The Risks and Returns of Mortgage-Backed Securities (MBS)

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Mortgage-backed securities (MBS) are groups of home mortgages that are sold by the issuing banks and then packaged together into “pools” and sold as a single security. This process is known as securitization.

When homeowners make the interest and principal payments, those cash flows pass through the MBS and through to bondholders (minus a fee for the entity that originates the mortgages). Mortgage-backed securities generally offer higher yields than U.S. Treasuries, but they also carry reinvestment risks, prepayment risk, and the risk of negative convexity.

Mortgage-Backed Security Prepayment Risk

The unique aspect of mortgage-backed securities (MBS) is the element of prepayment risk. This is the risk investors take when mortgagees decide to pay the principal on their mortgages ahead of schedule. The result, for investors in MBSs, is an early return of principal or a reduction in interest income if the mortgagee makes larger payments to pay the mortgage down more quickly.

MBSs played a role in the sub-prime mortgage crises of 2007 to 2010. Sub-prime loans were packed into MBSs. When the loans began defaulting en-masse, investors and lenders lost tremendous amounts of money when MBS values plummeted.

This means that the principal value of the underlying security shrinks over time, which in turn leads to a gradual reduction in interest income. Prepayment risk is typically highest when interest rates are falling since this leads homeowners to refinance their mortgages.

Interest Rate and Average Life of MBSs

Prepayments reduce the mortgage value to investors, not only because the interest income on the investment is reduced but the investor is then forced to reinvest at lower rates when the mortgage is paid off.

The average life of an MBS declines more rapidly when rates are falling (since homeowners refinance more when rates are going down), and it drops more slowly when rates are rising (higher rates generally reduce the amount of refinancing actions). This can lead to uncertain cash flows from individual MBSs, as well as the tendency for negative convexity.

Negative Convexity

Mortgages act similarly to bonds in that when rates go up, prices go down. However, mortgage-backed securities prices tend to increase at a decreasing rate when bond rates are falling; in turn, their prices tend to decrease at an increasing rate when rates are rising. This is known as negative convexity and is one reason why MBSs offer higher yields than U.S. Treasuries.

Mortgage-backed securities are sometimes used to hedge the overall risk of an investor's fixed income portfolio due to negative convexity.

In short, investors expect to be paid more to take on this added uncertainty. It should be noted that mortgage-backed securities tend to generate their best relative performance when prevailing rates are stable.

Agency Versus Non-Agency MBS

Mortgage pools can be created by private entities (in most cases) or by the three quasi-governmental agencies that issue MBSs: Government National Mortgage Association (known as GNMA or Ginnie Mae), Federal National Mortgage (FNMA or Fannie Mae), and Federal Home Loan Mortgage Corp. (Freddie Mac).

The most concise explanation of the differences among the three comes from the U.S. Securities and Exchange Commission (SEC):

"Ginnie Mae, backed by the full faith and credit of the U.S. government, guarantees that investors receive timely payments. Fannie Mae and Freddie Mac also provide certain guarantees and, while not backed by the full faith and credit of the U.S. government, have special authority to borrow from the U.S. Treasury. Some private institutions, such as brokerage firms, banks, and homebuilders, also securitize mortgages, known as "private-label" mortgage securities."

MBSs backed by Ginnie Mae aren’t at risk of default, but there is a small degree of default risk for a bond issued by Fannie Mae and Freddie Mac. Still, Freddie and Fannie's bonds have a stronger element of backing than they appear to since both were taken over by the federal government in the wake of the 2008 financial crisis.

How to Invest in an MBS

Investors can buy individual mortgage-backed securities through a broker, but this option is limited to those with the time and sophistication to conduct their own fundamental research regarding the average age, geographic location, and credit profile of the underlying mortgages.

Most investors who own a broad-based bond mutual fund or exchange-traded fund have some exposure to this sector since it is such a large portion of the market—therefore it is one that is heavily represented in diversified funds. Investors can also opt for funds that are dedicated solely to MBSs. Some of the exchange-traded funds (ETFs) that invest in this space are:

  • Barclays Agency Bond Fund (AGZ)
  • iShares Barclays MBS Fixed-Rate Bond Fund (MBB)
  • Mortgage-Backed Securities ETF (VMBS)
  • iShares Barclays GNMA Bond Fund (GNMA)
  • SPDR Barclays Capital Mortgage Backed Bond ETF (MBG)