Key Takeaways
- A mortgage-backed security passes the payments from a pool of home loans through to investors. You are not lending to one borrower, you are receiving a share of thousands of monthly payments.
- The payments include principal, not just interest. An MBS returns capital continuously from the first month, which is the single biggest structural difference from a conventional bond.
- Borrowers can repay early at any time, and that option belongs to them. When rates fall they refinance and your principal comes back to be reinvested at the new lower rate. That is prepayment risk.
- When rates rise, borrowers stop refinancing and principal comes back more slowly, extending the security’s life exactly when you would want it shorter. That is extension risk.
- Those two together produce negative convexity: the security’s price rises less when yields fall than it falls when yields rise. The MBS holder is short an option.
- Agency MBS carry a guarantee on the payments. FHFA describes Fannie Mae and Freddie Mac as packaging mortgages into MBS and guaranteeing the timely payment of principal and interest on the underlying mortgages. Non-agency MBS carry no such guarantee and depend entirely on the loans themselves.
- Asset-backed securities apply the same machinery to auto loans, credit card receivables, equipment leases and student loans. The disclosure regime for both was revised by the SEC’s 2014 rule on asset-backed securities disclosure and registration.
What Is a Mortgage-Backed Security?
A mortgage-backed security is a claim on the payments made by a pool of mortgage borrowers. A lender originates home loans, thousands of them are collected into a pool, and securities are issued that entitle their holders to a proportional share of everything that pool collects. Every month the borrowers pay, the servicer collects, fees are deducted, and the rest is distributed to the security holders.
The plainest version of this structure is called a pass-through, and the name is accurate. Whatever the pool receives is passed through. That includes the scheduled interest, the scheduled principal repayment built into every amortising mortgage payment, and any extra principal a borrower chooses to pay early.
That last sentence contains the whole subject. A conventional bond pays interest on a schedule and returns the full principal on one known date. A mortgage-backed security returns principal from the very first month, in an amount that nobody knows in advance, because it depends on how many borrowers move house, refinance, or simply pay extra. An MBS does not have a maturity date in the sense a corporate bond does. It has a final legal maturity that is unlikely to be reached and a weighted average life that changes with borrower behaviour.
Why the structure exists is worth a sentence, because it explains why the market is so large. A bank that keeps every mortgage it writes runs out of capital quickly. Securitisation moves the loans off the originator’s balance sheet and sells them to investors, which frees the originator to lend again. FHFA describes the mechanism from the government-sponsored enterprise side: by packaging mortgages into MBS and guaranteeing the timely payment of principal and interest on the underlying mortgages, Fannie Mae and Freddie Mac attract investors to the secondary mortgage market.
For the contract terms this structure varies from, see bond basics. Everything below concerns what changes when the payments come from a pool of consumers rather than from a single issuer.
Agency and Non-Agency: Two Very Different Securities
The most important division in this market is who, if anyone, stands behind the payments when borrowers default.
| Agency MBS | Non-agency MBS | |
|---|---|---|
| Who issues or guarantees | Government National Mortgage Association, or the government-sponsored enterprises Fannie Mae and Freddie Mac | Private financial institutions, with no government or GSE guarantee |
| Credit risk to the holder | Substantially transferred to the guarantor. FHFA describes the Enterprises as guaranteeing timely payment of principal and interest on the underlying mortgages | Borne entirely by the holder, structured through subordination between tranches |
| Underlying loans | Loans meeting the relevant programme or conforming criteria | Loans that fall outside those criteria, including larger balances and non-standard documentation |
| Dominant risk for the holder | Prepayment and interest rate risk | Prepayment and interest rate risk, plus full credit risk |
| Liquidity | Generally deep and actively traded | Generally much thinner, deal by deal |
Two clarifications matter here, and both are commonly muddled.
First, a guarantee on payments is not a guarantee on price. It means the holder receives principal and interest even when borrowers default. It says nothing about what the security is worth if sold before those payments arrive. The SEC makes the same point about government-guaranteed bonds generally: the guarantee covers the payments, not the market price if you sell before maturity. An agency MBS can lose substantial market value in a rising rate environment while every payment arrives exactly as promised.
