What it means
Central banks traditionally buy government bonds to steer interest rates, and the agency MBS purchase extends that toolkit to the housing market. The central bank buys securities built from bundled home loans guaranteed by agencies such as Fannie Mae and Freddie Mac.
The instrument matters because housing finance is enormous: mortgage-backed securities pool thousands of home loans into tradable bonds, and their yields set the price of new mortgages for households across the country. The transmission is direct.
When the central bank buys agency MBS in size, demand for the securities rises, their yields fall, and lenders pass the cheaper funding through as lower mortgage rates for borrowers. The purchases are funded by creating reserves, so the central bank pays the selling banks with newly created central bank money, expanding its balance sheet and adding liquidity to the banking system in one stroke.
The tool rose to prominence in crises. The US Federal Reserve began large-scale agency MBS purchases in the 2008 financial crisis to unfreeze mortgage markets, and expanded them again in later downturns as part of quantitative easing.
Its agency MBS holdings grew from zero before 2008 into one of the largest single portfolios in the world, measured in trillions, making its purchase and runoff decisions market events in themselves. The portfolio's composition matters as much as its size.
Purchases concentrate in newly issued securities, so the programme directly lowers the funding cost of new mortgages rather than simply supporting old bonds. For markets, the purchase pace is a watched number, because weekly agency MBS purchase schedules signal the central bank's stance and changes in that pace have moved bond and mortgage markets within minutes.
The policy has critics on two fronts. Buying housing-linked securities steers credit toward property rather than leaving allocation to markets, and the accumulated portfolio becomes politically difficult to unwind without upsetting mortgage rates.
Unwinding works two ways: the central bank can let securities mature without replacing them, shrinking the portfolio passively, or it can sell holdings outright, each approach pressing upward on mortgage rates at different speeds. For a manager, the concept explains why mortgage and corporate borrowing costs sometimes fall even when the economy looks weak: the central bank is in the market as a buyer of exactly the securities your lender funds itself with.
Watching the published purchase schedule is therefore part of anticipating borrowing costs.
In practice
Real-world examples.
Example
During a crisis the central bank announces monthly purchases of agency MBS, and average quoted mortgage rates fall by more than a percentage point within months.
Example
A lender prices new home loans off MBS yields, so a week of heavy central bank purchases shows up directly in the rates offered at the branch.
Example
The central bank stops reinvesting maturing agency MBS, its portfolio shrinks gradually, and mortgage rates drift upward even though the policy rate has not changed.
Formula
Calculation
There is no single formula, but the transmission can be followed step by step: reserves created = securities purchased; MBS yields fall as demand rises; lenders reprice new mortgages off those yields; the banking system holds the injected reserves.
Suppose a central bank buys $20,000,000,000 of agency MBS a month for six months. Securities purchased are 6 x $20,000,000,000 = $120,000,000,000, and the same amount of new reserves is credited to the selling banks.
Now suppose the added demand lowers quoted mortgage rates from 6.5% to 6.0% and a lender passes the full fall through. On a $300,000 mortgage, interest on the opening balance in the first year (ignoring repayments) falls from 6.5% x $300,000 = $19,500 to 6.0% x $300,000 = $18,000. That is a saving of $1,500 for the borrower in the first year.Case study
Seen in the real world.
This case study is fictional and illustrative. A made-up building society, Marlow Hill, watches its funding costs fall as the central bank's agency MBS purchases compress mortgage bond yields. It reopens a fixed-rate product it had withdrawn, wins a record month of remortgage business, and hedges its pipeline in case the purchase programme tapers. Its fixed rate falls from 6.5% to 6.0%, so a typical $300,000 borrower saves $1,500 of first-year interest.
Enquiries roughly double within weeks, and the sales team has to extend its opening hours to keep up. The finance director then models what happens if the central bank stops reinvesting maturing securities. She concludes that funding costs would drift upwards even with an unchanged policy rate, and sets pricing floors accordingly. The society and figures are invented for illustration only.
Watch out
Common mistakes.
- Confusing agency MBS with government bonds; the securities carry agency guarantees rather than the full sovereign promise, which is why their yields sit above government debt.
- Assuming purchases only help borrowers; the programme compresses returns for investors holding MBS, and unwinding it pushes mortgage rates up even when the policy rate is steady.
- Ignoring the signal in the schedule; changes in the published purchase pace move markets immediately, so treasury teams track the calendar as closely as rate decisions.
Questions
People also ask.
What is an agency MBS purchase?
A central bank's purchase of mortgage-backed securities guaranteed by government-sponsored housing agencies. It injects reserves into the banking system and lowers the yields that set mortgage and long-term borrowing rates.
Why does a central bank buy mortgage-backed securities?
To ease financial conditions specifically in housing finance. Heavy purchases push MBS yields down, and lenders pass the cheaper funding through as lower mortgage rates, supporting the housing market and the wider economy.
How are agency MBS purchases unwound?
Either passively, by letting maturing securities run off without replacement, or actively, by selling holdings. Both reduce the central bank's balance sheet and tend to put upward pressure on mortgage rates.
From the founder's library
Take it further with the book.
Build your financial confidence beyond this definition. Shihan's full-length guide, Accounting Fundamentals, takes the same plain-English approach and turns it into a complete, practical playbook for non-finance managers, business owners and students - with chapter-end quiz answers and presentation slides included.
25% off with code MMHQ25, applied at checkout. Priced in USD - checkout may show the equivalent in your local currency.
View the book and save 25%