Residential Mortgage-Backed Securities (RMBS) are bonds backed by pools of residential mortgage loans — from traditional 30-year fixed rate conforming mortgages to jumbo loans, adjustable-rate mortgages, and non-qualified mortgages. The RMBS market is vast and diverse, encompassing both agency-backed securities (issued or guaranteed by Fannie Mae, Freddie Mac, or Ginnie Mae) and non-agency or "private-label" securities without government backing.
Agency RMBS — guaranteed by Fannie Mae, Freddie Mac, or Ginnie Mae — carry negligible credit risk because the guarantee makes these securities equivalent to government obligations for practical investment purposes. Agency MBS is one of the largest and most liquid fixed income markets in the world. The primary risks in agency MBS are prepayment risk (borrowers refinancing or moving, returning principal earlier than expected) and extension risk (the opposite — rates rise and prepayments slow, extending the life of the security beyond expectations). Duration management for agency MBS portfolios is primarily a function of modeling these prepayment dynamics.
Non-agency RMBS (also called private-label RMBS) carries credit risk because there is no government guarantee. The pool's performance depends on borrower repayment, property values (which affect loss severity in default), and the servicer's ability to manage delinquent loans. Non-agency RMBS includes: Jumbo loans (high-balance mortgages exceeding conforming loan limits), Non-QM loans (mortgages that don't meet Qualified Mortgage standards), and historically, the Alt-A and subprime categories that featured prominently in the 2008 financial crisis.
RMBS analysis requires understanding the collateral characteristics: weighted average loan-to-value (WALTV), weighted average FICO score, geographic concentration, loan purpose (purchase vs. refinance), loan size, and occupancy type. Each factor affects default probability and loss severity. The servicer's performance — in advancing delinquent payments, managing foreclosures, and minimizing losses — significantly affects investor outcomes.
The 2008 financial crisis demonstrated catastrophically the risks of poor underwriting quality, insufficient credit enhancement, and ratings agency failures in the non-agency RMBS market. Post-crisis regulatory reform (Dodd-Frank, QM/QRM rules, risk retention requirements) substantially reshaped the non-agency RMBS market, though the market has been rebuilt on more conservative underwriting standards since approximately 2015.