A portfolio built to fixed proportions does not stay there, because its holdings grow at different rates. Rebalancing restores the original weights, and its purpose is often misunderstood.
Drift changes risk without any decision
A portfolio split evenly between shares and bonds will tilt toward shares over time if shares rise faster, and the tilt compounds.
After a long rise, an investor who chose a moderate allocation may hold something considerably more aggressive than they intended, without ever having agreed to it.
The drift is largest exactly when it matters most, because the concentration builds up before a fall rather than after one.
The mechanics are contrarian by construction
Restoring target weights requires selling the asset that has performed well and buying the one that has lagged, which runs against the direction of recent results.
This is uncomfortable in practice. Rebalancing after a sharp decline means adding to the asset that has just caused the loss.
The discipline works because it is rule-based. An investor deciding case by case will usually find reasons to postpone the uncomfortable side of the trade.
The return effect is smaller than claimed
Rebalancing is sometimes described as a source of extra return, on the basis that it systematically sells high and buys low.
That holds when assets fluctuate around similar long-run returns, but where one asset genuinely outperforms over decades, rebalancing away from it reduces the outcome.
The dependable benefit is that the portfolio keeps the risk profile it was designed to have, which is a control function rather than a performance one.
Frequency and cost interact
Rebalancing too often generates transaction costs and, in taxable accounts, realised gains that would otherwise have been deferred.
Rebalancing too rarely allows drift to accumulate. Most approaches use either a fixed interval or a tolerance band that triggers action only after a weight moves far enough.
Bands tend to be more efficient than calendars, because they act when the portfolio has actually moved rather than when a date has arrived.
Using contributions instead of trades
An investor still adding money can direct new contributions toward the underweight asset, which shifts weights without selling anything.
That avoids both transaction costs and tax consequences, and for a portfolio that is still growing it can handle most of the adjustment.
Only when contributions become small relative to the portfolio does explicit selling become the main mechanism available.