Second, a credit guarantee does not remove prepayment risk. If anything it isolates it. Strip out the possibility of loss from default and what remains is entirely a question of when borrowers return your money, which is the dominant risk in agency MBS and the subject of the next two sections.
Non-agency securities handle credit differently, through structure rather than guarantee. The pool’s cash flows are divided into tranches with a defined order of priority: senior tranches are paid first and absorb losses last, while subordinate tranches absorb losses first in exchange for a higher yield. The logic is the same seniority logic that governs corporate bonds, applied to a pool of consumer loans instead of a company.
Prepayment Risk: The Option You Are Short
Every residential mortgage borrower in the United States can repay early without penalty. They exercise that right constantly, for reasons that have nothing to do with your portfolio: moving house, refinancing, receiving an inheritance, selling after a divorce. Each early repayment sends a slice of principal back to the security holder ahead of schedule.
The problem is not that the money comes back. It is when it comes back. Refinancing is driven by interest rates, so prepayments accelerate when rates fall. That means the MBS holder receives a large amount of principal to reinvest precisely when reinvestment rates are worst.
Frame it as an option and it becomes obvious. The borrower holds an option to repay early. The MBS investor has sold that option and receives a higher yield as the premium. The borrower exercises it rationally, when it is valuable to them, which is exactly when it is costly to you. This is the same structure as a callable bond, with one difference that makes it harder: a callable bond has a stated call schedule you can read, while a mortgage pool has thousands of independent decision-makers and no schedule at all.
The mirror image is extension risk. When rates rise, refinancing stops being attractive and borrowers stay put. Prepayments slow, principal comes back more slowly than expected, and the security’s effective life extends. So the holder ends up with a longer bond in a rising rate environment and a shorter one in a falling rate environment, which is the opposite of what any investor would choose.
Put the two together and you have negative convexity, the defining characteristic of mortgage-backed securities. A conventional bond has positive convexity: it gains slightly more from a fall in yields than it loses from an equal rise. An MBS behaves in reverse, because the prepayment option caps the upside while the extension effect leaves the downside intact.
| What rates do | Borrower behaviour | Effect on the MBS holder |
|---|---|---|
| Rates fall sharply | Refinancing surges, prepayments accelerate | Principal returns early and must be reinvested at the new lower rate. Price appreciation is capped near the price at which borrowers refinance away. |
| Rates stable | Prepayments run at a background rate from moves and life events | Cash flows behave roughly as modelled. |
| Rates rise sharply | Refinancing stops, borrowers stay in place | Principal returns more slowly, average life extends, and the price falls as a longer-duration security would. |
Bond duration explained covers convexity in the conventional case, which is the necessary background for seeing why the mortgage version is unusual.
Worked Example: Where the Cash Actually Comes From
This example is hypothetical and computed from the stated assumptions. It is not a market quote, a projection or a recommendation, and it simplifies by treating a whole pool as a single 30 year loan and by applying any prepayment as a single event at the end of the year.
Assume 100,000 dollars of a pass-through backed by 30 year fixed rate mortgages at 5.00 percent, with payments monthly. The level monthly payment on a 100,000 dollar 30 year loan at 5.00 percent is 536.82 dollars, so the pool distributes 6,441.86 dollars over the first twelve months.
| First twelve months | Amount |
|---|---|
| Total cash received | 6,441.86 dollars |
| Of which interest | 4,966.49 dollars |
| Of which scheduled principal | 1,475.37 dollars |
| Remaining balance after twelve months | 98,524.63 dollars |
Compare that against a conventional 100,000 dollar bond with a 5.00 percent coupon. That bond pays 5,000 dollars of interest in year one and returns zero principal, leaving the full 100,000 dollars outstanding. The MBS has already handed back 1,475.37 dollars that must be reinvested somewhere, in year one, with no decision from you.
