Mortgage-Backed Securities (MBS): Definition, Types & How They Work
Understanding mortgage-backed securities: How mortgages become investment opportunities.
What Are Mortgage-Backed Securities (MBS)?
Mortgage-backed securities (MBS) are investment securities that are secured by pools of mortgages. These securities represent claims on the cash flows generated by mortgages, typically residential mortgages, that have been originated by banks and other financial institutions. When homeowners make monthly mortgage payments, the principal and interest portions flow through to the investors who hold the MBS.
MBS have become a cornerstone of the modern financial system, enabling lenders to free up capital and originate new mortgages by selling existing ones. This securitization process involves bundling together hundreds or thousands of individual mortgages into tradeable securities that can be purchased by institutional investors, mutual funds, pension funds, and other market participants.
How Mortgage-Backed Securities Work
The process of creating and operating mortgage-backed securities involves several key steps:
- Mortgage Origination: Banks and mortgage lenders originate individual residential mortgages to borrowers who meet lending criteria.
- Mortgage Pooling: Lenders sell these mortgages to investment banks or government-sponsored enterprises (GSEs) like Fannie Mae, Freddie Mac, or Ginnie Mae.
- Securitization: These mortgages are then bundled together into pools, and securities are issued that represent ownership claims on the cash flows from these mortgage pools.
- Distribution to Investors: The MBS are sold to various investors, who receive monthly payments of principal and interest as homeowners pay their mortgages.
- Servicing: A mortgage servicer continues to collect payments from borrowers and passes them along to MBS investors, minus servicing fees.
Investors in MBS receive monthly cash flows consisting of both principal repayment and interest payments. The timing and amount of these payments depend on the mortgage terms and the borrowers’ behavior, particularly prepayment rates.
Types of Mortgage-Backed Securities
There are several categories of mortgage-backed securities, each with distinct characteristics:
Government-Sponsored Enterprise (GSE) MBS
These MBS are issued or guaranteed by government-sponsored enterprises such as Fannie Mae (Federal National Mortgage Association), Freddie Mac (Federal Home Loan Mortgage Corporation), or Ginnie Mae (Government National Mortgage Association). GSE MBS carry implicit or explicit government backing, which reduces credit risk for investors. Most conforming mortgages—those meeting GSE standards—are packaged into these securities.
Non-Agency MBS
Also called private-label MBS, these securities are not backed by government guarantees. They typically contain mortgages that exceed conforming loan limits (jumbo mortgages) or borrowers with less-than-prime credit profiles. Non-agency MBS offer higher yields but carry greater credit risk.
Commercial Mortgage-Backed Securities (CMBS)
Rather than residential mortgages, CMBS are secured by commercial real estate properties such as office buildings, shopping centers, and apartment complexes. These securities are structured differently and carry different risks than residential MBS.
Stripped MBS
These are derivatives created from conventional MBS by separating principal and interest payments into distinct securities. Interest-only (IO) strips pay investors only the interest portion of mortgage payments, while principal-only (PO) strips pay only the principal portion.
Key Characteristics of MBS
| Characteristic | Description |
|---|---|
| Yield | MBS typically offer higher yields than U.S. Treasury bonds but lower than corporate bonds, reflecting their intermediate credit risk. |
| Duration Risk | MBS exhibit negative convexity due to prepayment risk, meaning their price sensitivity is asymmetric. |
| Liquidity | GSE MBS are highly liquid with active secondary markets, while non-agency MBS may be less liquid. |
| Payment Frequency | MBS typically pay investors monthly, matching the mortgage payment cycle. |
| Maturity | While MBS may have nominal maturities of 15 or 30 years, actual maturity depends on prepayment rates. |
Risks Associated with Mortgage-Backed Securities
Prepayment Risk
Prepayment risk occurs when homeowners refinance or pay off mortgages early, typically when interest rates decline. When borrowers prepay, investors lose the opportunity to earn higher yields on their principal. This creates negative convexity in MBS pricing—investors experience capital losses when rates fall (unlike traditional bonds), as mortgages are prepaid at par value.
Interest Rate Risk
MBS values decline when interest rates rise, just as with other fixed-income securities. However, the relationship is complicated by prepayment risk. When rates fall, the value of MBS may increase less than comparable bonds due to prepayment concerns.
Credit Risk
For non-agency MBS, there is genuine default risk if mortgage borrowers fail to pay. Even GSE MBS carry some credit risk in extreme scenarios, though government backing substantially mitigates this risk.
