At its core, a mortgage backed security is a financial asset that pools together home loans and sells slices of that pool to investors. This process transforms an illiquid asset, a 30-year mortgage payment stream, into a tradeable security that Wall Street and global markets can buy and sell. By repackaging individual mortgages, banks free up capital to originate new loans, while investors gain access to the interest payments generated by the underlying residential or commercial property debt.
How Mortgage Backed Securities Work
The creation of a mortgage backed security begins with the origination of thousands of individual home loans by banks and lenders. These loans are then grouped together based on shared characteristics, such as interest rate, geographic location, and credit quality, to form a mortgage pool. A government-sponsored enterprise or a private investment bank acts as an intermediary, purchasing these pools and securitizing them. The pooled payments are then sliced into different tranches, which are sold to institutional and retail investors seeking exposure to the housing market without directly owning physical property.
The Role of Credit Enhancement
To ensure investor confidence, these securities often rely on layers of credit enhancement. This can include over-collateralization, where the value of the underlying mortgages exceeds the value of the bonds issued, or third-party insurance products that cover losses from borrower defaults. The goal is to assign a credit rating to the security that reflects the risk of the underlying mortgages, making it easier to price the asset in the secondary market. Without this structure, the perceived risk of holding thousands of unknown borrowers’ debt would make the securities significantly less attractive.
Types of Mortgage Backed Securities
Not all mortgage backed securities are created equal, and the market is generally divided into two primary categories: agency and non-agency securities. Agency MBS are issued by government-sponsored enterprises like Fannie Mae and Freddie Mac, which provide a government implicit guarantee. Non-agency MBS, also known as private label securities, are created by financial institutions and carry the credit risk of the issuing firm, typically offering higher yields to compensate for that additional risk.
Pass-Through vs. Collateralized Mortgage Obligations
The most basic type is the pass-through security, where mortgage payments "pass through" to investors proportionally each month. However, the market has evolved into more complex structures known as Collateralized Mortgage Obligations, or CMOs. CMOs divide the cash flows into different tranches with varying maturities and risk profiles. This allows investors to choose between safer, shorter-duration tranches or riskier, longer-duration tranches that offer higher interest rates to account for prepayment and extension risk.
Why Investors Purchase These Securities
For investors, the appeal of a mortgage backed security lies in the potential for a steady stream of income. Because the underlying asset is a mortgage, the cash flow consists of both principal and interest payments, unlike a corporate bond which might only pay interest. This makes them a popular choice for institutions like pension funds and insurance companies that require predictable cash flows to match their long-term liabilities. Furthermore, because these securities are often backed by real estate, they provide a level of diversification from traditional equity markets.
Risks to Consider
Despite the steady income, investing in these vehicles is not without risk. The most significant dangers include prepayment risk, where borrowers pay off their mortgages early, forcing investors to reinvest the principal at lower prevailing rates, and default risk, where homeowners fail to make payments. The Global Financial Crisis of 2008 highlighted the dangers of poorly understood complex securities, leading to stricter regulations and transparency requirements. Modern investors analyze detailed data on loan performance, borrower credit scores, and geographic trends to mitigate these risks.