Now add prepayments. Assume that at the end of the first year, 10 percent of the remaining pool balance is prepaid because a wave of borrowers refinanced after rates fell.
| End of year one | No prepayment | 10 percent prepayment |
|---|---|---|
| Principal returned during the year | 1,475.37 dollars | 11,327.83 dollars |
| Balance carried into year two | 98,524.63 dollars | 88,672.17 dollars |
| Approximate interest in year two at 5.00 percent on that balance | About 4,890 dollars | About 4,400 dollars |
Roughly 490 dollars of annual interest income disappeared, and 9,852.46 dollars of capital came back needing a home. Both happened because rates fell, which is to say both happened at the worst possible moment for a reinvestor. Nothing went wrong. No borrower defaulted, no guarantee failed, and the security performed exactly as designed.
That is the lesson worth taking from the arithmetic. The risk in an agency mortgage-backed security is not that you will not be paid. It is that you will be paid on a schedule chosen by other people, in response to the same rate moves that determine what you can do with the money. The reinvestment problem this creates is the same one covered at reinvestment risk, in a form where the investor has no control over the timing at all.
CMOs: Slicing the Pool Into Tranches
A pass-through gives every holder the same proportional share of the same uncertain cash flows. Some investors want a shorter, more predictable stream; others will accept more uncertainty for more yield. A collateralised mortgage obligation restructures the pool’s payments into a sequence of tranches to serve both.
The basic technique is a payment waterfall. Interest is paid to all tranches, but principal is directed to the first tranche until it is fully retired, then to the second, and so on. The earliest tranche therefore has a short and relatively predictable life, and the later ones absorb most of the timing uncertainty.
Variants extend the idea:
- Sequential pay tranches retire in order, converting one pool with an uncertain average life into several securities with different expected lives.
- Planned amortisation class tranches receive a scheduled principal stream that holds as long as prepayments stay within a specified band. The stability is real, and it is purchased from another tranche in the same deal.
- Support or companion tranches absorb the variability the planned amortisation class was protected from. They pay more and carry substantially more timing risk.
- Interest-only and principal-only strips separate the two components entirely, producing securities whose values move in opposite directions as prepayment expectations change.
One principle is worth stating flatly, because it prevents most misreadings of a CMO. Tranching redistributes risk; it does not reduce it. The pool generates whatever cash it generates. If one tranche has been made more predictable, another tranche in the same deal is holding the unpredictability. A yield that looks generous for its stated average life is usually a support tranche, and the extra yield is payment for absorbing somebody else’s timing risk.
The same reasoning applies to credit in non-agency deals. Senior tranches are protected because subordinate tranches take losses first. The protection is real, and it is finite: it lasts until the subordination is exhausted.
Asset-Backed Securities: The Same Machinery, Different Collateral
Securitisation is not specific to mortgages. Any pool of receivables that produces predictable payments can be packaged the same way, and the resulting securities are called asset-backed securities. Mortgage-backed securities are, in the broadest sense, a large and specialised subset of the same family.
| Collateral type | Typical life | What drives the risk |
|---|---|---|
| Auto loans and leases | Short, commonly a few years | Borrower credit quality and used vehicle values, which set recovery on repossession. Prepayment matters far less than in mortgages because the balances are small and the terms short. |
| Credit card receivables | Revolving, with a defined amortisation period at the end | Payment rates, charge-off rates and the health of the sponsoring bank. The pool is continuously replenished rather than static. |
| Equipment loans and leases | Medium | Business credit quality and the resale value of the specific equipment. |
| Student loans | Long | Programme rules, deferment and forbearance behaviour, and any government guarantee attached to the loan type. |
| Residential mortgages | Long, with a highly variable effective life | Prepayment and extension behaviour above all, plus credit risk where there is no guarantee. |
Three structural features run through all of them, and they are the right checklist for reading any securitisation:
- The collateral pool. What the loans actually are, who the borrowers are, and how concentrated the pool is by geography, vintage or obligor.
- The credit enhancement. Subordination, overcollateralisation, excess spread and reserve accounts. This is what stands between a loss in the pool and a loss to your tranche, and its size is stated in the deal documents.
