What Is Quantitative Easing?
Quantitative easing (QE) is an unconventional monetary policy in which a central bank creates new reserves to buy financial assets - primarily government bonds and (sometimes) mortgage-backed securities - from commercial banks and other institutions.
The mechanical effect: the central bank's balance sheet grows. The composition of bank assets changes. Bond yields fall. Risk asset prices rise.
The intended economic effect: lower long-term borrowing costs, more bank lending, increased asset prices, increased spending and investment, eventually higher inflation and growth.
Whether QE actually achieves these economic effects is debated. Whether it changes asset prices is not - it clearly does.
For broader macro context, see our LTCM collapse and Flash Crash explainers.
How QE Works Mechanically
Step by step:
Step 1: Central bank decides to ease
A central bank (Federal Reserve, European Central Bank, Bank of England, Bank of Japan) determines that conditions warrant additional stimulus beyond conventional rate cuts. Typically this is when policy rates are already at zero ("zero lower bound") and inflation/growth need more support.
Step 2: Central bank announces a purchase programme
The central bank announces a specific size and pace - e.g., "$120 billion per month of Treasuries and MBS for the next 12 months."
Step 3: Central bank credits dealer accounts
When the central bank buys a bond from a primary dealer (a major bank like JPMorgan or Goldman Sachs), it pays by creating new reserves in the dealer bank's account at the central bank. These reserves don't come from anywhere - they're newly created.
Step 4: Bonds move to the central bank balance sheet
The central bank now holds the bonds. The dealer bank now has more reserves and fewer bonds. The total amount of money in the financial system has expanded.
Step 5: Markets adjust
- Bond prices rise (yields fall) because there's a new large buyer
- Banks have more reserves and incentives to lend or buy other assets
- Risk asset prices rise (broader portfolio rebalancing)
- The dollar typically weakens against other currencies
Important: QE is not "printing money" in the everyday sense
The newly-created reserves are held by banks at the central bank. They don't directly enter consumer hands. Whether they translate into broader money supply growth depends on whether banks lend out the reserves and what borrowers do with the money.
A Brief History of QE
QE1: November 2008 - June 2010
The Fed's first major QE programme launched in response to the 2008 financial crisis. Initial focus was on mortgage-backed securities to support housing markets. Fed balance sheet grew from ~2.1T.
Effect:
- Mortgage rates fell sharply
- Bank balance sheets stabilised
- Equity markets bottomed in March 2009 and began a multi-year bull run
QE2: November 2010 - June 2011
Smaller programme focused on long-term Treasury purchases. Designed to flatten the yield curve and support inflation expectations.
Effect:
- Modest yield reduction
- Boost to risk assets continued
- Some critics argued QE was creating asset bubbles without supporting real economy
QE3 ("QE Infinity"): September 2012 - October 2014
Open-ended programme: 45B/month Treasuries, with no fixed end date. Fed balance sheet grew to ~$4.5T.
Effect:
- US equities continued strong rally
- Housing recovery accelerated
- Unemployment fell from 8% to 5.7%
QE4 ("Pandemic QE"): March 2020 - March 2022
Massive programme in response to COVID-19. Fed balance sheet grew from ~8.9T - approximately 40% of US GDP.
Other central banks did similar:
- ECB balance sheet grew from €4.7T to €8.5T
- BOJ balance sheet exceeded 100% of Japan's GDP
- BOE balance sheet doubled
Effect:
- Risk assets boomed despite economic recession
- Bond yields hit historic lows globally
- Eventually contributed to the 2022 inflation surge
Quantitative Tightening (QT): The Unwinding
QT is the reverse of QE: the central bank reduces its balance sheet by either:
- Letting bonds mature without reinvesting (passive runoff)
- Actively selling bonds back into the market (rare; more disruptive)
Fed QT (June 2022 - present)
The Fed began QT in June 2022, allowing 8.9T to ~$7.0T.
