If you own a total bond market index fund, a large share of it is home loans. Those holdings are agency mortgage-backed securities, and they behave unlike anything else in the fund. Here is what they are, why their prices move in a strange direction, and why the ones you own are not the ones that blew up in 2008.

What an Agency Is

An agency bond is debt issued by a US federal agency, or by a government-sponsored enterprise: a private company Congress set up to do one public job. Five of them borrow regularly. Fannie Mae, Freddie Mac, the Federal Home Loan Banks, the Farm Credit System, and Farmer Mac.

They sell two different things.

The first is plain agency debt, sometimes called a debenture. The agency borrows, pays you a coupon, and returns your principal on a set date. It behaves like a Treasury bond and usually yields a bit more.

The second is an agency mortgage-backed security, or MBS. That is not a loan to the agency at all. It is a claim on a pool of somebody else's home loans, with the agency standing behind it. The rest of this article is about the second kind.

Ginnie Mae's Guarantee, and the Weaker One Behind Fannie and Freddie

Three names dominate housing, and one is not like the other two.

Ginnie Mae is a government corporation inside the Department of Housing and Urban Development, created in 1968. It buys no loans and issues no securities. It only guarantees them. Approved lenders pool federally backed mortgages (FHA, VA, rural housing, and public and Indian housing loans) and issue the security themselves. Ginnie Mae promises timely payment of principal and interest, backed by the full faith and credit of the United States. That is the same legal promise a Treasury bond carries.

Fannie Mae and Freddie Mac are shareholder-owned companies. They buy loans from lenders and issue their own securities, and the guarantee on those is the company's, not the government's. Their offering documents say it outright: the securities are not debt of the United States and are not federally guaranteed. Investors bought them anyway, assuming Washington would never let either firm fail. That assumption is the implicit guarantee.

In September 2008 the assumption got tested. Their regulator put both companies into conservatorship, a legal takeover that hands control to the regulator, and the Treasury signed agreements committing hundreds of billions of dollars to keep them solvent. They are still in conservatorship. No law changed, so the guarantee is still technically implicit. With a standing Treasury commitment behind it, though, the market now treats the credit risk on their securities as government risk.

How a Pass-Through Actually Works

Say a lender writes 1,000 mortgages averaging $300,000 each. That is a $300 million pool. The lender sends it to Fannie Mae, which guarantees the pool and issues a security against it. You buy a slice.

Every month, those 1,000 households make a mortgage payment. The money flows one way. The company that collects the payments keeps a servicing fee, the agency keeps a guarantee fee, and the rest passes through to you. That gap is why your coupon is lower than the rate any of those homeowners pays.

Notice what arrives. A Treasury pays interest twice a year and returns your principal on one day. A pass-through sends you interest and principal every month, because part of every mortgage payment is principal. The investment shrinks as it pays you.

Since 2019, fixed-rate Fannie and Freddie pools trade as one interchangeable security, so a buyer no longer has to care which company issued it.

Prepayment Risk, the Defining Feature

A homeowner can pay a mortgage off whenever they like, with no penalty. They sell the house. They refinance. They put an extra $200 in some months. Each of those sends your principal back early.

So the 30-year label on the pool is a ceiling, not a schedule. What matters is average life, and nobody knows it in advance.

The trouble runs in both directions.

You buy a pool paying 5%. Rates fall to 3%, and a big share of those homeowners refinance within the year. Your money comes back fast, and the only place to put it is a 3% market. You lost the good coupon exactly when good coupons got scarce.

Now flip it. Rates rise to 7%. Nobody sells and nobody refinances, so the pool you expected back in seven years takes twelve. You sit on 5% while new bonds pay 7%.

Both moves work against you. That is the structure, not bad luck.

Negative Convexity, and Why You Get Paid for It

Duration measures how far a bond's price moves when rates move, and for an ordinary bond it hardly depends on which way they go.

An MBS is different. Its duration shrinks when rates fall, because prepayments speed up, and stretches when rates rise, because prepayments stop. You want long duration in a rally and short duration in a selloff. You get the reverse of both.

Negative convexity is the name for that. Plot price against yield and the curve bends the wrong way, so the security gains less in a large rally than it loses in an equally large selloff.

Nobody takes that deal for nothing. Agency MBS yield more than Treasuries of similar length, and the extra yield is payment for not knowing when your money comes back. It is not payment for default risk, which is a separate question with its own letter grades. Whether the extra yield covers the cost depends on how homeowners actually behave.

Agency MBS Is Not What Broke in 2008

The securities at the center of that crisis were mostly private-label: pools assembled by Wall Street banks with no agency guarantee attached. When borrowers stopped paying, the losses landed on the buyer, cushioned only by a slicing structure and a credit rating that proved wrong.

An agency pool works differently. If a borrower defaults, the guarantor buys that loan out of the pool at face value and your principal comes back. The credit risk does not vanish. It moves off you and onto the guarantor.

That is what played out. Fannie and Freddie nearly collapsed because they were carrying that risk on millions of loans at once, and it took a federal takeover to keep them upright. The securities they had guaranteed kept paying.

So the question to ask about any mortgage bond is who guarantees it, and whether that guarantor could survive a bad decade.

Where You Already Own These

A total bond market index fund tracks a broad investment-grade US benchmark, and agency MBS are a large share of it, second only to Treasuries. You do not have to buy one on purpose to end up holding thousands.

That is why a bond fund page quotes average effective duration rather than the plain kind. Only the effective version models prepayments, so it is the only one that describes a mortgage holding honestly.

It is also what happened to mortgage holdings in 2022. Rates rose, refinancing stopped, the pools stretched out, and the duration of what people held grew while the price was already falling.

One label repays a second look. A fund described as government is not always all Treasuries. Agency mortgage pools qualify on the strength of their guarantee, and they do not behave like a Treasury of the same stated maturity.