- The waterfall. The order in which cash is applied, and any triggers that redirect it when performance deteriorates.
Disclosure for these securities is a regulated matter. The SEC adopted a final rule titled Asset-Backed Securities Disclosure and Registration, Release Nos. 33-9638 and 34-72982, on 4 September 2014, effective 24 November 2014 and published at 79 FR 58674, revising the registration, disclosure and reporting requirements for asset-backed securities. Offering documents and periodic distribution reports for registered deals are filed with the SEC and searchable through EDGAR full-text search.
One caution about ABS more generally. The collateral in a consumer receivables deal is short-lived and granular, which makes historical performance data genuinely informative in normal conditions and much less so when consumer behaviour shifts. A pool’s past charge-off rate describes the environment it lived through, not the one ahead of it.
How Do Individual Investors Actually Hold MBS?
Almost always through a fund, and there are good structural reasons for that rather than merely convenient ones.
- Position size. Individual mortgage-backed securities trade in institutional sizes. A single position is typically far beyond what a retail portfolio can hold without concentrating.
- Modelling. Valuing an MBS means forming a view on prepayment behaviour across a range of rate paths. That is a modelling exercise, not a document-reading exercise, and it is where the professional edge in this market lives.
- Cash flow administration. A pass-through returns principal every month in varying amounts. Reinvesting those small irregular sums is impractical by hand and is exactly what a fund does automatically.
- Diversification across pools. Prepayment behaviour varies with loan vintage, coupon, geography and servicer. A single pool is a concentrated bet on one set of borrowers.
Most broad bond index funds already contain a substantial allocation to agency mortgage-backed securities, because agency MBS make up a large share of the investment grade U.S. bond market. That is worth knowing even if you never buy an MBS deliberately: if you hold a total bond market fund, you already hold negative convexity, and part of your fund’s behaviour in a sharp rate move comes from it.
Dedicated mortgage funds concentrate that exposure. Before buying one, the questions worth asking are what mix of agency and non-agency it holds, how it is positioned for extension risk, and whether the yield it advertises reflects income that assumes a prepayment speed that may not persist. Bond ETF mechanics covers how a bond fund’s share price relates to its underlying holdings and where the two can separate.
Common Mistakes and Misconceptions
- Reading an agency guarantee as protection against loss. It covers the payments. It does not cover the market price, and it does nothing at all about prepayment timing.
- Treating the stated maturity as the real one. A 30 year pass-through has a final legal maturity it will almost certainly never reach. Weighted average life, which moves with rates, is the meaningful number.
- Spending the monthly distribution. Part of every payment is your own principal being returned. Treating the whole distribution as income depletes the capital without it being obvious.
- Assuming falling rates are good news. For most bonds they are. For an MBS they trigger the prepayments that take away the coupon you wanted to keep, which is why the price appreciation is capped.
- Ignoring extension risk. The security you thought had a five year average life can become a nine year one after a rate rise, precisely when a longer duration hurts most.
- Believing a CMO tranche removes risk. Tranching moves risk between tranches in the same deal. A high yield on a short stated average life usually means the tranche is absorbing someone else’s timing uncertainty.
- Judging an ABS deal by the sponsor’s brand. The securities are claims on a specific pool held in a specific structure, not general obligations of the sponsoring institution.
- Extrapolating historical pool performance. A pool’s past charge-off and prepayment record describes the conditions it experienced, not the ones ahead.
Putting It Together
Mortgage-backed and asset-backed securities are the part of fixed income where the usual questions stop working. There is no single issuer whose accounts you can read, no maturity date to hold to, and no covenant package to enforce. What there is instead is a pool of loans, a legal structure that directs their cash, and a large population of borrowers who will act in their own interest at times you cannot predict. Understanding these securities means shifting from credit analysis to cash flow analysis.
