Rebalancing: Keeping the Plan
Left alone, a portfolio quietly drifts into something riskier than you chose. Rebalancing is the simple rule that pulls it back — and makes you buy low by design.
You carefully built a diversified portfolio (lessons 30, 15) with sensible proportions — say a mix of stocks and bonds chosen for a risk level you’re comfortable with. Then you do nothing, and time passes. Here’s the quiet trap: a portfolio left alone does not stay the portfolio you built. Its pieces grow at different rates, and it silently drifts into a different, usually riskier mix than you ever chose — often becoming most aggressive right before it matters most. There’s a simple, almost mechanical habit that fixes this and, as a bonus, forces good behavior. Hold the question: if you never touch a diversified portfolio, why would its risk level change on its own?
Portfolios drift toward risk
Your chosen proportions are your target weights — say 60% stocks, 40% bonds. But holdings grow at different rates, and the winners swell while the laggards shrink as a share of the whole. After a strong run for stocks, that 60/40 might quietly become 75/25 — you now own a much riskier portfolio than you signed up for, without making a single decision. This is portfolio drift, and its direction is treacherous: it loads you up on whatever has run up most, precisely the thing most likely to fall hardest in a downturn. Drift means your actual risk creeps upward exactly when you’re least prepared for it — undoing the careful risk level diversification gave you (lessons 30–31).
Rebalancing restores the plan
Rebalancing is the fix: periodically sell some of what grew and buy some of what shrank to bring the mix back to your target weights. Drifted to 75/25? Sell enough stocks and buy enough bonds to return to 60/40. That’s it — it’s maintenance, not prediction. Notice what it mechanically forces you to do: trim the thing that’s expensive after a run and add to the thing that’s cheap after a fall — “sell high, buy low” by rule, with no forecasting required. And crucially, it makes you act against the crowd and against your own fear/greed (recall the behavioral traps of lesson 20): buying more of what everyone’s fleeing, trimming what everyone’s chasing. The discipline is in the rule, which is exactly why it works when raw willpower wouldn’t.
A 60/40 portfolio, rebalanced: • Start: $60k stocks / $40k bonds. • After a stock boom: $90k stocks / $40k bonds = ~69/31. Riskier than intended. • Rebalance to 60/40 of the $130k total → $78k stocks / $52k bonds: sell $12k of the run-up stocks, buy $12k bonds. • You just trimmed the expensive winner and topped up the cheaper holding — automatically, by rule, without guessing what’s next.
How, and the honest caveats
Rebalancing is usually done on a rule, not a whim: on a schedule (e.g. once a year) or when a weight drifts past a threshold (e.g. more than 5 points off target). The rule matters more than the specifics — its whole power is removing emotion and forecasting from the decision. Two honest caveats so you’re not naïve about it: rebalancing can trigger transaction costs and taxes (you’re selling), so you don’t want to do it too often or thoughtlessly; and it is not a return-maximizer — in a long one-way bull run, never trimming your winners would have made more. Its real job is risk control: keeping your portfolio at the risk level you actually chose, and enforcing buy-low/sell-high discipline. It’s the fitting close to this module — sizing, diversification, drawdown-awareness, and risk-adjusted thinking all serve the same goal, and rebalancing is the habit that keeps them in force over time. (Principle, not advice — your own schedule, thresholds, and mix depend on your situation.)
Think of a garden bed planted in careful proportions. Leave it a season and the vigorous plants overgrow, crowding out the rest until the bed is nearly all one aggressive species — not the balanced garden you planted. Rebalancing is the gardener who periodically trims back whatever’s taken over and gives room to what’s been squeezed, restoring the original design. The gardener isn’t predicting which plant will thrive next; they’re just keeping the balance you chose — and, in doing so, naturally cutting back the overgrown and nurturing the sparse, which is exactly “sell high, buy low” in leaf and soil.
Why the rule beats willpower: 1. After a huge rally, an investor’s stocks have ballooned to 80% of the portfolio. Everyone’s euphoric; selling any feels crazy. 2. A rebalancing rule says “back to 60%,” so they trim stocks and add bonds — acting against the euphoria, by rule, not by nerve. 3. When the market later falls, they were less over-exposed to stocks, so their drawdown is milder — and the rule now tells them to buy stocks cheap while everyone panics. 4. The discipline lived in the rule; no forecasting, no willpower heroics — just maintenance that quietly did the hard, counter-cyclical thing.
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