What Is Rebalancing?
What Is Rebalancing?
Rebalancing is the systematic process of restoring your portfolio to its target allocation weights. When some holdings grow faster than others—or lose value—the percentages you originally chose drift away. Rebalancing brings them back.
Key takeaways
- Rebalancing restores the target weights you wrote into your Investment Policy Statement.
- Market movements cause "drift"—the percentage of your portfolio in each asset class shifts over time.
- The act of rebalancing forces you to sell winners and buy losers, mechanically enforcing "buy low, sell high."
- A portfolio allowed to drift indefinitely becomes gradually riskier or riskier than you intended.
- Rebalancing is not trading for profit; it is maintenance for risk control.
The mechanics of drift
Start with a simple two-asset portfolio: 60% stocks, 40% bonds. You invest £100,000—£60,000 in equities, £40,000 in bonds. For the next year, stocks return 15% and bonds return 2%. Your portfolio is now worth approximately £102,300: £69,000 in stocks (67%), £33,300 in bonds (33%). You never made a conscious decision to become a 67/33 investor, yet that is what happened. This is drift.
Drift accelerates in bull markets (stocks overshoot their target) and creates unintended risk. A 60/40 portfolio that drifts to 70/30 is noticeably more volatile. Over decades, drift can transform a conservative allocation into an aggressive one without your knowledge or consent.
Why your Investment Policy Statement matters
When you crafted your Investment Policy Statement (IPS)—the document describing your goals, time horizon, and risk tolerance—you chose specific target weights. Those weights encode your actual risk appetite and your path to your financial goal. The 60/40 allocation you selected was calibrated for your situation, not someone else's. Rebalancing is the discipline that keeps you honest to that choice.
Ignoring drift is equivalent to saying, "Markets know better what allocation I need than my own financial plan does." Markets do not know your bills, your mortgage, your job security, or your required return. Your IPS does. Rebalancing is the mechanism that forces you to stay true to it.
The mechanical "buy low, sell high"
Rebalancing is a subtle but powerful form of market timing—not in the active-trading sense, but in the disciplined sense. When you rebalance, you are always selling the assets that have done well relative to your target (they are now overweight) and buying the assets that have done poorly (they are underweight). You sell after a bull run, buy after a bear run. This is the opposite of chasing performance.
Academic research, including work by Bogle and Ibbotson, has found that rebalancing adds a small but consistent return enhancement above the passive holding of the original allocation. This is not because you are cleverer than the market; it is because you are forcing yourself to do the hardest thing: sell strength and buy weakness.
However, rebalancing is not free. It incurs transaction costs (trading commissions, bid-ask spreads), and in taxable accounts it can trigger capital gains tax. The net benefit of rebalancing depends on how much the allocation drifts, how often you trade, what costs you pay, and your tax situation. In later articles, we will examine these trade-offs.
When rebalancing becomes non-negotiable
Rebalancing is not optional for portfolios with large contributions or withdrawals. If you are early in retirement, living off your portfolio, or making substantial annual pension contributions, your contributions themselves can drift you back toward your target without active rebalancing. But even in those cases, a sudden market crash (stocks fall 30%, bonds fall 5%) can push your allocation dangerously off-target. Rebalancing is your safety valve.
For a buy-and-hold investor in accumulation (still working, adding to the portfolio regularly), rebalancing via directing new money is painless and tax-efficient. For someone in drawdown (spending from the portfolio), or someone whose portfolio has drifted significantly, rebalancing often requires selling.
The three rebalancing methods
At the highest level, you have three ways to rebalance:
- Direct new contributions to underweight assets (simplest, most tax-efficient).
- Redirect dividends and distributions to underweight assets (free rebalancing fuel).
- Sell overweight assets and buy underweight ones (necessary when contributions and dividends cannot close the gap).
In tax-advantaged accounts (IRAs, 401(k)s, ISAs), all three methods are frictionless. In taxable accounts, methods 1 and 2 are often preferable because they avoid realising capital gains.
Rebalancing is a means, not an end
Rebalancing is not an investment strategy. It is not a way to beat the market. It is a maintenance procedure—like an oil change or a structural inspection. It is the mechanism that keeps your portfolio aligned with your plan. Without it, markets will, over time, build a portfolio for you—one that no longer matches your circumstances, your goals, or your tolerance for volatility.
The decision to rebalance is binary: yes or no. But once you have decided yes, the next questions are about how: what triggering mechanism, what frequency, what costs, what tax consequences. That is the terrain of the remaining articles in this chapter.
Process overview
Next
The next article examines the evidence for rebalancing: whether it actually improves returns, reduces risk, or both. We will look at historical data and explore the difference between risk control (which rebalancing does accomplish) and return enhancement (which is smaller and more uncertain).