You should now be able to read one in the right order. Establish whether the payments are guaranteed and by whom, because agency and non-agency are genuinely different securities rather than points on a spectrum. Recognise that principal returns from month one, so the distribution is not income and the balance is not static. Understand which way you are exposed to rates: prepayments accelerate when rates fall and stall when rates rise, so the security shortens when you want length and lengthens when you want brevity. If it is a structured deal, find your tranche in the waterfall and find out what stands between a loss in the pool and a loss to you. And if you cannot answer the prepayment question, that is a reason to hold the exposure through a fund rather than directly.
The failure that costs individual investors most here is not credit loss. It is misreading the cash flow. Someone buys a mortgage fund for its yield, spends the monthly distributions as though they were interest, and does not notice that a portion of every payment was their own capital coming home. Rates then fall, prepayments surge, the yield drops, and the principal that came back has been reinvested at a worse rate or consumed. Nothing in that sequence involves a default. It is simply what the instrument does, and it is entirely visible in advance to anyone who reads the payment as principal plus interest rather than as a number.
Where to go next depends on the gap. If the option framing is the useful thread, callable bonds and yield to worst covers the same short-an-option problem in a form with a published schedule, which makes it easier to see. If convexity is the unfamiliar term, bond duration explained builds it from the conventional case first. And if the practical decision is which fund to hold, bond ETF mechanics is the right next step.
Frequently Asked Questions
What is a mortgage-backed security?
A mortgage-backed security is a claim on the payments made by a pool of mortgage borrowers. Home loans are collected into a pool and securities are issued entitling holders to a proportional share of what the pool collects each month. The simplest form is a pass-through, which passes along scheduled interest, scheduled principal, and any extra principal borrowers choose to repay early. The holder is not lending to one borrower but receiving a share of thousands of monthly payments.
How is an MBS different from a regular bond?
Two structural differences. First, an MBS returns principal from the very first month, because every amortising mortgage payment contains principal, whereas a conventional bond returns principal in full on one known date. Second, the amount and timing of that principal are unknown in advance, because borrowers can repay early whenever they choose. An MBS has a final legal maturity it is unlikely to reach and a weighted average life that changes with borrower behaviour.
What is prepayment risk?
Prepayment risk is the risk that borrowers repay their mortgages early and return your principal sooner than expected. Because refinancing is driven by interest rates, prepayments accelerate when rates fall, which means capital comes back to be reinvested exactly when reinvestment rates are worst. In option terms the borrower holds the right to repay early, the MBS investor has sold that right and is paid a higher yield as the premium, and the borrower exercises it when it is most costly to the investor.
What is extension risk on a mortgage-backed security?
Extension risk is the mirror image of prepayment risk. When rates rise, refinancing stops being attractive and borrowers stay in place, so prepayments slow and principal returns more slowly than expected. The security’s effective life extends, which means the holder is left with a longer-duration bond in a rising rate environment, precisely when a longer duration is most damaging to price.
What is negative convexity, and why do MBS have it?
Negative convexity means a security gains less when yields fall than it loses when yields rise by the same amount. A conventional bond has the opposite property. Mortgage-backed securities have negative convexity because the borrower’s prepayment option caps the upside: when rates fall far enough, borrowers refinance away and the price cannot rise much beyond the level at which that happens, while extension risk leaves the downside from rising rates fully intact.
What is the difference between agency and non-agency MBS?
Agency MBS are issued or guaranteed through Ginnie Mae or the government-sponsored enterprises. FHFA describes Fannie Mae and Freddie Mac as packaging mortgages into MBS and guaranteeing the timely payment of principal and interest on the underlying mortgages in order to attract investors to the secondary mortgage market. Non-agency MBS are issued by private institutions with no such guarantee, so the holder bears the full credit risk of the loan pool, managed through subordination between tranches rather than by a guarantor.
Does an agency guarantee mean an MBS cannot lose money?
No. A guarantee on payments is not a guarantee on price. It means the holder receives principal and interest even when borrowers default, and says nothing about what the security is worth if sold before those payments arrive. The SEC makes the general version of this point about government-guaranteed bonds: the guarantee covers the payments, not the market price if you sell before maturity. An agency MBS can also lose value to extension risk while every payment arrives on time.
How much principal does a mortgage pass-through return in the first year?