Extension Risk
Conversely, when interest rates rise, homeowners tend to hold mortgages longer, extending the duration of MBS investments. This exposes investors to the risk of receiving promised cash flows over a longer period than anticipated.
Liquidity Risk
While GSE MBS are generally liquid, non-agency and specialized MBS may face liquidity challenges, particularly during market stress.
MBS Versus Other Investments
MBS vs. U.S. Treasuries: MBS offer higher yields than Treasury bonds but carry additional risks, including prepayment risk and credit risk (for non-agency MBS). Treasuries provide greater certainty about cash flows and maturity dates.
MBS vs. Corporate Bonds: MBS typically offer lower yields than corporate bonds but lower credit risk for GSE-backed securities. Corporate bonds provide more predictable maturity dates and lack prepayment risk.
MBS vs. Bond Funds: Individual MBS securities provide defined cash flows, while bond mutual funds and ETFs offer diversification and professional management but involve ongoing fees.
The Role of MBS in Financial Markets
Mortgage-backed securities play a crucial role in the mortgage market and broader economy. By enabling banks to sell mortgages, MBS free up capital that lenders can use to originate new mortgages, increasing the availability of mortgage credit. This has historically helped keep mortgage rates competitive and accessible to borrowers.
During the 2008 financial crisis, the MBS market experienced significant turmoil as housing prices declined and mortgage defaults surged. This highlighted the risks inherent in mortgage securitization when underwriting standards are compromised. Subsequent regulatory reforms, including the Dodd-Frank Act, sought to improve MBS market oversight and transparency.
Today, MBS remain an important component of fixed-income portfolios and continue to be actively traded in secondary markets. The Federal Reserve has used MBS purchases as a monetary policy tool during periods of economic stress.
How to Invest in MBS
Investors can gain exposure to mortgage-backed securities through several avenues:
- Direct Purchase: Institutional investors can purchase MBS directly through securities dealers.
- Mutual Funds: Bond mutual funds often hold MBS as part of diversified portfolios.
- Exchange-Traded Funds (ETFs): MBS ETFs provide convenient access to mortgage-backed securities with daily liquidity.
- Bond Ladders: Investors can construct laddered portfolios of individual MBS with staggered maturities.
Frequently Asked Questions (FAQs)
Q: Are mortgage-backed securities safe investments?
A: GSE-backed MBS are relatively safe due to government backing, though they carry prepayment and interest rate risks. Non-agency MBS carry greater credit risk. Like all investments, MBS involve risk and are suitable for different investors depending on their risk tolerance and investment objectives.
Q: What is prepayment risk in MBS?
A: Prepayment risk is the risk that homeowners will pay off mortgages early, typically when interest rates decline. This forces investors to reinvest principal at lower rates, reducing overall returns.
Q: How do MBS differ from bonds?
A: Unlike traditional bonds with fixed maturity dates, MBS have uncertain maturity dates due to prepayment risk. MBS also have monthly payment schedules rather than semi-annual coupons typical of bonds.
Q: Can individual investors buy MBS directly?
A: While individual investors can purchase MBS through brokerage accounts, most individual investors gain MBS exposure through mutual funds or ETFs, which provide easier access and diversification.
Q: How did MBS contribute to the 2008 financial crisis?
A: The 2008 crisis resulted from deteriorating mortgage underwriting standards, complex MBS structures that obscured risks, and subsequent housing market collapse. When home prices fell and defaults surged, MBS values collapsed, triggering broader financial instability.
Q: What is negative convexity in MBS?
A: Negative convexity means MBS exhibit asymmetric price behavior. When rates fall, MBS prices increase less than comparable bonds due to prepayment expectations. When rates rise, MBS prices fall more dramatically.
References
- An Introduction to Mortgage-Backed Securities — U.S. Securities and Exchange Commission (SEC). 2024. https://www.sec.gov/investor/alerts/investorbasics.htm
- Mortgage-Backed Securities: Understanding the Market and Its Risks — Federal Reserve Board. 2024. https://www.federalreserve.gov
- Global Securitization and Structured Finance — International Capital Market Association (ICMA). 2024. https://www.icmagroup.org
- Fixed Income Securities: Tools for Today’s Markets — Global Association of Risk Professionals (GARP). 2023. https://www.garp.org
- The Role of Government-Sponsored Enterprises in Mortgage Markets — Congressional Research Service. 2023. https://www.congress.gov
- Mortgage Securitization and Market Stability — Bank for International Settlements (BIS). 2024. https://www.bis.org
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