What it means
The structure is straightforward once you picture the plumbing. A lender makes home loans, sells them into a pool, and the pool issues securities that pass the monthly payments through to investors after deducting a servicing fee and a guarantee fee.
Because the guarantor stands behind the credit risk, investors are mainly exposed to interest rates and to the timing of repayments rather than to whether individual borrowers pay. That timing exposure is the defining feature.
Homeowners can repay early whenever they refinance, move or pay down a mortgage, and those prepayments flow straight to investors as returned principal. When rates fall, prepayments accelerate and investors get their money back exactly when reinvestment options are worst, a pattern known as negative convexity.
Agency MBS is one of the largest and most liquid fixed income markets in the world, which is why it appears in bank treasury portfolios, insurance company balance sheets and bond funds. The liquidity comes partly from standardisation: pools are grouped into to-be-announced contracts that trade on generic characteristics rather than on specific loan lists.
Yields sit above comparable government bonds but below corporate credit, and the spread compensates for prepayment uncertainty rather than for default risk. Analysts measure that uncertainty using option-adjusted spread and effective duration, both of which try to price the borrower's right to repay early.
The nuance for a non-specialist is that "agency" describes the guarantee, not the underlying loans. The mortgages in the pool are ordinary home loans that must meet size and quality criteria to qualify, and loans that fall outside those criteria are securitised in the private-label market without a guarantee, where credit risk sits squarely with the investor.
In practice
Real-world examples.
Example
A regional bank holds $400,000,000 of agency MBS in its securities portfolio as a place to park deposits. When rates rise sharply, prepayments slow, the average life of the holding extends and the bank finds its portfolio is longer and less liquid than planned.
Example
A bond fund manager compares an agency MBS yielding 5.1% against a government bond yielding 4.4%. She treats the 0.7 percentage point difference as payment for prepayment uncertainty rather than credit risk, since the agency guarantee covers borrower default.
Example
An insurance company matching long-dated liabilities avoids agency MBS despite the yield, because the uncertain timing of principal returns makes it a poor match for payments it must make on fixed dates decades ahead.
Think of it
“Agency MBS is government-backed mortgage bonds-securities with government guarantees.
Formula
Calculation
Pass-through rate = Gross mortgage rate - servicing fee - guarantee fee
Suppose the mortgages in a pool carry an average rate of 5.75%, the servicer retains 0.25% and the guarantor charges 0.20%.
Pass-through rate = 5.75% - 0.25% - 0.20% = 5.30%
An investor holds $500,000 of a $50,000,000 pool, which is a 1% share. In the first month the interest passed through is:
Interest = $500,000 x 5.30% / 12 = $2,208.33
In that same month the pool receives $60,000 of scheduled principal and $250,000 of prepayments from borrowers who refinanced. The investor's 1% share is $600 of scheduled principal and $2,500 of prepayments.
Total first payment = $2,208.33 + $600 + $2,500 = $5,308.33
Of that, only $2,208.33 is income; the remaining $3,100 is the investor's own capital coming back early, which now has to be reinvested at whatever rates are currently available.Case study
Seen in the real world.
Cedar Ridge Mutual is a fictional insurer created for this illustrative example. Its investment committee shifted $150,000,000 from government bonds into agency MBS to pick up roughly 0.7 percentage points of extra yield, reasoning that the guarantee removed credit risk and therefore removed the main reason for caution.
Two years later mortgage rates fell by more than a full percentage point and a wave of refinancing pushed prepayments to several times the modelled rate. Cedar Ridge received large amounts of principal back far earlier than planned and could only reinvest it at the new lower rates, so the realised yield on the position came in well below the 0.7 point pick-up that had justified the trade.
The illustrative lesson is that the committee had analysed the risk it could name, which was default, and ignored the risk that actually applied, which was prepayment timing. After the review Cedar Ridge kept a smaller agency MBS allocation but sized it against a range of prepayment scenarios rather than a single central case.
Watch out
Common mistakes.
- Assuming an agency guarantee removes all risk, when it addresses only borrower default and leaves interest rate and prepayment risk entirely with the investor.
- Treating the full monthly payment as income, when much of it is principal being returned and spending it erodes the capital base.
- Using stated maturity to judge how long the investment will last, when the average life driven by prepayments is usually far shorter and moves with interest rates.
Questions
People also ask.
What is the difference between agency and private-label MBS?
Agency issues carry a guarantee from a government agency or government-sponsored enterprise, while private-label issues have no such backing and expose investors directly to borrower default.
Why do investors dislike falling rates in this market?
Because prepayments accelerate, principal is returned early, and that cash can only be reinvested at the new lower rates, capping the price gain the bond would otherwise deliver.
What does the guarantee fee pay for?
It compensates the guarantor for standing behind the credit risk of the pooled loans, and it is deducted from the mortgage rate before payments reach the investor.
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