Understanding Mortgage-Backed Securities

Mortgage-backed securities (MBS) are one of the biggest and most important parts of the global bond market — financial instruments that turn pools of home loans into tradable securities. They channel money to mortgage lending and give investors a way to earn returns from it, but they also played a central role in the 2008 financial crisis. This guide explains what mortgage-backed securities are, how securitisation works, the main types, and their benefits and risks — in clear, plain language. It complements our deeper guide on MBS and prepayment and our guide to tranches, and is relevant to anyone studying fixed income or finance.

What are mortgage-backed securities?

A mortgage-backed security is a bond-like investment backed by a pool of mortgages. Many individual home loans are bundled together, and securities are issued whose payments come from the homeowners' monthly mortgage payments — principal and interest. Investors who buy these securities effectively receive a share of those payments. In this way, MBS transform thousands of illiquid individual loans into standardised, tradable securities, connecting borrowers in the housing market with investors in the capital markets. The result is a vast, liquid market that links Main Street mortgages to global investors right around the world.

How securitisation works

The process of creating an MBS is called securitisation. It typically runs as follows: a lender (the originator) makes many mortgage loans; these loans are gathered into a pool and sold to a special-purpose vehicle (SPV) or agency; the SPV then issues securities backed by the pool's cash flows and sells them to investors. The mortgage payments flow through this structure to the security holders. Securitisation lets the original lender remove the loans from its balance sheet and recover its capital quickly — freeing it to make new loans — while investors gain exposure to mortgage returns without having to lend directly.

The main types of MBS

There are several important varieties:

• Pass-through securities — the simplest form, where mortgage payments are passed straight through the pool to investors (minus a servicing fee).
• Collateralised mortgage obligations (CMOs) — more complex structures that divide the cash flows into tranches with different risk, return and maturity profiles, to suit different investors.
• Agency vs non-agency — agency MBS are issued or guaranteed by government-linked bodies (in the US, Ginnie Mae, Fannie Mae and Freddie Mac), carrying little credit risk; non-agency (private-label) MBS are issued by private institutions and carry more credit risk.
• RMBS vs CMBS — residential mortgage-backed securities are backed by home loans, while commercial mortgage-backed securities are backed by loans on commercial property.

These variations let MBS serve a wide range of investor needs and risk appetites.

The benefits of mortgage-backed securities

MBS bring several benefits to the financial system. They provide liquidity to mortgage lenders, who can sell loans and recycle their capital into new lending — helping make mortgages more available and affordable. They give investors access to the returns of mortgage lending without having to originate or service loans themselves. And they distribute risk across many investors rather than concentrating it on a single lender. In principle, this spreading of risk and freeing-up of capital makes the housing-finance system more efficient and helps keep borrowing costs lower for homeowners.

The risks of mortgage-backed securities

MBS also carry significant risks. The most distinctive is prepayment risk — because homeowners can repay early (especially when rates fall), the timing of cash flows is uncertain (explored fully in our MBS and prepayment guide). There's credit risk too, especially for non-agency MBS, if borrowers default. And there's interest-rate risk, as with any bond. The dangers were dramatically exposed in the 2008 financial crisis, when MBS backed by poor-quality "subprime" mortgages suffered huge losses, contributing to a global crisis. The episode showed that MBS are only as sound as the loans beneath them, and that complexity can obscure real risk.

Frequently asked questions

What are mortgage-backed securities?

Bond-like investments backed by a pool of mortgages, where investors receive a share of the homeowners' monthly principal and interest payments.

How are mortgage-backed securities created?

Through securitisation: a lender pools many mortgages and sells them to a special-purpose vehicle or agency, which issues securities backed by the pool's cash flows to investors.

What are the main types of MBS?

Pass-through securities, collateralised mortgage obligations (CMOs, which use tranches), agency versus non-agency MBS, and residential (RMBS) versus commercial (CMBS) MBS.

What are the risks of mortgage-backed securities?

Prepayment risk (uncertain cash-flow timing), credit risk (especially non-agency), and interest-rate risk — risks starkly exposed by the 2008 subprime crisis.

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