Effects:
- Bond yields rose (along with rate hikes - hard to disentangle)
- Bank reserves declined
- Some episodes of repo market stress (echoing September 2019)
What's different in 2026
After the 2024-2025 disinflation, central banks globally have begun easing policy rates again, but most are continuing QT (or maintaining roughly stable balance sheets). The interaction between rate cuts and QT creates novel dynamics for markets to digest.
Why QE Matters to Quants
1. Yield curve dynamics changed
Pre-QE, the long end of the yield curve was set primarily by inflation expectations and term premium. Post-QE, central bank purchases became a major direct influence. This affects:
- Fixed income strategies (relative value across the curve)
- Carry trades
- Mortgage strategies (negative convexity hedging)
2. Volatility regimes shifted
QE compressed implied vol across asset classes - the "Fed put" idea. Vol-selling strategies performed unusually well during QE periods; vol-buying strategies underperformed. The reversal in 2022 caused significant losses for vol-short strategies that had become institutionalised.
3. Cross-asset correlations changed
Stock-bond correlation shifted from negative (the historical norm) to positive during QE periods, then back to negative in 2022 inflation. Risk parity strategies depended heavily on this correlation; their 2022 drawdowns were directly linked to this regime shift.
4. Liquidity dynamics
Banks holding excess reserves behave differently. Repo markets behave differently. The 2019 repo spike and 2023 SVB collapse both reflect structural changes from QE/QT cycles.
5. Carry trades become asymmetric
When central banks suppress rates, currencies become correlated with their central bank's QE stance. The yen carry trade unwind in August 2024 reflected divergence between BOJ and Fed policies.
Quants' Models Need to Account for QE
If you're building a strategy, consider:
- Sample period selection. A strategy backtest from 2010-2021 is largely a "QE era" backtest. It may not generalise.
- Regime classification. Many strategies benefit from explicit regime detection (high-vol vs low-vol, QE vs QT, etc.)
- Tail correlation in stress. QE compressed but didn't eliminate stress correlation. When liquidity reverses, correlations can spike (see LTCM for the historical analog).
- Forward inflation expectations. TIPS-derived breakeven inflation is a more useful signal than historical inflation in QE periods.
For relevant interview-prep coverage, see:
- Quant research interview questions (macro section)
- Time series interview questions
- Derivatives pricing interview questions (rates section)
Common Misconceptions
"QE is money printing that causes inflation." Mostly false historically. QE creates bank reserves, not consumer cash. The 2010-2020 QE didn't generate inflation. The 2020-2022 QE coincided with significant fiscal transfers (CARES Act, etc.) - the fiscal-monetary combination was inflationary, not the QE alone.
"QE was a one-time emergency tool." False. QE has been used by every major central bank multiple times since 2008. It's now part of the standard toolkit.
"QE only helps banks and rich people." Mixed. QE supports asset prices, which benefits asset holders disproportionately. But QE also lowers borrowing costs (mortgages, business loans) which has broader benefits. The distributional effects are debated.
"Central banks lose money during QT." Partly true. As bonds mature and rates have risen, central banks face accounting losses on their bond portfolios. The Fed has reported significant operating losses since 2022. Whether this matters economically is debated.
What's Likely Next
In 2026, the global QE/QT picture is mixed:
- The Fed is gradually reducing its balance sheet
- The ECB is roughly flat after years of expansion
- The BOJ is still moving slowly
- Emerging market central banks are largely independent
For quant strategies, key dynamics to watch:
- US fiscal trajectory (deficits affect Treasury supply)
- Inflation persistence (matters for term premium)
- Bank reserve dynamics (matters for funding markets)
- Cross-central-bank divergence (matters for FX)
Further Reading
- The Lords of Finance by Liaquat Ahamed - history of central banking, won Pulitzer
- The Courage to Act by Ben Bernanke - first-person account of QE1 design
- Reform by Mervyn King - BoE perspective on monetary policy
- The Power of Independence by Tim Geithner - financial crisis policy memoir
- Federal Reserve research papers on QE effectiveness (search "FEDS Working Papers")
For related topics:
- LTCM collapse - earlier monetary policy era
- Flash Crash of 2010 - market structure under QE
- Greatest quant traders of all time
- Best quant finance books - for further macro reading
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