Rebalancing is a decision you make once

“Stay the course” is good advice that gets heard as “do nothing.” A long-term plan has trades in it: bringing a drifted mix back to its targets, trimming the position that grew past its cap, adding on the drop you said you would buy. The research on real investors says the trouble is rarely whether they trade. It is when.

What timing costs

Every year Morningstar compares what US mutual funds and ETFs earned with what the dollars invested in them earned, which depends on when investors bought and sold. Over the ten years to the end of 2025, the funds returned 9.9% a year. The average dollar in them returned 8.7%.

The gap is not even across funds. In every rolling ten-year window Morningstar reports, from the decade ended 2021 to the decade ended 2025, investors in allocation funds kept more of their funds’ returns than the average investor: they gave up 0.1 to 0.8 points a year, against 1.1 to 1.7 overall. Allocation funds rebalance on their own and are often bought through regular contributions, so the trades in them happen on a schedule rather than on a feeling. Plain US stock funds had narrow gaps too, and Morningstar’s own reading is that investors have fared better with simple options than with complex building blocks. Simplicity and a schedule do the same work: they take the timing out of the investor’s hands.

Drift is the default

Left alone, a portfolio drifts toward whatever has been rising. Laurent Calvet, John Campbell and Paolo Sodini followed the finances of every household in Sweden and found that when market moves pushed a household’s share of risky assets up or down, the household’s own trading undid only about half of that change. The other half stayed. Nobody chose the new mix; the market chose it, and inertia kept it.

When investors do act, they often act in the wrong direction. Terrance Odean’s study of 10,000 brokerage accounts found they sold winners about one and a half times as readily as losers. It was not foresight: over the following year, the winners they sold beat the losers they kept by about three percentage points. Trimming a winner can be exactly right, when your plan says so. The problem was that these sales followed the feeling of locking in a gain, and the losers stayed because selling them felt like admitting a mistake.

How often is a rule, not a mood

There is no perfect rebalancing schedule, but there is a reasonable range. Vanguard modelled rebalancing across a wide range of simulated markets, trading costs included, and found the best methods were neither too frequent (monthly or quarterly) nor too rare (every two years). For investors who are not harvesting tax losses or tracking a benchmark closely, rebalancing about once a year came out on top. Others prefer a band: act only when a holding drifts a set distance from its target.

Either is defensible. What none of this research supports is rebalancing when it feels urgent, because the feeling tends to arrive just after the move the rebalance was meant to prepare for.

Write it as if-then

Psychologists have studied the difference between deciding to do something and deciding exactly when you will. Peter Gollwitzer and Paschal Sheeran pooled 94 tests of what they call implementation intentions, plans written as “if this happens, I will do that.” People who made them reached their goals more often than people who only set the goal, by a margin the authors describe as medium to large. The effect showed up in getting started, in staying on track when distracted, and in changing course when the original plan stopped working.

A rebalancing rule is an implementation intention with numbers in it. “If any holding passes my cap, I trim it back.” “If my mix drifts past my band, I rebalance to my targets.” “Every January, I bring it back.” The numbers are yours to choose. The research is about the shape: the moment and the action, decided together and in advance, so that when the moment arrives the decision is already made.

What this means for a portfolio

Holding still and trading on schedule are the same discipline. Both come down to one thing: your trades happen when your plan says, not when the market makes you feel something. A plan that only lives in your head gets rewritten by every big move. A plan written as rules gets followed through.

That is what Cetagon is for. You write the rule, the action you decided on and the reason you decided it, and when the line is crossed, all three come back to you. Whether it says hold, trim or buy, the decision is the one you already made. Write your first rule →