“Buy and hold” is one of the most useful pieces of investing advice ever given. It keeps investors in the market through volatility, suppresses trading costs, and reduces the tax drag of short-term gains. But somewhere along the way, “buy and hold” became confused with “buy and ignore.” Those are not the same thing — and the difference has a real cost.
A portfolio left entirely alone will drift. Over time, the assets that perform well grow into a larger share of the whole, and the assets that lag fall below their intended weights. The result is a portfolio that no longer reflects the investor’s original risk tolerance or investment plan. The investor believes they are in a moderate-risk portfolio. They are actually in something rather more aggressive.
Rebalancing is the act of correcting that drift: periodically selling what has grown beyond its target weight and buying what has fallen below it, restoring the portfolio to the allocations originally chosen. It is not market timing. It is not active management. It is maintenance, and like most maintenance, the cost of neglecting it tends to accumulate quietly until it suddenly becomes visible.
How Drift Happens
Asset prices move at different rates. In a sustained equity bull market, a portfolio initially set at 60% equities and 40% fixed income can drift to 75/25 or beyond without a single trade being made. The investor did nothing wrong in the conventional sense — they stayed invested, resisted panic, and let compounding work. But the composition of what they own has changed substantially.
For a visual representation of how significant this allocation drift can be have a look at this previous post on building a risk premia portfolio.
The mathematics are straightforward. If equities return 15% in a year and bonds return 2%, the equity share of a 60/40 portfolio grows to roughly 66% before rebalancing. Over a multi-year bull run the drift compounds. Research from the CFA Institute illustrates how the risk profile shifts materially when moving from a 60/40 to an 80/20 allocation; a shift that drift can produce entirely without intention.
The practical implication: an investor who set their portfolio at the start of a decade-long equity expansion and never rebalanced may have arrived at retirement holding a portfolio far riskier than the one they intended to hold — with the worst possible timing for a correction.
The Risk Dimension
Risk is the part of portfolio drift that gets the least attention. The discussion tends to focus on return: whether rebalancing adds or costs performance. But the more fundamental issue is that an investor who started with a moderate-risk allocation and allowed it to drift into high-risk territory is now taking risk they never agreed to take.
Every investment plan implies a risk budget: the level of volatility and potential drawdown the investor can absorb, financially and emotionally, without abandoning the strategy. Drift erodes that budget silently. When a correction arrives, the investor discovers that the portfolio they hold is not the portfolio they thought they held; and the gap between the two is measured in lost sleep and, often, in poorly timed redemptions at the worst point in the cycle.
Rebalancing is, at its core, a risk management discipline. The return considerations are secondary.
Three Approaches to Rebalancing
There is no single correct rebalancing method. The right approach depends on the investor’s tax situation, the size of the portfolio, and the cost structure of their accounts. Three methods dominate practice:
Calendar-based rebalancing
The portfolio is reviewed and reset to target on a fixed schedule — annually, semi-annually, or quarterly. Its advantages are simplicity and predictability:
- No monitoring required between review dates
- Easy to integrate with an annual financial review
- Operationally straightforward for advisers and institutions
The limitation is responsiveness. In a sharp market move — the kind seen in early 2020, for example — a quarterly or annual rebalancer may allow drift to reach 10% or more from target before acting.
Threshold-based rebalancing
The portfolio is monitored continuously (or periodically), and rebalancing is triggered only when an asset class drifts beyond a defined band, for example commonly 5% from its target weight. Vanguard research finds that threshold-based rebalancing generates higher expected returns and better risk control than calendar-based approaches for a 60/40 portfolio, primarily because it reduces unnecessary transaction costs while acting more quickly when drift is meaningful.
The trade-off: this approach requires more monitoring and is less predictable in its trading frequency.
Rebalancing through new contributions
New money added to the portfolio — regular contributions, dividends, or interest — is directed to underweight asset classes rather than invested proportionally. This approach:
- Corrects drift incrementally without triggering a sale
- Avoids realizing gains in taxable accounts
- Works particularly well for investors in the accumulation phase who are adding capital regularly
Its limitation is that contributions may not be large enough to correct significant drift on their own, requiring a supplementary sell-and-buy rebalance in extreme cases.
The Counterintuitive Discipline
Rebalancing requires selling assets that have performed well and buying assets that have performed poorly. That is the opposite of what feels instinctively right. The equity allocation is overweight because equities have been rising; buying more fixed income means stepping off what looks like a winning ride. Yet this is precisely what the discipline of “buy low, sell high” demands at the portfolio level.