More than people expect. In a hypothetical 100,000 dollar pool of 30 year fixed rate mortgages at 5.00 percent, the level monthly payment is 536.82 dollars, so the pool distributes 6,441.86 dollars over twelve months, of which 4,966.49 dollars is interest and 1,475.37 dollars is scheduled principal, leaving a balance of 98,524.63 dollars. A conventional 100,000 dollar bond with a 5.00 percent coupon would pay 5,000 dollars of interest and return no principal at all in the same period.
What is a CMO and does tranching reduce risk?
A collateralised mortgage obligation restructures a pool’s payments into a sequence of tranches through a payment waterfall, directing principal to the first tranche until it retires, then the second, and so on. Sequential pay, planned amortisation class, support and strip tranches all serve different timing preferences. Tranching redistributes risk rather than reducing it: the pool generates whatever it generates, so if one tranche has been made more predictable, another tranche in the same deal is holding the uncertainty.
What are asset-backed securities and how do they differ from MBS?
Asset-backed securities apply the same securitisation machinery to non-mortgage receivables: auto loans and leases, credit card balances, equipment leases and student loans. The structural checklist is identical (the collateral pool, the credit enhancement, and the waterfall), but the risks differ by collateral. Auto ABS depend on borrower credit and used vehicle values with little prepayment sensitivity. Credit card ABS revolve and depend on payment and charge-off rates. Mortgages are dominated by prepayment and extension behaviour.
How are asset-backed securities regulated and disclosed?
Registered securitisations are offered through documents filed with the SEC and are searchable on EDGAR, alongside periodic distribution reports. The SEC adopted a final rule titled Asset-Backed Securities Disclosure and Registration, Release Nos. 33-9638 and 34-72982, on 4 September 2014, effective 24 November 2014 and published at 79 FR 58674, which revised the registration, disclosure and reporting requirements for asset-backed securities.
Can individual investors buy mortgage-backed securities directly?
Rarely, and usually it is not advisable. Individual MBS trade in institutional sizes far larger than a retail portfolio can hold without concentrating, valuing one requires modelling prepayment behaviour across a range of rate paths rather than reading documents, and a pass-through returns small irregular amounts of principal every month that are impractical to reinvest by hand. Most investors hold this exposure through funds, and most broad bond index funds already contain a substantial agency MBS allocation.
References
This guide is based on U.S. regulator and federal agency publications, each retrieved and verified on 22 August 2026:
- FHFA: About Fannie Mae and Freddie Mac: the description of the Enterprises packaging mortgages into MBS and guaranteeing the timely payment of principal and interest on the underlying mortgages in order to attract investors to the secondary mortgage market.
- SEC: Asset-Backed Securities Disclosure and Registration (Release Nos. 33-9638 and 34-72982): the SEC final rule adopted 4 September 2014 and effective 24 November 2014, published at 79 FR 58674, which revised the registration, disclosure and reporting regime for asset-backed securities.
- SEC: The Laws That Govern the Securities Industry: the Securities Act registration and prospectus framework that applies to publicly offered securitisations, and the Trust Indenture Act standards for publicly offered debt.
- SEC: EDGAR Full-Text Search: the filing archive where registered asset-backed securities offering documents and periodic distribution reports are found.
- SEC Office of Investor Education and Advocacy: Investor Bulletin, Fixed Income Investments, When Interest Rates Go Up, Prices of Fixed-Rate Bonds Fall: the inverse price and rate relationship, the greater rate sensitivity of longer maturities, and the point that a federal guarantee covers the payments rather than the market price before maturity.
- FINRA: Bonds: the fixed income taxonomy including mortgage-backed and asset-backed products, and duration as a measure of price sensitivity to yield changes.
The cash flow tables are original, hypothetical calculations from the stated assumptions, simplified by treating a pool as a single 30 year loan and applying prepayment as a single year-end event. The rates are round numbers chosen so the arithmetic can be checked, not observed market levels, and nothing here is a projection or a recommendation. This is educational content, not personalised investment, tax, or legal advice.