Portfolio Drift and Rebalancing
How a portfolio's mix changes on its own as markets move, what that did to a 60/40 portfolio in real US data, and the main ways to bring it back to target.
The short version
Over time, whatever grows fastest becomes a bigger share of your portfolio. That's drift, and it quietly changes your risk. A 60% stock, 40% bond portfolio left alone through the 1990s ended the decade at about 80% stocks, then fell about twice as far as a rebalanced one from 2000 to 2002. Rebalancing means bringing the mix back to target, by selling some of what has grown, buying what has lagged, or directing new money. It controls risk; it doesn't always raise returns.
By the end of this lesson, you'll be able to
- Explain what portfolio drift is and why it changes risk
- Calculate the trades needed to rebalance a portfolio
- Compare calendar and threshold rebalancing, including costs and taxes
Why this matters
You chose an allocation for a reason: it fits the goal's time horizon and the losses you can live with. But markets don't hold your mix still. After a few strong years for stocks, a portfolio built to be moderate can become aggressive without you making a single decision. Rebalancing is how you keep the risk you actually chose.
What drift looks like
Portfolio drift. The gradual change in a portfolio's mix as some holdings grow faster than others. Glossary happens because different holdings grow at different rates. The fastest grower becomes a bigger share of the whole. The SEC's guide gives an example: you decide stocks should be 60% of your portfolio, and after a stock market rise they're 80%.1
$100,000 is invested at the start of 1990: 60% in the S&P 500, 40% in 10-year Treasury bonds. One version is never rebalanced. The other is rebalanced to 60/40 at the start of every year.
| Never rebalanced | Rebalanced yearly | |
|---|---|---|
| Stocks as a share, end of 1999 | 79.5% | 60% |
| Value at the end of 1999 | $396,900 | $368,700 |
| Change from 2000 to 2002 | −21.2% | −10.6% |
| Value at the end of 2002 | $312,900 | $329,700 |
Calculated from Damodaran's yearly returns, before fees, taxes, and inflation.2 Past results don't predict future ones.
Two lessons sit in that table. During the 1990s boom, letting stocks run earned more, because stocks did very well. But by 1999, the "60/40" investor really owned an 80/20 portfolio, and when stocks fell, they lost about twice as much as the rebalanced investor, ending up behind. Drift had silently turned a moderate portfolio into an aggressive one at the worst possible time.
The opposite can happen too. Over the long bull market from 2009 to 2021, the same calculation shows a never-rebalanced 60/40 portfolio drifting to about 88% stocks and ending well ahead of the rebalanced one.2 Rebalancing isn't a way to earn more. It's a way to keep your risk where you chose to put it.
The rest of this lesson is for members
Still to come in this lesson: How to rebalance, When to rebalance, the key takeaways, and 2 quiz questions with explanations.
Membership opens every lesson in the curriculum, the capstone projects, the interactive tools, and the members-only channels in the Hyperion Entities Discord. The first lesson of every module stays free.
Go deeper
- Rebalancing Strategies in DepthPortfolio Theory and Risk (depth track)
- Asset LocationPart 6: Strategy, Behavior, Accounts, and Taxes
- Asset AllocationPart 5: Building a Portfolio
Notes and sources
- Investor.gov, U.S. Securities and Exchange Commission. Beginners' Guide to Asset Allocation, Diversification, and Rebalancing. Checked 2026-09-26. Back to text
- Aswath Damodaran, NYU Stern School of Business. Historical Returns on Stocks, Bonds and Bills, with annual data through 2025 (updated January 2026). Checked 2026-10-12. Back to text