Rebalancing is the rare portfolio decision that is both mechanically simple and psychologically difficult. It asks you to sell what has been working and buy what has not — which is precisely why most investors quietly stop doing it at exactly the moments it matters most.
What rebalancing is actually for
The common claim is that rebalancing boosts returns. Sometimes it does, but that is not its job and it is not reliably true. In a long equity bull market, a portfolio that never rebalances will usually outperform one that does — because it drifts toward the asset that happens to be winning.
Rebalancing is a risk control. Its purpose is to keep the portfolio you own resembling the portfolio you chose. Left alone, a 60/40 portfolio that started in 2009 would have drifted well past 80/20 within a decade. The investor never decided to take substantially more risk; the market decided for them, and they found out during the next drawdown.
Calendar, threshold, or both
Three approaches dominate practice:
- Calendar rebalancing — check on a fixed schedule, typically annually. Simple, predictable, easy to automate. Its weakness is that it ignores what markets did between checkpoints.
- Threshold rebalancing — act when an allocation drifts more than a set amount from target, commonly five percentage points absolute or 25% relative. More responsive, and it trades only when there is something worth trading.
- Combined — check on a schedule, act only if a threshold is breached. This captures most of the benefit of threshold rebalancing without requiring you to watch daily.
The evidence does not strongly favor one over the others. It favors having a rule and following it. The gap between a good policy and a mediocre policy is small. The gap between any policy and improvisation is large.
Rebalance with cash flow first
The cheapest rebalancing trade is the one you do not have to make. If you are still contributing, direct new money to the underweight asset. If you are drawing income, take withdrawals from the overweight one. In taxable accounts, this alone can handle most drift without realizing a single capital gain.
Where trades are unavoidable, sequence matters for tax:
- Rebalance inside tax-deferred and tax-free accounts, where trades have no immediate tax cost.
- Use dividends and interest in taxable accounts rather than automatically reinvesting them.
- Pair any necessary gain realization with available losses.
- Only then consider a taxable sale on its own merits.
The behavioral part is the hard part
Rebalancing in March 2009 meant buying equities while headlines said the financial system was ending. Rebalancing in late 2021 meant trimming the positions everyone was celebrating. Both were correct. Both felt wrong.
A rebalancing policy written down in advance is a decision made by a calm person on behalf of a future anxious one.
This is the strongest argument for automation: not that a computer is smarter, but that it is not frightened. Write the rule while markets are unremarkable, commit to it, and let the schedule rather than the mood decide when to act.
A workable default
For most diversified portfolios: review semi-annually, act when any major asset class drifts five percentage points from target, use contributions and withdrawals as the first tool, and confine trades to sheltered accounts wherever possible. That is unglamorous, and it is enough.
