Mortgage-Backed Securities

What Are Mortgage-Backed Securities?

Mortgage-backed securities (MBS) are bonds created by pooling thousands of individual home loans — primarily residential mortgages — into a single tradeable instrument. As homeowners make their monthly payments, that cash flow passes through to MBS bondholders in the form of interest and principal. The mechanics introduce two features that distinguish MBS from ordinary corporate or Treasury bonds: prepayment risk (homeowners refinancing or selling faster than expected) and extension risk (homeowners holding loans longer than anticipated when rates rise). These cash-flow uncertainties require investors to model a range of prepayment scenarios — typically expressed as a percentage of the PSA prepayment model — rather than a fixed yield to maturity.

Collateralized Mortgage Obligations (CMOs) and Real Estate Mortgage Investment Conduits (REMICs) take the process further by slicing a pool into tranches with different priority, duration, and risk profiles, giving investors the ability to target specific parts of the cash-flow waterfall.

Agency vs. Non-Agency MBS

Agency MBS are issued or guaranteed by Fannie Mae, Freddie Mac, or Ginnie Mae. Ginnie Mae carries an explicit full-faith-and-credit guarantee from the U.S. government; Fannie and Freddie carry an implicit (and since 2008 effectively explicit) guarantee following their conservatorship. This near-sovereign credit quality makes agency MBS a near-perfect substitute for Treasuries in terms of credit risk, but with added prepayment complexity and a liquidity premium that generates a yield spread over comparable Treasury bonds — the option-adjusted spread (OAS).

Non-agency MBS, sometimes called private-label MBS, are issued without government backing by banks, investment banks, or other private conduits. These securities bear credit risk tied directly to the underlying borrowers and loan quality. The rapid expansion of non-agency MBS backed by subprime and Alt-A mortgages — with notional issuance exceeding $1 trillion in 2006 alone — was central to the 2008 financial crisis. Today's non-agency market is a fraction of its pre-crisis size, dominated by high-quality jumbo loans and re-performing loan pools.

The Fed and MBS

The Federal Reserve first purchased agency MBS in late 2008 as part of its initial quantitative easing programme, eventually accumulating a peak balance of approximately $2.74 trillion by early 2022. This buying was not incidental — by suppressing MBS yields, the Fed directly reduced 30-year fixed mortgage rates, stimulating the housing market as part of its broader monetary transmission mechanism.

Once the Fed shifted to quantitative tightening (QT) in June 2022, it capped MBS runoff at $35 billion per month (alongside $60 billion in Treasuries). However, the passive runoff structure means the Fed does not sell MBS outright — it simply reinvests only the principal cash flows that exceed the monthly cap. Because prepayments collapsed as 30-year mortgage rates surged past 7% in 2022–2023, actual MBS runoff consistently fell well below the $35 billion cap, averaging closer to $15–20 billion monthly. This created a notable asymmetry in QT: Treasury runoff ran at or near its cap, while MBS runoff lagged significantly, slowing the overall pace of balance sheet reduction.

Why It Matters for Traders

For macro traders, MBS represent one of the most direct links between monetary policy and the real economy. The MBS-Treasury spread — typically the 30-year current coupon MBS yield minus the 10-year Treasury yield — is a real-time indicator of mortgage market stress and Fed policy transmission. When this spread widens, as it did sharply in March 2020 (exceeding 150 basis points) and again in mid-2022, it signals deteriorating housing affordability and tightening financial conditions beyond what benchmark yields alone suggest.

Heavy agency MBS supply — from new issuance or potential Fed sales — tends to cheapen MBS relative to Treasuries, pushing mortgage rates higher even if the federal funds rate is unchanged. Conversely, any signal that the Fed might begin selling MBS outright (rather than passive runoff) would likely widen spreads materially, acting as an additional, incremental tightening of financial conditions.

How to Read and Interpret It

Practitioners track several key metrics: the current coupon MBS yield, the OAS on the Bloomberg U.S. MBS Index (a tightening OAS signals strong investor demand; a widening OAS signals stress or excess supply), and the 30-year mortgage rate published weekly by Freddie Mac's Primary Mortgage Market Survey. As a rough threshold, when the 30-year mortgage rate rises more than 50 basis points above its trailing 6-month average, housing starts and existing home sales tend to soften within two to three months — a leading indicator for GDP revisions and construction sector earnings.

Monitor the Fed's weekly H.4.1 balance sheet release for the actual pace of MBS runoff. A persistent gap between the $35 billion cap and actual runoff signals that prepayments remain depressed — itself confirming a locked-in housing market where homeowners with low-rate mortgages refuse to move.

Historical Context

The most instructive episode remains the 2008–2009 financial crisis, when non-agency MBS backed by subprime mortgages collapsed in value, with AAA-rated tranches losing 20–60% of face value. The ABX.HE index, which tracked credit default swaps on subprime MBS, fell from par to below 40 cents on the dollar for the 2006-2 vintage by early 2008 — a real-time signal of systemic stress that preceded the Lehman bankruptcy by six months.

More recently, in March 2020, agency MBS spreads gapped wider by roughly 80 basis points in a matter of days as leveraged mortgage REITs faced margin calls and were forced to liquidate. The Fed's emergency purchase announcement on March 15, 2020, and subsequent daily MBS purchases totalling over $300 billion in six weeks, compressed spreads back to near-historical tights by June 2020.

Limitations and Caveats

MBS signals can mislead when the Fed is an active buyer or seller, since central bank demand distorts the price discovery that spreads would otherwise reflect. During QE periods, artificially compressed MBS spreads overstated the health of the housing finance system. Additionally, prepayment models are notoriously difficult to calibrate: in novel rate environments (such as rates rising 500 basis points in 14 months), historical prepayment relationships break down, making duration estimates unreliable. Finally, agency MBS credit quality tells you nothing about broader consumer health — that requires monitoring non-agency, auto loan, and credit card ABS spreads alongside.

What to Watch

• Weekly Fed H.4.1: Track actual MBS runoff versus the $35 billion cap — persistent shortfalls signal a frozen housing market.
• MBS-Treasury spread (current coupon OAS): Widening above 50–60 bps historically precedes mortgage rate spikes and housing slowdowns.
• 30-year mortgage rate relative to 10-year Treasury: The "mortgage basis" — normally around 170 bps — widening toward 300 bps (as it did in late 2023) signals acute market stress.
• Fed communication on MBS sales: Any shift from passive runoff to active sales would be a significant incremental tightening signal worth trading in both rates and housing-related equities.