Mortgage-backed security

Mortgage backed securities, which include Ginnie Mae Securities (GNMA) and Collateralized Debt Instruments (CDO), are among the most complex securities in the fixed income asset class. Vanguard provides a basic description of mortgage backed securities in the prospectus to its bond funds: Mortgage-backed securities represent interests in underlying pools of mortgages. Unlike ordinary bonds, which generally pay a fixed rate of interest at regular intervals and then repay principal upon maturity, mortgage-backed securities pass through both interest and principal from underlying mortgages as part of their regular payments. Because the mortgages underlying the securities can be prepaid at any time by homeowners or by corporate borrowers, mortgage-backed securities are subject to prepayment risk. These types of securities are issued by a number of government agencies, including the GNMA, the FHLMC, and the FNMA, [as well as by private issuers]. As a rule, when interest rates rise, bond prices fall. The opposite is also true: Bond prices go up when interest rates fall. Mortgage-backed securities are different. In general, declining interest rates will not lift the prices of mortgage-backed securities—such as GNMAs—as much as the prices of comparable bonds. Why? Because when interest rates fall, the bond market tends to discount the prices of mortgage-backed securities for prepayment risk—the possibility that homeowners will refinance their mortgages at lower rates and cause the bonds to be paid off prior to maturity. In part to compensate for this prepayment possibility, mortgage-backed securities tend to offer higher yields than other bonds of comparable credit quality and maturity.

Definitions
Some definitions useful in understanding mortgage backed securities:


 * Mortgage Backed Security (MBS)
 * Collateralized Mortgage Obligation (CMO)
 * Ginnie Mae - Government National Mortgage Association (GNMA)
 * Pass Through Security
 * Real Estate Mortgage Investment Conduits (REMIC)
 * Tranches

Risks
In addition to the normal risks of bonds, mortgage-backed securities exhibit negative convexity. This negative convexity results from contraction risk--the tendency of homeowners to refinance as interest rates fall--and from extension risk--the tendency of homeowners to delay pre-payments as interest rates rise.

Determining the exact extent of this negative convexity is impossible, as it depends on how mortgagors behave. In general, this behavior is non-linear: small changes make little difference in homeowner behavior but large changes cause homeowners to overcome the hassle and to refinance in larger numbers. The extent of negative convexity can be approximated with statistical modeling, although behavior with large interest rate changes is relatively hard to predict. Bondholders are compensated for this risk by the "option-adjusted spread," which can be interpreted as the market's assessment of the size of prepayment risk.

Detailed discussions can be found at InvestinginBonds.com.

Links

 * Solving The Mortgage Mystery by Barclays Global Investors, Investment Insights, 11.05

Academic Papers

 * Mason, Joseph R. and Rosner, Josh, "How Resilient are Mortgage Backed Securities to Collateralized Debt Obligation Market Disruptions?" (February 13, 2007). Available at SSRN: http://ssrn.com/abstract=1027472
 * Mason, Joseph R. and Rosner, Josh, "Where Did the Risk Go? How Misapplied Bond Ratings Cause Mortgage Backed Securities and Collateralized Debt Obligation Market Disruptions" (May 3, 2007). Available at SSRN: http://ssrn.com/abstract=1027475
 * Stein, Harvey J., Belikoff, Alexander L., Levin, Kirill and Tian, Xusheng, "Analysis of Mortgage Backed Securities" (January 5, 2007). Available at SSRN: http://ssrn.com/abstract=955358