Agency vs Non-Agency Mortgage Securities

Two mortgage bonds can hold economically similar home loans and trade for completely different reasons. One's price moves almost entirely with interest rates and how fast homeowners refinance; the other's price moves with those things and with whether the underlying borrowers actually keep paying. The dividing line is whether the security is agency or non-agency — whether it carries a guarantee from Fannie Mae, Freddie Mac, or Ginnie Mae against homeowner default.

What the guarantee actually does

Fannie Mae and Freddie Mac (government-sponsored enterprises) and Ginnie Mae (a full-faith-and-credit federal agency) buy pools of mortgages that meet strict underwriting rules — loan size caps, documentation standards, borrower credit thresholds — and guarantee investors will receive scheduled principal and interest even if individual homeowners default. That guarantee is priced into the mortgage rate borrowers pay and effectively removes credit risk from the security entirely; the only things left to worry about are how fast rates move and how fast homeowners prepay. Loans that don't meet agency underwriting standards — jumbo loans above the size cap, loans with thin documentation, borrowers below the credit threshold — get securitized without a guarantee, as non-agency deals, and are structured instead with the tranching and subordination tools used across securitization generally.

non-agency MBS no guarantee tranches absorb loss directly

Worked example

A $300 million agency MBS pool has homeowner defaults totaling $4 million over a year. Because Fannie Mae guarantees the pool, it advances the missed principal and interest, and investors receive their scheduled cash flows unaffected — the $4 million loss is absorbed by the GSE, not the security. A $300 million non-agency pool with the same $4 million of defaults has no guarantor: the loss flows into the deal's waterfall, hitting the equity tranche first, then mezzanine, exactly as in any other securitization without agency backing.

What this means in practice

Analysts pricing an agency MBS focus almost entirely on prepayment modeling — how fast borrowers refinance as rates move — because credit risk has effectively been outsourced to the guarantor. Analysts pricing a non-agency deal need full credit underwriting of the loan pool itself, on top of the same prepayment analysis, because there's no backstop standing between weak loans and investor losses.