Every investor wants to buy low and sell high. Almost none do it naturally. Left to instinct, most investors do the opposite: buying assets after they’ve risen and become exciting, selling after they’ve fallen and become frightening. Rebalancing is the mechanism that forces the better behaviour, whether you feel like it or not.
Why portfolios drift
No portfolio stays where you put it. Different assets grow at different rates, and over time this pulls your actual allocation away from your intended one.
Imagine a portfolio that starts at 60% equities and 40% bonds. After a strong multi-year equity bull run, that same portfolio might become 75% equities and 25% bonds, with no single decision responsible for the shift. No additional shares were bought. The value of the shares simply grew faster than the bonds.
The investor still thinks of themselves as running a moderate, 60/40 risk profile. In reality, they are now carrying meaningfully more risk than they signed up for and they will not find out until the next downturn arrives and the losses are larger than expected.
Rebalancing exists to close that gap: periodically trimming what has grown above its target weight and topping up what has fallen below it, restoring the portfolio to the risk level it was actually built for.
The behavioural case for rebalancing
The mechanical explanation is only half the story. The more important case for rebalancing is behavioural.
Selling an asset that has just performed well feels wrong. Buying more of an asset that has just fallen in value feels worse. Every instinct says to let the winners run and avoid the losers. Rebalancing overrides that instinct by design, forcing a systematic version of buying low and selling high rather than leaving the decision to sentiment in the moment.
This is precisely why chasing performance is one of the most reliably damaging patterns in investing. Overweighting whatever did best last year means buying in at the point valuations are most stretched, just as mean reversion becomes most likely. Rebalancing is the disciplined antidote: it takes profits from what has become expensive and reinvests in what has become comparatively cheap, systematically, without requiring the investor to predict anything or trust their own nerve in the moment.
Rebalancing as a valuation tool, not just a maintenance task
A well-managed portfolio treats rebalancing as more than housekeeping. Done properly, it becomes an active tool for both risk management and return enhancement.
Trimming a position that has run hard is not simply about restoring a target percentage. It is also, often, about reducing exposure to an asset whose valuation has become stretched relative to its underlying fundamentals. Adding to a position that has lagged is not simply filling a gap. It can mean increasing exposure to something that has become more attractively priced.
Used this way, rebalancing keeps a portfolio aligned not just with its strategic targets, but with the prevailing opportunity set: buying more of what now offers better value, and less of what no longer does.
“Be fearful when others are greedy, and greedy when others are fearful.” Warren Buffett
Rebalancing is, in effect, a systematic way of applying that principle without needing to time markets or predict the future. It doesn’t require knowing when a rally will end or a downturn will reverse. It only requires the discipline to act when drift becomes meaningful, in either direction.
Knowing when to act
Rebalancing should not be triggered by every market wiggle. Constant trading introduces its own costs: brokerage, bid-offer spreads, and potential tax consequences from realising gains, all of which quietly erode the very returns rebalancing is meant to protect.
Two broad approaches are common:
Calendar-based rebalancing reviews the portfolio at fixed intervals, such as quarterly or annually, and adjusts back to target weights regardless of how far it has drifted.
Threshold-based rebalancing monitors the portfolio continuously but only acts once an asset class has moved a meaningful distance from its target, for example five percentage points. This tends to be more responsive to genuine drift while avoiding unnecessary transaction activity during periods of normal market noise.
Either approach can work. What matters is having a method and applying it consistently rather than deciding case by case in the heat of the moment, which is exactly when judgement is least reliable.
The discipline behind the discipline
Rebalancing sounds simple in theory. In practice, it asks an investor to do the two hardest things in investing at once: sell something that has been making them money and buy something that has been losing it.
That discomfort is the point. A rebalancing strategy that never feels uncomfortable to execute probably isn’t doing very much. The investors who benefit most from rebalancing are rarely the ones who enjoy doing it. They are the ones who do it anyway, on schedule, because the framework asks them to rather than because it feels good in the moment.
Over one year, the effect of rebalancing is usually modest. Over twenty, applied consistently through multiple market cycles, it becomes one of the more dependable sources of long-term outperformance. This is not because it predicts the future, but because it enforces the one behaviour that consistently proves difficult for us to enforce ourselves.