Investors who understand this can reframe rebalancing not as a constraint, but as a systematic mechanism for enforcing what they already believe in theory: that maintaining exposure to undervalued or underperforming assets and trimming overvalued ones is rational behavior. The rebalancing trigger removes the emotional element from the decision. There is no need to predict the market; there is only the question of whether the current allocation still matches the plan.
This is one of the places where an investment plan earns its keep. Without a defined target allocation, there is nothing to rebalance toward. The act of rebalancing is only possible — and only meaningful — if the investor has already decided what their portfolio is supposed to look like.
Tax and Cost Considerations
Rebalancing in a taxable account involves selling appreciated assets, which triggers a taxable event. This does not make rebalancing wrong, but it does make thoughtful sequencing important:
- Rebalance inside tax-advantaged accounts (pension wrappers, ISAs, 401(k)s, SIPPs, and their equivalents) first — trades within these accounts do not generate taxable gains
- Direct new contributions to underweight asset classes to minimize the amount of selling required in taxable accounts
Use losses in taxable accounts to offset gains where available (tax-loss harvesting). Vanguard’s rebalancing research estimates that prioritizing rebalancing within tax-advantaged accounts before taxable accounts can add roughly 44 basis points of annualized return without increasing portfolio volatility.
The cost dimension also includes transaction fees and bid-ask spreads, particularly relevant in portfolios holding individual securities or less liquid instruments. For low-cost index fund portfolios, transaction costs are generally minor relative to the risk control benefit. The calculation is less clear for portfolios with meaningful friction.
The Sweet Spot: When Not to Over-Rebalance
More frequent rebalancing is not always better. A portfolio rebalanced daily back to its exact target allocations would incur substantial transaction costs and, in taxable accounts, a relentless stream of realised gains. The friction would erode most of the risk-management benefit. Every rebalancing trade has a cost; that cost must be weighed against the cost of drift.
CFA-level analysis frames this as the central trade-off: tighter tolerance bands reduce drift risk but increase trading frequency and cost; wider bands reduce costs but allow greater deviation from target. The practical consensus across institutions is that a 5% threshold band or an annual calendar review — ideally combined — represents a reasonable middle ground for most long-term portfolios.
A few signs that a portfolio may be over-rebalanced:
- Rebalancing trades are consistently very small (say, less than 1% of portfolio value), suggesting drift is within a range that poses no material risk concern
- Transaction costs or tax bills from rebalancing activity are rising as a proportion of portfolio income
- The same asset classes are being bought and sold in rapid succession, indicating the portfolio is oscillating around its target rather than drifting meaningfully from it
Connecting Rebalancing to the Investment Plan
Portfolio rebalancing is not a standalone activity. It is the operational expression of an investment plan. Without a defined target allocation — one grounded in the investor’s time horizon, risk tolerance, and financial objectives — there is nothing to rebalance toward. The discipline of rebalancing assumes that the target allocation was chosen deliberately and that it remains appropriate. If either of those conditions has changed, the right response is to revise the plan, not to rebalance blindly.
This is why passive investing and hands-off investing are not the same thing. A passive investor chooses low-cost, diversified index exposure and avoids the futile exercise of picking securities or timing markets. But they still need to decide what allocation to hold, review whether that allocation continues to match their circumstances, and periodically reset the portfolio when drift has carried it away from the target. The “passive” in passive investing describes the strategy; it does not describe the level of engagement required to maintain it.
What, then, is the price of doing nothing? For many investors, the answer is: more risk than they intended, compounded quietly over time, made visible only when a correction arrives.
Final Thoughts
- “Buy and hold” is not the same as “buy and ignore”: portfolios drift away from their target allocations over time as different assets grow at different rates.
- Drift is primarily a risk problem: a moderate-risk portfolio left unrebalanced through a bull market can silently become a high-risk one.
- Three practical approaches — calendar-based, threshold-based, and contribution-directed — each have merits, and many investors benefit from combining elements of all three.
- Rebalancing forces a counterintuitive action: selling winners and buying laggards, which is, mechanically, what “buy low, sell high” requires at the portfolio level.
- In taxable accounts, sequence matters: use tax-advantaged accounts and new contributions first to minimize unnecessary gain realization; the research suggests this sequencing can add meaningful annualized return.
- Over-rebalancing carries its own costs; the goal is maintaining the risk profile, not achieving perfection on every allocation line. A 5% tolerance band or annual review is a reasonable starting point for most portfolios.
If you reviewed your portfolio today, would the allocation you actually hold still match the allocation you originally chose? Or has drift already decided that question for you?