence influence borrower behavior and loan performance.
- Government Policy: Actions by the Federal Reserve, Treasury Department, and regulatory agencies can affect MBS supply, demand, and pricing.
- Credit Quality of Underlying Loans: While agency MBS benefit from guarantor protection, the credit profile of the mortgage pool still affects market perception and pricing, particularly during periods of stress.
Why MBS Matter to Homebuyers and Investors
For homebuyers, the MBS market helps determine the availability and cost of mortgage credit. A liquid and stable MBS market enables lenders to offer competitive rates and maintain consistent lending standards across geographic regions.
For investors, MBS provide access to a sector that offers yield advantages over traditional government bonds, albeit with additional risks such as prepayment and interest rate volatility. Institutional investors including pension funds, insurance companies, and asset managers frequently allocate to agency MBS as part of diversified fixed-income portfolios.
As of April 2026, the MBS market continues to evolve in response to changing monetary policy, housing affordability challenges, and technological advancements in trading and data analytics. Ongoing efforts to enhance transparency, improve risk modeling, and expand access to underserved communities remain central to the mission of the GSEs and their regulators.
Frequently Asked Questions
What is the main purpose of mortgage-backed securities?
The primary purpose of MBS is to enable lenders to replenish their capital after issuing home loans, thereby allowing them to make additional mortgages available to borrowers. This process increases liquidity in the housing finance system.
Are mortgage-backed securities guaranteed by the U.S. Government?
No. While Fannie Mae and Freddie Mac guarantee the timely payment of principal and interest on their MBS, these securities are not backed by the full faith and credit of the United States government, unlike U.S. Treasury securities.
How do changes in the 10-year Treasury yield affect mortgage rates?
Mortgage rates tend to move in the same direction as the 10-year Treasury yield. When the yield rises, mortgage rates typically increase; when the yield falls, mortgage rates often decrease. This correlation exists because both instruments compete for similar investor capital and have comparable sensitivity to interest rate expectations.
What is a “Super” MBS?
A Super MBS is a single-class resecuritization of Uniform Mortgage-Backed Securities (UMBS) or other eligible collateral issued by Fannie Mae or Freddie Mac. Supers are designed to be TBA-eligible with a 55-day payment delay and can be backed by a combination of UMBS, other Supers, or existing TBA-eligible MBS.
Why do MBS prices and mortgage rates move in opposite directions?
MBS prices and mortgage rates have an inverse relationship because higher demand for MBS pushes up their prices, which lowers their yield. Lower MBS yields translate to lower borrowing costs for mortgages, as lenders base their rates in part on the returns required by MBS investors.